Introduce a new API for thread-local blk_io_plug() that does not
traverse the block graph. The goal is to make blk_io_plug() multi-queue
friendly.

Instead of having block drivers track whether or not we're in a plugged
section, provide an API that allows them to defer a function call until
we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
called multiple times with the same fn/opaque pair, then fn() is only
called once at the end of the function - resulting in batching.

This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
because the plug state is now thread-local.

Later patches convert block drivers to blk_io_plug_call() and then we
can finally remove .bdrv_co_io_plug() once all block drivers have been
converted.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 MAINTAINERS                       |   1 +
 include/sysemu/block-backend-io.h |  13 +--
 block/block-backend.c             |  22 -----
 block/plug.c                      | 159 ++++++++++++++++++++++++++++++
 hw/block/dataplane/xen-block.c    |   8 +-
 hw/block/virtio-blk.c             |   4 +-
 hw/scsi/virtio-scsi.c             |   6 +-
 block/meson.build                 |   1 +
 8 files changed, 173 insertions(+), 41 deletions(-)
 create mode 100644 block/plug.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 50585117a0..574202295c 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2644,6 +2644,7 @@ F: util/aio-*.c
 F: util/aio-*.h
 F: util/fdmon-*.c
 F: block/io.c
+F: block/plug.c
 F: migration/block*
 F: include/block/aio.h
 F: include/block/aio-wait.h
diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h
index d62a7ee773..be4dcef59d 100644
--- a/include/sysemu/block-backend-io.h
+++ b/include/sysemu/block-backend-io.h
@@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error);
 int blk_get_max_iov(BlockBackend *blk);
 int blk_get_max_hw_iov(BlockBackend *blk);
 
-/*
- * blk_io_plug/unplug are thread-local operations. This means that multiple
- * IOThreads can simultaneously call plug/unplug, but the caller must ensure
- * that each unplug() is called in the same IOThread of the matching plug().
- */
-void coroutine_fn blk_co_io_plug(BlockBackend *blk);
-void co_wrapper blk_io_plug(BlockBackend *blk);
-
-void coroutine_fn blk_co_io_unplug(BlockBackend *blk);
-void co_wrapper blk_io_unplug(BlockBackend *blk);
+void blk_io_plug(void);
+void blk_io_unplug(void);
+void blk_io_plug_call(void (*fn)(void *), void *opaque);
 
 AioContext *blk_get_aio_context(BlockBackend *blk);
 BlockAcctStats *blk_get_stats(BlockBackend *blk);
diff --git a/block/block-backend.c b/block/block-backend.c
index ca537cd0ad..1f1d226ba6 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify)
     notifier_list_add(&blk->insert_bs_notifiers, notify);
 }
 
