From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37789) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aT6kp-0006ts-Ax for qemu-devel@nongnu.org; Tue, 09 Feb 2016 06:46:48 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aT6km-0005Kr-1o for qemu-devel@nongnu.org; Tue, 09 Feb 2016 06:46:47 -0500 Received: from mx1.redhat.com ([209.132.183.28]:57014) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aT6kl-0005Kk-RU for qemu-devel@nongnu.org; Tue, 09 Feb 2016 06:46:44 -0500 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) by mx1.redhat.com (Postfix) with ESMTPS id 7363B8E3F2 for ; Tue, 9 Feb 2016 11:46:43 +0000 (UTC) From: Paolo Bonzini Date: Tue, 9 Feb 2016 12:46:13 +0100 Message-Id: <1455018374-4706-16-git-send-email-pbonzini@redhat.com> In-Reply-To: <1455018374-4706-1-git-send-email-pbonzini@redhat.com> References: <1455018374-4706-1-git-send-email-pbonzini@redhat.com> Subject: [Qemu-devel] [PATCH 15/16] aio: document locking List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: stefanha@redhat.com Signed-off-by: Paolo Bonzini --- docs/multiple-iothreads.txt | 5 ++--- include/block/aio.h | 32 ++++++++++++++++---------------- 2 files changed, 18 insertions(+), 19 deletions(-) diff --git a/docs/multiple-iothreads.txt b/docs/multiple-iothreads.txt index c5d38e9..8ec3777 100644 --- a/docs/multiple-iothreads.txt +++ b/docs/multiple-iothreads.txt @@ -84,9 +84,8 @@ How to synchronize with an IOThread AioContext is not thread-safe so some rules must be followed when using file descriptors, event notifiers, timers, or BHs across threads: -1. AioContext functions can be called safely from file descriptor, event -notifier, timer, or BH callbacks invoked by the AioContext. No locking is -necessary. +1. AioContext functions can always be called safely. They handle their +own locking internally. 2. Other threads wishing to access the AioContext must take the QEMU global mutex *as well as* call aio_context_acquire()/aio_context_release() for diff --git a/include/block/aio.h b/include/block/aio.h index fb0ff21..cea92b0 100644 --- a/include/block/aio.h +++ b/include/block/aio.h @@ -50,18 +50,12 @@ typedef void IOHandler(void *opaque); struct AioContext { GSource source; - /* Protects all fields from multi-threaded access */ + /* Used by AioContext users to protect from multi-threaded access. */ QemuRecMutex lock; - /* The list of registered AIO handlers */ + /* The list of registered AIO handlers. Protected by ctx->list_lock. */ QLIST_HEAD(, AioHandler) aio_handlers; - /* This is a simple lock used to protect the aio_handlers list. - * Specifically, it's used to ensure that no callbacks are removed while - * we're walking and dispatching callbacks. - */ - int walking_handlers; - /* Used to avoid unnecessary event_notifier_set calls in aio_notify; * accessed with atomic primitives. If this field is 0, everything * (file descriptors, bottom halves, timers) will be re-evaluated @@ -87,9 +81,9 @@ struct AioContext { */ uint32_t notify_me; - /* A lock to protect between bh's adders and deleter, and to ensure - * that no callbacks are removed while we're walking and dispatching - * them. + /* A lock to protect between QEMUBH and AioHandler adders and deleter, + * and to ensure that no callbacks are removed while we're walking and + * dispatching them. */ QemuLockCnt list_lock; @@ -111,10 +105,14 @@ struct AioContext { bool notified; EventNotifier notifier; - /* Thread pool for performing work and receiving completion callbacks */ + /* Thread pool for performing work and receiving completion callbacks. + * Has its own locking. + */ struct ThreadPool *thread_pool; - /* TimerLists for calling timers - one per clock type */ + /* TimerLists for calling timers - one per clock type. Has its own + * locking. + */ QEMUTimerListGroup tlg; int external_disable_cnt; @@ -162,9 +160,11 @@ void aio_context_unref(AioContext *ctx); * automatically takes care of calling aio_context_acquire and * aio_context_release. * - * Access to timers and BHs from a thread that has not acquired AioContext - * is possible. Access to callbacks for now must be done while the AioContext - * is owned by the thread (FIXME). + * Note that this is separate from bdrv_drained_begin/bdrv_drained_end. A + * thread still has to call those to avoid being interrupted by the guest. + * + * Bottom halves, timers and callbacks can be created or removed without + * acquiring the AioContext. */ void aio_context_acquire(AioContext *ctx); -- 2.5.0