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bh=DCK+lnfLAYtw1mOtOApL/BAriyzaovAFIXtqHAPBx7E=; b=UR/YYC4MefkayGtPdGINbBaadX1I/DGoWbk6x7FoEAdKxdgq7frK+M9RjAIeB1zne3i6D9 ArTUv0v7TGecTvMHyCraDz2KSkeZux3+7reA7+jVxvuY18ECAxtyYFzqDwfGfnVBDV77qn dtZlgE6s5FKj5c1A9dF9mH7BkL3/RKY= X-MC-Unique: GEOy8nXNO9aTyOXA76zMLA-1 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Cc: "Dr. David Alan Gilbert" , Eduardo Habkost , pbonzini@redhat.com, Markus Armbruster , Eric Blake , kwolf@redhat.com, Maxim Levitsky , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Stefan Hajnoczi Subject: [RFC 2/3] rcu: add drain_call_rcu_co() API Date: Wed, 6 Sep 2023 15:01:40 -0400 Message-ID: <20230906190141.1286893-3-stefanha@redhat.com> In-Reply-To: <20230906190141.1286893-1-stefanha@redhat.com> References: <20230906190141.1286893-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.1 on 10.11.54.7 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1694026993990100001 Content-Type: text/plain; charset="utf-8" call_drain_rcu() has limitations that make it unsuitable for use in qmp_device_add(). Introduce a new coroutine version of drain_call_rcu() with the same functionality but that does not drop the BQL. The next patch will use it to fix qmp_device_add(). Signed-off-by: Stefan Hajnoczi --- MAINTAINERS | 2 ++ docs/devel/rcu.txt | 21 +++++++++++++++++ include/qemu/rcu.h | 1 + util/rcu-internal.h | 8 +++++++ util/rcu-co.c | 55 +++++++++++++++++++++++++++++++++++++++++++++ util/rcu.c | 3 ++- util/meson.build | 2 +- 7 files changed, 90 insertions(+), 2 deletions(-) create mode 100644 util/rcu-internal.h create mode 100644 util/rcu-co.c diff --git a/MAINTAINERS b/MAINTAINERS index 3b29568ed4..7f98253bda 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -2908,6 +2908,8 @@ F: include/qemu/rcu*.h F: tests/unit/rcutorture.c F: tests/unit/test-rcu-*.c F: util/rcu.c +F: util/rcu-co.c +F: util/rcu-internal.h =20 Human Monitor (HMP) M: Dr. David Alan Gilbert diff --git a/docs/devel/rcu.txt b/docs/devel/rcu.txt index 2e6cc607a1..344764527f 100644 --- a/docs/devel/rcu.txt +++ b/docs/devel/rcu.txt @@ -130,6 +130,27 @@ The core RCU API is small: =20 g_free_rcu(&foo, rcu); =20 + void coroutine_fn drain_call_rcu_co(void); + + drain_call_rcu_co() yields until the reclamation phase is finished. + Reclaimer functions previously submitted with call_rcu1() in this + thread will have finished by the time drain_call_rcu_co() returns. + + void drain_call_rcu(void); + + drain_call_rcu() releases the Big QEMU Lock (BQL), if held, waits = until + the reclamation phase is finished, and then re-acquires the BQL, if + previously held. Reclaimer functions previously submitted with + call_rcu1() in this thread will have finished by the time + drain_call_rcu() returns. + + drain_call_rcu() has the following limitations: + 1. It deadlocks when called within an RCU read-side critical secti= on. + 2. All functions on the call stack must be designed to handle drop= ping + the BQL. + + Prefer drain_call_rcu_co() over drain_call_rcu(). + typeof(*p) qatomic_rcu_read(p); =20 qatomic_rcu_read() is similar to qatomic_load_acquire(), but it ma= kes diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h index fea058aa9f..53055df1dc 100644 --- a/include/qemu/rcu.h +++ b/include/qemu/rcu.h @@ -141,6 +141,7 @@ struct rcu_head { }; =20 void call_rcu1(struct rcu_head *head, RCUCBFunc *func); +void coroutine_fn drain_call_rcu_co(void); void drain_call_rcu(void); =20 /* The operands of the minus operator must have the same type, diff --git a/util/rcu-internal.h b/util/rcu-internal.h new file mode 100644 index 0000000000..7d85366d54 --- /dev/null +++ b/util/rcu-internal.h @@ -0,0 +1,8 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ + +#ifndef RCU_INTERNAL_H +#define RCU_INTERNAL_H + +extern int in_drain_call_rcu; + +#endif /* RCU_INTERNAL_H */ diff --git a/util/rcu-co.c b/util/rcu-co.c new file mode 100644 index 0000000000..920fcacb7a --- /dev/null +++ b/util/rcu-co.c @@ -0,0 +1,55 @@ +/* SPDX-License-Identifier: LGPL-2.1-or-later */ +/* + * RCU APIs for coroutines + * + * The RCU coroutine APIs are kept separate from the main RCU code to avoid + * depending on AioContext APIs in rcu.c. This is necessary because at lea= st + * tests/unit/ptimer-test.c has replacement functions for AioContext APIs = that + * conflict with the real functions. + * + * It's also nice to logically separate the core RCU code from the corouti= ne + * APIs :). + */ +#include "qemu/osdep.h" +#include "block/aio.h" +#include "qemu/atomic.h" +#include "qemu/coroutine.h" +#include "qemu/rcu.h" +#include "rcu-internal.h" + +typedef struct { + struct rcu_head rcu; + Coroutine *co; +} RcuDrainCo; + +static void drain_call_rcu_co_bh(void *opaque) +{ + RcuDrainCo *data =3D opaque; + + /* Re-enter drain_call_rcu_co() where it yielded */ + aio_co_wake(data->co); +} + +static void drain_call_rcu_co_cb(struct rcu_head *node) +{ + RcuDrainCo *data =3D container_of(node, RcuDrainCo, rcu); + AioContext *ctx =3D qemu_coroutine_get_aio_context(data->co); + + /* + * drain_call_rcu_co() might still be running in its thread, so schedu= le a + * BH in its thread. The BH only runs after the coroutine has yielded. + */ + aio_bh_schedule_oneshot(ctx, drain_call_rcu_co_bh, data); +} + +void coroutine_fn drain_call_rcu_co(void) +{ + RcuDrainCo data =3D { + .co =3D qemu_coroutine_self(), + }; + + qatomic_inc(&in_drain_call_rcu); + call_rcu1(&data.rcu, drain_call_rcu_co_cb); + qemu_coroutine_yield(); /* wait for drain_rcu_co_bh() */ + qatomic_dec(&in_drain_call_rcu); +} diff --git a/util/rcu.c b/util/rcu.c index e587bcc483..2519bd7d5c 100644 --- a/util/rcu.c +++ b/util/rcu.c @@ -32,6 +32,7 @@ #include "qemu/thread.h" #include "qemu/main-loop.h" #include "qemu/lockable.h" +#include "rcu-internal.h" #if defined(CONFIG_MALLOC_TRIM) #include #endif @@ -46,7 +47,7 @@ unsigned long rcu_gp_ctr =3D RCU_GP_LOCKED; =20 QemuEvent rcu_gp_event; -static int in_drain_call_rcu; +int in_drain_call_rcu; static QemuMutex rcu_registry_lock; static QemuMutex rcu_sync_lock; =20 diff --git a/util/meson.build b/util/meson.build index a375160286..849d56f756 100644 --- a/util/meson.build +++ b/util/meson.build @@ -43,7 +43,7 @@ util_ss.add(files('keyval.c')) util_ss.add(files('crc32c.c')) util_ss.add(files('uuid.c')) util_ss.add(files('getauxval.c')) -util_ss.add(files('rcu.c')) +util_ss.add(files('rcu.c', 'rcu-co.c')) if have_membarrier util_ss.add(files('sys_membarrier.c')) endif --=20 2.41.0