-void coroutine_fn blk_co_io_plug(BlockBackend *blk)
-{
-    BlockDriverState *bs = blk_bs(blk);
-    IO_CODE();
-    GRAPH_RDLOCK_GUARD();
-
-    if (bs) {
-        bdrv_co_io_plug(bs);
-    }
-}
-
-void coroutine_fn blk_co_io_unplug(BlockBackend *blk)
-{
-    BlockDriverState *bs = blk_bs(blk);
-    IO_CODE();
-    GRAPH_RDLOCK_GUARD();
-
-    if (bs) {
-        bdrv_co_io_unplug(bs);
-    }
-}
-
 BlockAcctStats *blk_get_stats(BlockBackend *blk)
 {
     IO_CODE();
diff --git a/block/plug.c b/block/plug.c
new file mode 100644
index 0000000000..6738a568ba
--- /dev/null
+++ b/block/plug.c
@@ -0,0 +1,159 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * Block I/O plugging
+ *
+ * Copyright Red Hat.
+ *
+ * This API defers a function call within a blk_io_plug()/blk_io_unplug()
+ * section, allowing multiple calls to batch up. This is a performance
+ * optimization that is used in the block layer to submit several I/O requests
+ * at once instead of individually:
+ *
+ *   blk_io_plug(); <-- start of plugged region
+ *   ...
+ *   blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call
+ *   blk_io_plug_call(my_func, my_obj); <-- another
+ *   blk_io_plug_call(my_func, my_obj); <-- another
+ *   ...
+ *   blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once
+ *
+ * This code is actually generic and not tied to the block layer. If another
+ * subsystem needs this functionality, it could be renamed.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/coroutine-tls.h"
+#include "qemu/notify.h"
+#include "qemu/thread.h"
+#include "sysemu/block-backend.h"
+
+/* A function call that has been deferred until unplug() */
+typedef struct {
+    void (*fn)(void *);
+    void *opaque;
+} UnplugFn;
+
+/* Per-thread state */
+typedef struct {
+    unsigned count;       /* how many times has plug() been called? */
+    GArray *unplug_fns;   /* functions to call at unplug time */
+} Plug;
+
+/* Use get_ptr_plug() to fetch this thread-local value */
+QEMU_DEFINE_STATIC_CO_TLS(Plug, plug);
+
+/* Called at thread cleanup time */
+static void blk_io_plug_atexit(Notifier *n, void *value)
+{
+    Plug *plug = get_ptr_plug();
+    g_array_free(plug->unplug_fns, TRUE);
+}
+
+/* This won't involve coroutines, so use __thread */
+static __thread Notifier blk_io_plug_atexit_notifier;
+
+/**
+ * blk_io_plug_call:
+ * @fn: a function pointer to be invoked
+ * @opaque: a user-defined argument to @fn()
+ *
+ * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
+ * section.
+ *
+ * Otherwise defer the call until the end of the outermost
+ * blk_io_plug()/blk_io_unplug() section in this thread. If the same
+ * @fn/@opaque pair has already been deferred, it will only be called once upon
+ * blk_io_unplug() so that accumulated calls are batched into a single call.
+ *
+ * The caller must ensure that @opaque is not be freed before @fn() is invoked.
+ */
+void blk_io_plug_call(void (*fn)(void *), void *opaque)
+{
+    Plug *plug = get_ptr_plug();
+
+    /* Call immediately if we're not plugged */
+    if (plug->count == 0) {
+        fn(opaque);
+        return;
+    }
+
+    GArray *array = plug->unplug_fns;
+    if (!array) {
+        array = g_array_new(FALSE, FALSE, sizeof(UnplugFn));
+        plug->unplug_fns = array;
+        blk_io_plug_atexit_notifier.notify = blk_io_plug_atexit;
+        qemu_thread_atexit_add(&blk_io_plug_atexit_notifier);
+    }
+
+    UnplugFn *fns = (UnplugFn *)array->data;
+    UnplugFn new_fn = {
+        .fn = fn,
+        .opaque = opaque,
+    };
+
+    /*
+     * There won't be many, so do a linear search. If this becomes a bottleneck
+     * then a binary search (glib 2.62+) or different data structure could be
+     * used.
+     */
+    for (guint i = 0; i < array->len; i++) {
+        if (memcmp(&fns[i], &new_fn, sizeof(new_fn)) == 0) {
+            return; /* already exists */
+        }
+    }
+
+    g_array_append_val(array, new_fn);
+}
+
+/**
+ * blk_io_plug: Defer blk_io_plug_call() functions until blk_io_unplug()
+ *
+ * blk_io_plug/unplug are thread-local operations. This means that multiple
+ * threads can simultaneously call plug/unplug, but the caller must ensure that
+ * each unplug() is called in the same thread of the matching plug().
+ *
+ * Nesting is supported. blk_io_plug_call() functions are only called at the
+ * outermost blk_io_unplug().
+ */
+void blk_io_plug(void)
+{
+    Plug *plug = get_ptr_plug();
+
+    assert(plug->count < UINT32_MAX);
+
+    plug->count++;
+}
+
+/**
+ * blk_io_unplug: Run any pending blk_io_plug_call() functions
+ *
+ * There must have been a matching blk_io_plug() call in the same thread prior
+ * to this blk_io_unplug() call.
+ */
+void blk_io_unplug(void)
+{
+    Plug *plug = get_ptr_plug();
+
+    assert(plug->count > 0);
+
+    if (--plug->count > 0) {
+        return;
+    }
+
+    GArray *array = plug->unplug_fns;
+    if (!array) {
+        return;
+    }
+
+    UnplugFn *fns = (UnplugFn *)array->data;
+
+    for (guint i = 0; i < array->len; i++) {
+        fns[i].fn(fns[i].opaque);
+    }
+
+    /*
+     * This resets the array without freeing memory so that appending is cheap
+     * in the future.
+     */
+    g_array_set_size(array, 0);
+}
diff --git a/hw/block/dataplane/xen-block.c b/hw/block/dataplane/xen-block.c
index d8bc39d359..e49c24f63d 100644
--- a/hw/block/dataplane/xen-block.c
+++ b/hw/block/dataplane/xen-block.c
@@ -537,7 +537,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
      * is below us.
      */
     if (inflight_atstart > IO_PLUG_THRESHOLD) {
-        blk_io_plug(dataplane->blk);
+        blk_io_plug();
     }
     while (rc != rp) {
         /* pull request from ring */
@@ -577,12 +577,12 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
 
         if (inflight_atstart > IO_PLUG_THRESHOLD &&
             batched >= inflight_atstart) {
-            blk_io_unplug(dataplane->blk);
+            blk_io_unplug();
         }
         xen_block_do_aio(request);
         if (inflight_atstart > IO_PLUG_THRESHOLD) {
             if (batched >= inflight_atstart) {
-                blk_io_plug(dataplane->blk);
+                blk_io_plug();
                 batched = 0;
             } else {
                 batched++;
@@ -590,7 +590,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
         }
     }
     if (inflight_atstart > IO_PLUG_THRESHOLD) {
-        blk_io_unplug(dataplane->blk);
+        blk_io_unplug();
     }
 
     return done_something;
diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
index 8f65ea4659..b4286424c1 100644
--- a/hw/block/virtio-blk.c
+++ b/hw/block/virtio-blk.c
@@ -1134,7 +1134,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
     bool suppress_notifications = virtio_queue_get_notification(vq);
 
     aio_context_acquire(blk_get_aio_context(s->blk));
-    blk_io_plug(s->blk);
+    blk_io_plug();
 
     do {
         if (suppress_notifications) {
@@ -1158,7 +1158,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
         virtio_blk_submit_multireq(s, &mrb);
     }
 
-    blk_io_unplug(s->blk);
+    blk_io_unplug();
     aio_context_release(blk_get_aio_context(s->blk));
 }
 
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 612c525d9d..534a44ee07 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -799,7 +799,7 @@ static int virtio_scsi_handle_cmd_req_prepare(VirtIOSCSI *s, VirtIOSCSIReq *req)
         return -ENOBUFS;
     }
     scsi_req_ref(req->sreq);
-    blk_io_plug(d->conf.blk);
+    blk_io_plug();
     object_unref(OBJECT(d));
     return 0;
 }
@@ -810,7 +810,7 @@ static void virtio_scsi_handle_cmd_req_submit(VirtIOSCSI *s, VirtIOSCSIReq *req)
     if (scsi_req_enqueue(sreq)) {
         scsi_req_continue(sreq);
     }
-    blk_io_unplug(sreq->dev->conf.blk);
+    blk_io_unplug();
     scsi_req_unref(sreq);
 }
 
