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bh=IwAh2HMTK47Tl0DWIW2+AnGGnj3M77DkVbDIB6bRVB8=; b=NlXK8pxsExj0gcj4plVSoEzKiQKDH4fAl24InCMfeDjZ+ZT5a7BYt51G/an46PyLhW2IZM IfxBkqz3yGEBRr5EGe5A0L9BF1i+LyOzgX9sJKhhUOHB7Nk9DGjbFeuFfmKHg+sqjvGlnE CQHcNi4bF661nv5PpMhgIQ+P37fJmqI= X-MC-Unique: zeqCgNDnOdmd-kxzAMy0nQ-1 From: Kevin Wolf To: qemu-block@nongnu.org Cc: kwolf@redhat.com, stefanha@redhat.com, qemu-devel@nongnu.org Subject: [PULL 25/33] docs: remove AioContext lock from IOThread docs Date: Thu, 21 Dec 2023 22:23:30 +0100 Message-ID: <20231221212339.164439-26-kwolf@redhat.com> In-Reply-To: <20231221212339.164439-1-kwolf@redhat.com> References: <20231221212339.164439-1-kwolf@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.8 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=170.10.129.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.061, 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, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable 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: 1703194024567100002 Content-Type: text/plain; charset="utf-8" From: Stefan Hajnoczi Encourage the use of locking primitives and stop mentioning the AioContext lock since it is being removed. Signed-off-by: Stefan Hajnoczi Reviewed-by: Eric Blake Message-ID: <20231205182011.1976568-12-stefanha@redhat.com> Reviewed-by: Kevin Wolf Signed-off-by: Kevin Wolf --- docs/devel/multiple-iothreads.txt | 47 +++++++++++-------------------- 1 file changed, 16 insertions(+), 31 deletions(-) diff --git a/docs/devel/multiple-iothreads.txt b/docs/devel/multiple-iothre= ads.txt index a3e949f6b3..4865196bde 100644 --- a/docs/devel/multiple-iothreads.txt +++ b/docs/devel/multiple-iothreads.txt @@ -88,27 +88,18 @@ loop, depending on which AioContext instance the caller= passes in. =20 How to synchronize with an IOThread ----------------------------------- -AioContext is not thread-safe so some rules must be followed when using fi= le -descriptors, event notifiers, timers, or BHs across threads: +Variables that can be accessed by multiple threads require some form of +synchronization such as qemu_mutex_lock(), rcu_read_lock(), etc. =20 -1. AioContext functions can always be called safely. They handle their -own locking internally. - -2. Other threads wishing to access the AioContext must use -aio_context_acquire()/aio_context_release() for mutual exclusion. Once the -context is acquired no other thread can access it or run event loop iterat= ions -in this AioContext. - -Legacy code sometimes nests aio_context_acquire()/aio_context_release() ca= lls. -Do not use nesting anymore, it is incompatible with the BDRV_POLL_WHILE() = macro -used in the block layer and can lead to hangs. - -There is currently no lock ordering rule if a thread needs to acquire mult= iple -AioContexts simultaneously. Therefore, it is only safe for code holding t= he -QEMU global mutex to acquire other AioContexts. +AioContext functions like aio_set_fd_handler(), aio_set_event_notifier(), +aio_bh_new(), and aio_timer_new() are thread-safe. They can be used to tri= gger +activity in an IOThread. =20 Side note: the best way to schedule a function call across threads is to c= all -aio_bh_schedule_oneshot(). No acquire/release or locking is needed. +aio_bh_schedule_oneshot(). + +The main loop thread can wait synchronously for a condition using +AIO_WAIT_WHILE(). =20 AioContext and the block layer ------------------------------ @@ -124,22 +115,16 @@ Block layer code must therefore expect to run in an I= OThread and avoid using old APIs that implicitly use the main loop. See the "How to program for IOThreads" above for information on how to do that. =20 -If main loop code such as a QMP function wishes to access a BlockDriverSta= te -it must first call aio_context_acquire(bdrv_get_aio_context(bs)) to ensure -that callbacks in the IOThread do not run in parallel. - Code running in the monitor typically needs to ensure that past requests from the guest are completed. When a block device is running in an IOThread, the IOThread can also process requests from the guest (via ioeventfd). To achieve both objects, wrap the code between bdrv_drained_begin() and bdrv_drained_end(), thus creating a "drained -section". The functions must be called between aio_context_acquire() -and aio_context_release(). You can freely release and re-acquire the -AioContext within a drained section. - -Long-running jobs (usually in the form of coroutines) are best scheduled in -the BlockDriverState's AioContext to avoid the need to acquire/release aro= und -each bdrv_*() call. The functions bdrv_add/remove_aio_context_notifier, -or alternatively blk_add/remove_aio_context_notifier if you use BlockBacke= nds, -can be used to get a notification whenever bdrv_try_change_aio_context() m= oves a +section". + +Long-running jobs (usually in the form of coroutines) are often scheduled = in +the BlockDriverState's AioContext. The functions +bdrv_add/remove_aio_context_notifier, or alternatively +blk_add/remove_aio_context_notifier if you use BlockBackends, can be used = to +get a notification whenever bdrv_try_change_aio_context() moves a BlockDriverState to a different AioContext. --=20 2.43.0