The following changes since commit 8bac3ba57eecc466b7e73dabf7d19328a59f684e:

Merge remote-tracking branch 'remotes/rth/tags/pull-rx-20200408' into staging (2020-04-09 13:23:30 +0100)

https://github.com/stefanha/qemu.git tags/block-pull-request

for you to fetch changes up to 5710a3e09f9b85801e5ce70797a4a511e5fc9e2c:

async: use explicit memory barriers (2020-04-09 16:17:14 +0100)

Paolo Bonzini (2):

aio-wait: delegate polling of main AioContext if BQL not held

async: use explicit memory barriers

Stefan Hajnoczi (1):

aio-posix: signal-proof fdmon-io_uring

include/block/aio-wait.h | 22 ++++++++++++++++++++++

include/block/aio.h | 29 ++++++++++-------------------

util/aio-posix.c | 16 ++++++++++++++--

util/aio-win32.c | 17 ++++++++++++++---

util/async.c | 16 ++++++++++++----

util/fdmon-io_uring.c | 10 ++++++++--

6 files changed, 80 insertions(+), 30 deletions(-)

2.25.1

From: Paolo Bonzini <pbonzini@redhat.com>

When using C11 atomics, non-seqcst reads and writes do not participate

in the total order of seqcst operations. In util/async.c and util/aio-posix.c,

in particular, the pattern that we use

write ctx->notify_me write bh->scheduled

read bh->scheduled read ctx->notify_me

if !bh->scheduled, sleep if ctx->notify_me, notify

needs to use seqcst operations for both the write and the read. In

general this is something that we do not want, because there can be

many sources that are polled in addition to bottom halves. The

alternative is to place a seqcst memory barrier between the write

and the read. This also comes with a disadvantage, in that the

memory barrier is implicit on strongly-ordered architectures and

it wastes a few dozen clock cycles.

Fortunately, ctx->notify_me is never written concurrently by two

threads, so we can assert that and relax the writes to ctx->notify_me.

The resulting solution works and performs well on both aarch64 and x86.

Note that the atomic_set/atomic_read combination is not an atomic

read-modify-write, and therefore it is even weaker than C11 ATOMIC_RELAXED;

on x86, ATOMIC_RELAXED compiles to a locked operation.

Analyzed-by: Ying Fang <fangying1@huawei.com>

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Tested-by: Ying Fang <fangying1@huawei.com>

Message-Id: <20200407140746.8041-6-pbonzini@redhat.com>

util/aio-posix.c | 16 ++++++++++++++--

util/aio-win32.c | 17 ++++++++++++++---

util/async.c | 16 ++++++++++++----

3 files changed, 40 insertions(+), 9 deletions(-)

diff --git a/util/aio-posix.c b/util/aio-posix.c

--- a/util/aio-posix.c

+++ b/util/aio-posix.c

@@ -XXX,XX +XXX,XX @@ bool aio_poll(AioContext *ctx, bool blocking)

int64_t timeout;

int64_t start = 0;

+ * There cannot be two concurrent aio_poll calls for the same AioContext (or

+ * an aio_poll concurrent with a GSource prepare/check/dispatch callback).

+ * We rely on this below to avoid slow locked accesses to ctx->notify_me.

assert(in_aio_context_home_thread(ctx));

/* aio_notify can avoid the expensive event_notifier_set if

@@ -XXX,XX +XXX,XX @@ bool aio_poll(AioContext *ctx, bool blocking)

* so disable the optimization now.

*/

if (blocking) {

- atomic_add(&ctx->notify_me, 2);

+ atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);

+ /*

+ * Write ctx->notify_me before computing the timeout

+ * (reading bottom half flags, etc.). Pairs with

+ * smp_mb in aio_notify().

+ */

+ smp_mb();

+ * There cannot be two concurrent aio_poll calls for the same AioContext (or

+ * an aio_poll concurrent with a GSource prepare/check/dispatch callback).

+ * We rely on this below to avoid slow locked accesses to ctx->notify_me.

+ assert(in_aio_context_home_thread(ctx));

progress = false;

/* aio_notify can avoid the expensive event_notifier_set if

@@ -XXX,XX +XXX,XX @@ bool aio_poll(AioContext *ctx, bool blocking)

* so disable the optimization now.

*/

if (blocking) {

- atomic_add(&ctx->notify_me, 2);

+ atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);

+ /*

+ * Write ctx->notify_me before computing the timeout

+ * (reading bottom half flags, etc.). Pairs with

+ * smp_mb in aio_notify().

+ */

+ smp_mb();

}

qemu_lockcnt_inc(&ctx->list_lock);

@@ -XXX,XX +XXX,XX @@ bool aio_poll(AioContext *ctx, bool blocking)

ret = WaitForMultipleObjects(count, events, FALSE, timeout);

if (blocking) {

assert(first);

- assert(in_aio_context_home_thread(ctx));

- atomic_sub(&ctx->notify_me, 2);

+ atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) - 2);

aio_notify_accept(ctx);

}

diff --git a/util/async.c b/util/async.c

index XXXXXXX..XXXXXXX 100644

--- a/util/async.c

+++ b/util/async.c

@@ -XXX,XX +XXX,XX @@ aio_ctx_prepare(GSource *source, gint *timeout)

{

AioContext *ctx = (AioContext *) source;

- atomic_or(&ctx->notify_me, 1);

+ atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) | 1);

+ * Write ctx->notify_me before computing the timeout

+ * (reading bottom half flags, etc.). Pairs with

+ * smp_mb in aio_notify().

+ smp_mb();

/* We assume there is no timeout already supplied */

*timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));

@@ -XXX,XX +XXX,XX @@ aio_ctx_check(GSource *source)

QEMUBH *bh;

BHListSlice *s;

- atomic_and(&ctx->notify_me, ~1);

+ /* Finish computing the timeout before clearing the flag. */

+ atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) & ~1);

aio_notify_accept(ctx);

QSLIST_FOREACH_RCU(bh, &ctx->bh_list, next) {

@@ -XXX,XX +XXX,XX @@ LuringState *aio_get_linux_io_uring(AioContext *ctx)

void aio_notify(AioContext *ctx)

{

/* Write e.g. bh->scheduled before reading ctx->notify_me. Pairs

- * with atomic_or in aio_ctx_prepare or atomic_add in aio_poll.

+ * with smp_mb in aio_ctx_prepare or aio_poll.

*/

smp_mb();

- if (ctx->notify_me) {

+ if (atomic_read(&ctx->notify_me)) {

event_notifier_set(&ctx->notifier);

atomic_mb_set(&ctx->notified, true);

}

2.25.1