@@ -836,7 +836,7 @@ static void virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
                 while (!QTAILQ_EMPTY(&reqs)) {
                     req = QTAILQ_FIRST(&reqs);
                     QTAILQ_REMOVE(&reqs, req, next);
-                    blk_io_unplug(req->sreq->dev->conf.blk);
+                    blk_io_unplug();
                     scsi_req_unref(req->sreq);
                     virtqueue_detach_element(req->vq, &req->elem, 0);
                     virtio_scsi_free_req(req);
diff --git a/block/meson.build b/block/meson.build
index 486dda8b85..fb4332bd66 100644
--- a/block/meson.build
+++ b/block/meson.build
@@ -23,6 +23,7 @@ block_ss.add(files(
   'mirror.c',
   'nbd.c',
   'null.c',
+  'plug.c',
   'qapi.c',
   'qcow2-bitmap.c',
   'qcow2-cache.c',
-- 
2.40.1

On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote:
>Introduce a new API for thread-local blk_io_plug() that does not
>traverse the block graph. The goal is to make blk_io_plug() multi-queue
>friendly.
>
>Instead of having block drivers track whether or not we're in a plugged
>section, provide an API that allows them to defer a function call until
>we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
>called multiple times with the same fn/opaque pair, then fn() is only
>called once at the end of the function - resulting in batching.
>
>This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
>blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
>because the plug state is now thread-local.
>
>Later patches convert block drivers to blk_io_plug_call() and then we
>can finally remove .bdrv_co_io_plug() once all block drivers have been
>converted.
>
>Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
>---
> MAINTAINERS                       |   1 +
> include/sysemu/block-backend-io.h |  13 +--
> block/block-backend.c             |  22 -----
> block/plug.c                      | 159 ++++++++++++++++++++++++++++++
> hw/block/dataplane/xen-block.c    |   8 +-
> hw/block/virtio-blk.c             |   4 +-
> hw/scsi/virtio-scsi.c             |   6 +-
> block/meson.build                 |   1 +
> 8 files changed, 173 insertions(+), 41 deletions(-)
> create mode 100644 block/plug.c
>
>diff --git a/MAINTAINERS b/MAINTAINERS
>index 50585117a0..574202295c 100644
>--- a/MAINTAINERS
>+++ b/MAINTAINERS
>@@ -2644,6 +2644,7 @@ F: util/aio-*.c
> F: util/aio-*.h
> F: util/fdmon-*.c
> F: block/io.c
>+F: block/plug.c
> F: migration/block*
> F: include/block/aio.h
> F: include/block/aio-wait.h
>diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h
>index d62a7ee773..be4dcef59d 100644
>--- a/include/sysemu/block-backend-io.h
>+++ b/include/sysemu/block-backend-io.h
>@@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error);
> int blk_get_max_iov(BlockBackend *blk);
> int blk_get_max_hw_iov(BlockBackend *blk);
>
>-/*
>- * blk_io_plug/unplug are thread-local operations. This means that multiple
>- * IOThreads can simultaneously call plug/unplug, but the caller must ensure
>- * that each unplug() is called in the same IOThread of the matching plug().
>- */
>-void coroutine_fn blk_co_io_plug(BlockBackend *blk);
>-void co_wrapper blk_io_plug(BlockBackend *blk);
>-
>-void coroutine_fn blk_co_io_unplug(BlockBackend *blk);
>-void co_wrapper blk_io_unplug(BlockBackend *blk);
>+void blk_io_plug(void);
>+void blk_io_unplug(void);
>+void blk_io_plug_call(void (*fn)(void *), void *opaque);
>
> AioContext *blk_get_aio_context(BlockBackend *blk);
> BlockAcctStats *blk_get_stats(BlockBackend *blk);
>diff --git a/block/block-backend.c b/block/block-backend.c
>index ca537cd0ad..1f1d226ba6 100644
>--- a/block/block-backend.c
>+++ b/block/block-backend.c
>@@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify)
>     notifier_list_add(&blk->insert_bs_notifiers, notify);
> }
>
>-void coroutine_fn blk_co_io_plug(BlockBackend *blk)
>-{
>-    BlockDriverState *bs = blk_bs(blk);
>-    IO_CODE();
>-    GRAPH_RDLOCK_GUARD();
>-
>-    if (bs) {
>-        bdrv_co_io_plug(bs);
>-    }
>-}
>-
>-void coroutine_fn blk_co_io_unplug(BlockBackend *blk)
>-{
>-    BlockDriverState *bs = blk_bs(blk);
>-    IO_CODE();
>-    GRAPH_RDLOCK_GUARD();
>-
>-    if (bs) {
>-        bdrv_co_io_unplug(bs);
>-    }
>-}
>-
> BlockAcctStats *blk_get_stats(BlockBackend *blk)
> {
>     IO_CODE();
>diff --git a/block/plug.c b/block/plug.c
>new file mode 100644
>index 0000000000..6738a568ba
>--- /dev/null
>+++ b/block/plug.c
>@@ -0,0 +1,159 @@
>+/* SPDX-License-Identifier: GPL-2.0-or-later */
>+/*
>+ * Block I/O plugging
>+ *
>+ * Copyright Red Hat.
>+ *
>+ * This API defers a function call within a blk_io_plug()/blk_io_unplug()
>+ * section, allowing multiple calls to batch up. This is a performance
>+ * optimization that is used in the block layer to submit several I/O requests
>+ * at once instead of individually:
>+ *
>+ *   blk_io_plug(); <-- start of plugged region
>+ *   ...
>+ *   blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call
>+ *   blk_io_plug_call(my_func, my_obj); <-- another
>+ *   blk_io_plug_call(my_func, my_obj); <-- another
>+ *   ...
>+ *   blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once
>+ *
>+ * This code is actually generic and not tied to the block layer. If another
>+ * subsystem needs this functionality, it could be renamed.
>+ */
>+
>+#include "qemu/osdep.h"
>+#include "qemu/coroutine-tls.h"
>+#include "qemu/notify.h"
>+#include "qemu/thread.h"
>+#include "sysemu/block-backend.h"
>+
>+/* A function call that has been deferred until unplug() */
>+typedef struct {
>+    void (*fn)(void *);
>+    void *opaque;
>+} UnplugFn;
>+
>+/* Per-thread state */
>+typedef struct {
>+    unsigned count;       /* how many times has plug() been called? */
>+    GArray *unplug_fns;   /* functions to call at unplug time */
>+} Plug;
>+
>+/* Use get_ptr_plug() to fetch this thread-local value */
>+QEMU_DEFINE_STATIC_CO_TLS(Plug, plug);
>+
>+/* Called at thread cleanup time */
>+static void blk_io_plug_atexit(Notifier *n, void *value)
>+{
>+    Plug *plug = get_ptr_plug();
>+    g_array_free(plug->unplug_fns, TRUE);
>+}
>+
>+/* This won't involve coroutines, so use __thread */
>+static __thread Notifier blk_io_plug_atexit_notifier;
>+
>+/**
>+ * blk_io_plug_call:
>+ * @fn: a function pointer to be invoked
>+ * @opaque: a user-defined argument to @fn()
>+ *
>+ * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
>+ * section.

Just to understand better, what if two BlockDrivers share the same
iothread but one calls blk_io_plug()/blk_io_unplug(), while the other
calls this function not in a blk_io_plug()/blk_io_unplug() section?

If the call is in the middle of the other BlockDriver's section, it is
deferred, right?

Is this situation possible?
Or should we prevent blk_io_plug_call() from being called out of a
blk_io_plug()/blk_io_unplug() section?

Thanks,
Stefano

>+ *
>+ * Otherwise defer the call until the end of the outermost
>+ * blk_io_plug()/blk_io_unplug() section in this thread. If the same
>+ * @fn/@opaque pair has already been deferred, it will only be called once upon
>+ * blk_io_unplug() so that accumulated calls are batched into a single call.
>+ *
>+ * The caller must ensure that @opaque is not be freed before @fn() is invoked.
>+ */
>+void blk_io_plug_call(void (*fn)(void *), void *opaque)
>+{
>+    Plug *plug = get_ptr_plug();
>+
>+    /* Call immediately if we're not plugged */
>+    if (plug->count == 0) {
>+        fn(opaque);
>+        return;
>+    }
>+
>+    GArray *array = plug->unplug_fns;
>+    if (!array) {
>+        array = g_array_new(FALSE, FALSE, sizeof(UnplugFn));
>+        plug->unplug_fns = array;
>+        blk_io_plug_atexit_notifier.notify = blk_io_plug_atexit;
>+        qemu_thread_atexit_add(&blk_io_plug_atexit_notifier);
>+    }
>+
>+    UnplugFn *fns = (UnplugFn *)array->data;
>+    UnplugFn new_fn = {
>+        .fn = fn,
>+        .opaque = opaque,
>+    };
>+
>+    /*
>+     * There won't be many, so do a linear search. If this becomes a bottleneck
>+     * then a binary search (glib 2.62+) or different data structure could be
>+     * used.
>+     */
>+    for (guint i = 0; i < array->len; i++) {
>+        if (memcmp(&fns[i], &new_fn, sizeof(new_fn)) == 0) {
>+            return; /* already exists */
>+        }
>+    }
>+
>+    g_array_append_val(array, new_fn);
>+}
>+
>+/**
>+ * blk_io_plug: Defer blk_io_plug_call() functions until blk_io_unplug()
>+ *
>+ * blk_io_plug/unplug are thread-local operations. This means that multiple
>+ * threads can simultaneously call plug/unplug, but the caller must ensure that
>+ * each unplug() is called in the same thread of the matching plug().
>+ *
>+ * Nesting is supported. blk_io_plug_call() functions are only called at the
>+ * outermost blk_io_unplug().
>+ */
>+void blk_io_plug(void)
>+{
>+    Plug *plug = get_ptr_plug();
>+
>+    assert(plug->count < UINT32_MAX);
>+
>+    plug->count++;
>+}
>+
>+/**
>+ * blk_io_unplug: Run any pending blk_io_plug_call() functions
>+ *
>+ * There must have been a matching blk_io_plug() call in the same thread prior
>+ * to this blk_io_unplug() call.
>+ */
>+void blk_io_unplug(void)
>+{
>+    Plug *plug = get_ptr_plug();
>+
>+    assert(plug->count > 0);
>+
>+    if (--plug->count > 0) {
>+        return;
>+    }
>+
>+    GArray *array = plug->unplug_fns;
>+    if (!array) {
>+        return;
>+    }
>+
>+    UnplugFn *fns = (UnplugFn *)array->data;
>+
>+    for (guint i = 0; i < array->len; i++) {
>+        fns[i].fn(fns[i].opaque);
>+    }
>+
>+    /*
>+     * This resets the array without freeing memory so that appending is cheap
>+     * in the future.
>+     */
>+    g_array_set_size(array, 0);
>+}
>diff --git a/hw/block/dataplane/xen-block.c b/hw/block/dataplane/xen-block.c
>index d8bc39d359..e49c24f63d 100644
>--- a/hw/block/dataplane/xen-block.c
>+++ b/hw/block/dataplane/xen-block.c
>@@ -537,7 +537,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
>      * is below us.
>      */
>     if (inflight_atstart > IO_PLUG_THRESHOLD) {
>-        blk_io_plug(dataplane->blk);
>+        blk_io_plug();
>     }
>     while (rc != rp) {
>         /* pull request from ring */
>@@ -577,12 +577,12 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
>
>         if (inflight_atstart > IO_PLUG_THRESHOLD &&
>             batched >= inflight_atstart) {
>-            blk_io_unplug(dataplane->blk);
>+            blk_io_unplug();
>         }
>         xen_block_do_aio(request);
>         if (inflight_atstart > IO_PLUG_THRESHOLD) {
>             if (batched >= inflight_atstart) {
>-                blk_io_plug(dataplane->blk);
>+                blk_io_plug();
>                 batched = 0;
>             } else {
>                 batched++;
>@@ -590,7 +590,7 @@ static bool xen_block_handle_requests(XenBlockDataPlane *dataplane)
>         }
>     }
>     if (inflight_atstart > IO_PLUG_THRESHOLD) {
>-        blk_io_unplug(dataplane->blk);
>+        blk_io_unplug();
>     }
>
>     return done_something;
>diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
>index 8f65ea4659..b4286424c1 100644
>--- a/hw/block/virtio-blk.c
>+++ b/hw/block/virtio-blk.c
>@@ -1134,7 +1134,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
>     bool suppress_notifications = virtio_queue_get_notification(vq);
>
>     aio_context_acquire(blk_get_aio_context(s->blk));
>-    blk_io_plug(s->blk);
>+    blk_io_plug();
>
>     do {
>         if (suppress_notifications) {
>@@ -1158,7 +1158,7 @@ void virtio_blk_handle_vq(VirtIOBlock *s, VirtQueue *vq)
>         virtio_blk_submit_multireq(s, &mrb);
>     }
>
>-    blk_io_unplug(s->blk);
>+    blk_io_unplug();
>     aio_context_release(blk_get_aio_context(s->blk));
> }
>
>diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
>index 612c525d9d..534a44ee07 100644
>--- a/hw/scsi/virtio-scsi.c
>+++ b/hw/scsi/virtio-scsi.c
>@@ -799,7 +799,7 @@ static int virtio_scsi_handle_cmd_req_prepare(VirtIOSCSI *s, VirtIOSCSIReq *req)
>         return -ENOBUFS;
>     }
>     scsi_req_ref(req->sreq);
>-    blk_io_plug(d->conf.blk);
>+    blk_io_plug();
>     object_unref(OBJECT(d));
>     return 0;
> }
>@@ -810,7 +810,7 @@ static void virtio_scsi_handle_cmd_req_submit(VirtIOSCSI *s, VirtIOSCSIReq *req)
>     if (scsi_req_enqueue(sreq)) {
>         scsi_req_continue(sreq);
>     }
>-    blk_io_unplug(sreq->dev->conf.blk);
>+    blk_io_unplug();
>     scsi_req_unref(sreq);
> }
>
>@@ -836,7 +836,7 @@ static void virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
>                 while (!QTAILQ_EMPTY(&reqs)) {
>                     req = QTAILQ_FIRST(&reqs);
>                     QTAILQ_REMOVE(&reqs, req, next);
>-                    blk_io_unplug(req->sreq->dev->conf.blk);
>+                    blk_io_unplug();
>                     scsi_req_unref(req->sreq);
>                     virtqueue_detach_element(req->vq, &req->elem, 0);
>                     virtio_scsi_free_req(req);
>diff --git a/block/meson.build b/block/meson.build
>index 486dda8b85..fb4332bd66 100644
>--- a/block/meson.build
>+++ b/block/meson.build
>@@ -23,6 +23,7 @@ block_ss.add(files(
>   'mirror.c',
>   'nbd.c',
>   'null.c',
>+  'plug.c',
>   'qapi.c',
>   'qcow2-bitmap.c',
>   'qcow2-cache.c',
>-- 
>2.40.1
>

On Fri, May 19, 2023 at 10:45:57AM +0200, Stefano Garzarella wrote:
> On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote:
> > Introduce a new API for thread-local blk_io_plug() that does not
> > traverse the block graph. The goal is to make blk_io_plug() multi-queue
> > friendly.
> > 
> > Instead of having block drivers track whether or not we're in a plugged
> > section, provide an API that allows them to defer a function call until
> > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
> > called multiple times with the same fn/opaque pair, then fn() is only
> > called once at the end of the function - resulting in batching.
> > 
> > This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
> > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
> > because the plug state is now thread-local.
> > 
> > Later patches convert block drivers to blk_io_plug_call() and then we
> > can finally remove .bdrv_co_io_plug() once all block drivers have been
> > converted.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > MAINTAINERS                       |   1 +
> > include/sysemu/block-backend-io.h |  13 +--
> > block/block-backend.c             |  22 -----
> > block/plug.c                      | 159 ++++++++++++++++++++++++++++++
> > hw/block/dataplane/xen-block.c    |   8 +-
> > hw/block/virtio-blk.c             |   4 +-
> > hw/scsi/virtio-scsi.c             |   6 +-
> > block/meson.build                 |   1 +
> > 8 files changed, 173 insertions(+), 41 deletions(-)
> > create mode 100644 block/plug.c
> > 
> > diff --git a/MAINTAINERS b/MAINTAINERS
> > index 50585117a0..574202295c 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -2644,6 +2644,7 @@ F: util/aio-*.c
> > F: util/aio-*.h
> > F: util/fdmon-*.c
> > F: block/io.c
> > +F: block/plug.c
> > F: migration/block*
> > F: include/block/aio.h
> > F: include/block/aio-wait.h
> > diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h
> > index d62a7ee773..be4dcef59d 100644
> > --- a/include/sysemu/block-backend-io.h
> > +++ b/include/sysemu/block-backend-io.h
> > @@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error);
> > int blk_get_max_iov(BlockBackend *blk);
> > int blk_get_max_hw_iov(BlockBackend *blk);
> > 
> > -/*
> > - * blk_io_plug/unplug are thread-local operations. This means that multiple
> > - * IOThreads can simultaneously call plug/unplug, but the caller must ensure
> > - * that each unplug() is called in the same IOThread of the matching plug().
> > - */
> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk);
> > -void co_wrapper blk_io_plug(BlockBackend *blk);
> > -
> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk);
> > -void co_wrapper blk_io_unplug(BlockBackend *blk);
> > +void blk_io_plug(void);
> > +void blk_io_unplug(void);
> > +void blk_io_plug_call(void (*fn)(void *), void *opaque);
> > 
> > AioContext *blk_get_aio_context(BlockBackend *blk);
> > BlockAcctStats *blk_get_stats(BlockBackend *blk);
> > diff --git a/block/block-backend.c b/block/block-backend.c
> > index ca537cd0ad..1f1d226ba6 100644
> > --- a/block/block-backend.c
> > +++ b/block/block-backend.c
> > @@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify)
> >     notifier_list_add(&blk->insert_bs_notifiers, notify);
> > }
> > 
> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk)
> > -{
> > -    BlockDriverState *bs = blk_bs(blk);
> > -    IO_CODE();
> > -    GRAPH_RDLOCK_GUARD();
> > -
> > -    if (bs) {
> > -        bdrv_co_io_plug(bs);
> > -    }
> > -}
> > -
> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk)
> > -{
> > -    BlockDriverState *bs = blk_bs(blk);
> > -    IO_CODE();
> > -    GRAPH_RDLOCK_GUARD();
> > -
> > -    if (bs) {
> > -        bdrv_co_io_unplug(bs);
> > -    }
> > -}
> > -
> > BlockAcctStats *blk_get_stats(BlockBackend *blk)
> > {
> >     IO_CODE();
> > diff --git a/block/plug.c b/block/plug.c
> > new file mode 100644
> > index 0000000000..6738a568ba
> > --- /dev/null
> > +++ b/block/plug.c
> > @@ -0,0 +1,159 @@
> > +/* SPDX-License-Identifier: GPL-2.0-or-later */
> > +/*
> > + * Block I/O plugging
> > + *
> > + * Copyright Red Hat.
> > + *
> > + * This API defers a function call within a blk_io_plug()/blk_io_unplug()
> > + * section, allowing multiple calls to batch up. This is a performance
> > + * optimization that is used in the block layer to submit several I/O requests
> > + * at once instead of individually:
> > + *
> > + *   blk_io_plug(); <-- start of plugged region
> > + *   ...
> > + *   blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call
> > + *   blk_io_plug_call(my_func, my_obj); <-- another
> > + *   blk_io_plug_call(my_func, my_obj); <-- another
> > + *   ...
> > + *   blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once
> > + *
> > + * This code is actually generic and not tied to the block layer. If another
> > + * subsystem needs this functionality, it could be renamed.
> > + */
> > +
> > +#include "qemu/osdep.h"
> > +#include "qemu/coroutine-tls.h"
> > +#include "qemu/notify.h"
> > +#include "qemu/thread.h"
> > +#include "sysemu/block-backend.h"
> > +
> > +/* A function call that has been deferred until unplug() */
> > +typedef struct {
> > +    void (*fn)(void *);
> > +    void *opaque;
> > +} UnplugFn;
> > +
> > +/* Per-thread state */
> > +typedef struct {
> > +    unsigned count;       /* how many times has plug() been called? */
> > +    GArray *unplug_fns;   /* functions to call at unplug time */
> > +} Plug;
> > +
> > +/* Use get_ptr_plug() to fetch this thread-local value */
> > +QEMU_DEFINE_STATIC_CO_TLS(Plug, plug);
> > +
> > +/* Called at thread cleanup time */
> > +static void blk_io_plug_atexit(Notifier *n, void *value)
> > +{
> > +    Plug *plug = get_ptr_plug();
> > +    g_array_free(plug->unplug_fns, TRUE);
> > +}
> > +
> > +/* This won't involve coroutines, so use __thread */
> > +static __thread Notifier blk_io_plug_atexit_notifier;
> > +
> > +/**
> > + * blk_io_plug_call:
> > + * @fn: a function pointer to be invoked
> > + * @opaque: a user-defined argument to @fn()
> > + *
> > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
> > + * section.
> 
> Just to understand better, what if two BlockDrivers share the same
> iothread but one calls blk_io_plug()/blk_io_unplug(), while the other
> calls this function not in a blk_io_plug()/blk_io_unplug() section?
> 
> If the call is in the middle of the other BlockDriver's section, it is
> deferred, right?

Yes, the call is deferred until blk_io_unplug().

> Is this situation possible?

One scenario I can think of is when aio_poll() is called between
plug/unplug. In that case, some I/O associated with device B might
happen while device A is with plug/unplug.

> Or should we prevent blk_io_plug_call() from being called out of a
> blk_io_plug()/blk_io_unplug() section?

blk_io_plug_call() is called outside blk_io_plug()/blk_io_unplug() when
device emulation doesn't use plug/unplug. For example, IDE doesn't use
it but still calls down into the same Linux AIO or io_uring code that
invokes blk_io_plug_call(). This is why blk_io_plug_call() calls fn()
immediately when invoked outside plug/unplug.

Stefan

On Tue, May 23, 2023 at 11:47:08AM -0400, Stefan Hajnoczi wrote:
>On Fri, May 19, 2023 at 10:45:57AM +0200, Stefano Garzarella wrote:
>> On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote:
>> > Introduce a new API for thread-local blk_io_plug() that does not
>> > traverse the block graph. The goal is to make blk_io_plug() multi-queue
>> > friendly.
>> >
>> > Instead of having block drivers track whether or not we're in a plugged
>> > section, provide an API that allows them to defer a function call until
>> > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
>> > called multiple times with the same fn/opaque pair, then fn() is only
>> > called once at the end of the function - resulting in batching.
>> >
>> > This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
>> > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
>> > because the plug state is now thread-local.
>> >
>> > Later patches convert block drivers to blk_io_plug_call() and then we
>> > can finally remove .bdrv_co_io_plug() once all block drivers have been
>> > converted.
>> >
>> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
>> > ---
>> > MAINTAINERS                       |   1 +
>> > include/sysemu/block-backend-io.h |  13 +--
>> > block/block-backend.c             |  22 -----
>> > block/plug.c                      | 159 ++++++++++++++++++++++++++++++
>> > hw/block/dataplane/xen-block.c    |   8 +-
>> > hw/block/virtio-blk.c             |   4 +-
>> > hw/scsi/virtio-scsi.c             |   6 +-
>> > block/meson.build                 |   1 +
>> > 8 files changed, 173 insertions(+), 41 deletions(-)
>> > create mode 100644 block/plug.c
>> >
>> > diff --git a/MAINTAINERS b/MAINTAINERS
>> > index 50585117a0..574202295c 100644
>> > --- a/MAINTAINERS
>> > +++ b/MAINTAINERS
>> > @@ -2644,6 +2644,7 @@ F: util/aio-*.c
>> > F: util/aio-*.h
>> > F: util/fdmon-*.c
>> > F: block/io.c
>> > +F: block/plug.c
>> > F: migration/block*
>> > F: include/block/aio.h
>> > F: include/block/aio-wait.h
>> > diff --git a/include/sysemu/block-backend-io.h b/include/sysemu/block-backend-io.h
>> > index d62a7ee773..be4dcef59d 100644
>> > --- a/include/sysemu/block-backend-io.h
>> > +++ b/include/sysemu/block-backend-io.h
>> > @@ -100,16 +100,9 @@ void blk_iostatus_set_err(BlockBackend *blk, int error);
>> > int blk_get_max_iov(BlockBackend *blk);
>> > int blk_get_max_hw_iov(BlockBackend *blk);
>> >
>> > -/*
>> > - * blk_io_plug/unplug are thread-local operations. This means that multiple
>> > - * IOThreads can simultaneously call plug/unplug, but the caller must ensure
>> > - * that each unplug() is called in the same IOThread of the matching plug().
>> > - */
>> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk);
>> > -void co_wrapper blk_io_plug(BlockBackend *blk);
>> > -
>> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk);
>> > -void co_wrapper blk_io_unplug(BlockBackend *blk);
>> > +void blk_io_plug(void);
>> > +void blk_io_unplug(void);
>> > +void blk_io_plug_call(void (*fn)(void *), void *opaque);
>> >
>> > AioContext *blk_get_aio_context(BlockBackend *blk);
>> > BlockAcctStats *blk_get_stats(BlockBackend *blk);
>> > diff --git a/block/block-backend.c b/block/block-backend.c
>> > index ca537cd0ad..1f1d226ba6 100644
>> > --- a/block/block-backend.c
>> > +++ b/block/block-backend.c
>> > @@ -2568,28 +2568,6 @@ void blk_add_insert_bs_notifier(BlockBackend *blk, Notifier *notify)
>> >     notifier_list_add(&blk->insert_bs_notifiers, notify);
>> > }
>> >
>> > -void coroutine_fn blk_co_io_plug(BlockBackend *blk)
>> > -{
>> > -    BlockDriverState *bs = blk_bs(blk);
>> > -    IO_CODE();
>> > -    GRAPH_RDLOCK_GUARD();
>> > -
>> > -    if (bs) {
>> > -        bdrv_co_io_plug(bs);
>> > -    }
>> > -}
>> > -
>> > -void coroutine_fn blk_co_io_unplug(BlockBackend *blk)
>> > -{
>> > -    BlockDriverState *bs = blk_bs(blk);
>> > -    IO_CODE();
>> > -    GRAPH_RDLOCK_GUARD();
>> > -
>> > -    if (bs) {
>> > -        bdrv_co_io_unplug(bs);
>> > -    }
>> > -}
>> > -
>> > BlockAcctStats *blk_get_stats(BlockBackend *blk)
>> > {
>> >     IO_CODE();
>> > diff --git a/block/plug.c b/block/plug.c
>> > new file mode 100644
>> > index 0000000000..6738a568ba
>> > --- /dev/null
>> > +++ b/block/plug.c
>> > @@ -0,0 +1,159 @@
>> > +/* SPDX-License-Identifier: GPL-2.0-or-later */
>> > +/*
>> > + * Block I/O plugging
>> > + *
>> > + * Copyright Red Hat.
>> > + *
>> > + * This API defers a function call within a blk_io_plug()/blk_io_unplug()
>> > + * section, allowing multiple calls to batch up. This is a performance
>> > + * optimization that is used in the block layer to submit several I/O requests
>> > + * at once instead of individually:
>> > + *
>> > + *   blk_io_plug(); <-- start of plugged region
>> > + *   ...
>> > + *   blk_io_plug_call(my_func, my_obj); <-- deferred my_func(my_obj) call
>> > + *   blk_io_plug_call(my_func, my_obj); <-- another
>> > + *   blk_io_plug_call(my_func, my_obj); <-- another
>> > + *   ...
>> > + *   blk_io_unplug(); <-- end of plugged region, my_func(my_obj) is called once
>> > + *
>> > + * This code is actually generic and not tied to the block layer. If another
>> > + * subsystem needs this functionality, it could be renamed.
>> > + */
>> > +
>> > +#include "qemu/osdep.h"
>> > +#include "qemu/coroutine-tls.h"
>> > +#include "qemu/notify.h"
>> > +#include "qemu/thread.h"
>> > +#include "sysemu/block-backend.h"
>> > +
>> > +/* A function call that has been deferred until unplug() */
>> > +typedef struct {
>> > +    void (*fn)(void *);
>> > +    void *opaque;
>> > +} UnplugFn;
>> > +
>> > +/* Per-thread state */
>> > +typedef struct {
>> > +    unsigned count;       /* how many times has plug() been called? */
>> > +    GArray *unplug_fns;   /* functions to call at unplug time */
>> > +} Plug;
>> > +
>> > +/* Use get_ptr_plug() to fetch this thread-local value */
>> > +QEMU_DEFINE_STATIC_CO_TLS(Plug, plug);
>> > +
>> > +/* Called at thread cleanup time */
>> > +static void blk_io_plug_atexit(Notifier *n, void *value)
>> > +{
>> > +    Plug *plug = get_ptr_plug();
>> > +    g_array_free(plug->unplug_fns, TRUE);
>> > +}
>> > +
>> > +/* This won't involve coroutines, so use __thread */
>> > +static __thread Notifier blk_io_plug_atexit_notifier;
>> > +
>> > +/**
>> > + * blk_io_plug_call:
>> > + * @fn: a function pointer to be invoked
>> > + * @opaque: a user-defined argument to @fn()
>> > + *
>> > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
>> > + * section.
>>
>> Just to understand better, what if two BlockDrivers share the same
>> iothread but one calls blk_io_plug()/blk_io_unplug(), while the other
>> calls this function not in a blk_io_plug()/blk_io_unplug() section?
>>
>> If the call is in the middle of the other BlockDriver's section, it is
>> deferred, right?
>
>Yes, the call is deferred until blk_io_unplug().
>
>> Is this situation possible?
>
>One scenario I can think of is when aio_poll() is called between
>plug/unplug. In that case, some I/O associated with device B might
>happen while device A is with plug/unplug.
>
>> Or should we prevent blk_io_plug_call() from being called out of a
>> blk_io_plug()/blk_io_unplug() section?
>
>blk_io_plug_call() is called outside blk_io_plug()/blk_io_unplug() when
>device emulation doesn't use plug/unplug. For example, IDE doesn't use
>it but still calls down into the same Linux AIO or io_uring code that
>invokes blk_io_plug_call(). This is why blk_io_plug_call() calls fn()
>immediately when invoked outside plug/unplug.

Got it, so it seems that everything should work properly.

Thanks,
Stefano

On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote:
> Introduce a new API for thread-local blk_io_plug() that does not
> traverse the block graph. The goal is to make blk_io_plug() multi-queue
> friendly.
> 
> Instead of having block drivers track whether or not we're in a plugged
> section, provide an API that allows them to defer a function call until
> we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
> called multiple times with the same fn/opaque pair, then fn() is only
> called once at the end of the function - resulting in batching.
> 
> This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
> blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
> because the plug state is now thread-local.
> 
> Later patches convert block drivers to blk_io_plug_call() and then we
> can finally remove .bdrv_co_io_plug() once all block drivers have been
> converted.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---

> +++ b/block/plug.c
> +
> +/**
> + * blk_io_plug_call:
> + * @fn: a function pointer to be invoked
> + * @opaque: a user-defined argument to @fn()
> + *
> + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
> + * section.
> + *
> + * Otherwise defer the call until the end of the outermost
> + * blk_io_plug()/blk_io_unplug() section in this thread. If the same
> + * @fn/@opaque pair has already been deferred, it will only be called once upon
> + * blk_io_unplug() so that accumulated calls are batched into a single call.
> + *
> + * The caller must ensure that @opaque is not be freed before @fn() is invoked.

s/be //

> + */
> +void blk_io_plug_call(void (*fn)(void *), void *opaque)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

On Thu, May 18, 2023 at 07:04:52PM -0500, Eric Blake wrote:
> On Wed, May 17, 2023 at 06:10:17PM -0400, Stefan Hajnoczi wrote:
> > Introduce a new API for thread-local blk_io_plug() that does not
> > traverse the block graph. The goal is to make blk_io_plug() multi-queue
> > friendly.
> > 
> > Instead of having block drivers track whether or not we're in a plugged
> > section, provide an API that allows them to defer a function call until
> > we're unplugged: blk_io_plug_call(fn, opaque). If blk_io_plug_call() is
> > called multiple times with the same fn/opaque pair, then fn() is only
> > called once at the end of the function - resulting in batching.
> > 
> > This patch introduces the API and changes blk_io_plug()/blk_io_unplug().
> > blk_io_plug()/blk_io_unplug() no longer require a BlockBackend argument
> > because the plug state is now thread-local.
> > 
> > Later patches convert block drivers to blk_io_plug_call() and then we
> > can finally remove .bdrv_co_io_plug() once all block drivers have been
> > converted.
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> 
> > +++ b/block/plug.c
> > +
> > +/**
> > + * blk_io_plug_call:
> > + * @fn: a function pointer to be invoked
> > + * @opaque: a user-defined argument to @fn()
> > + *
> > + * Call @fn(@opaque) immediately if not within a blk_io_plug()/blk_io_unplug()
> > + * section.
> > + *
> > + * Otherwise defer the call until the end of the outermost
> > + * blk_io_plug()/blk_io_unplug() section in this thread. If the same
> > + * @fn/@opaque pair has already been deferred, it will only be called once upon
> > + * blk_io_unplug() so that accumulated calls are batched into a single call.
> > + *
> > + * The caller must ensure that @opaque is not be freed before @fn() is invoked.
> 
> s/be //

Will fix, thanks!

Stefan