From nobody Sun May 11 12:55:33 2025 Delivered-To: importer2@patchew.org Authentication-Results: mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer2=patchew.org@nongnu.org; dmarc=pass(p=none dis=none) header.from=redhat.com ARC-Seal: i=1; a=rsa-sha256; t=1701800589; cv=none; d=zohomail.com; s=zohoarc; b=naFIsd1XohkvqlQo8MtBi+XzySH99TpMEyduELLU9kFFfnxyqLGRYfpw661+xDu3YVCUZGQHVUiqwh/F/cSiokl4y4RzD30mH5vZvfxK6tTzs2fe6yoS4v+2h4PCXi0m2AwxNFSLrei5t2cZQoPrzmQuW04VZple911CpHp/D6I= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1701800589; h=Content-Transfer-Encoding:Cc:Cc:Date:Date:From:From:In-Reply-To:List-Subscribe:List-Post:List-Id:List-Archive:List-Help:List-Unsubscribe:MIME-Version:Message-ID:References:Sender:Subject:Subject:To:To:Message-Id:Reply-To; bh=8/B7zc0XB6x72qbuhQc1b2o1g2AgjtE1qkTNJwgvUlY=; b=Nw4XGdF+oXDRyszwjs/S8JS3PK05JU4qW6jSskMjRbAQp9AtjrRybd68pIxm8UYpLdGRTPYatendHAhPJCP4i+Kt4LS1KfPDG2jkVW6W/m5IydiXNNHn/LvqMaQqXERWG2UTts0D1rGnsVkSG1+mkTGX00DAbAMUHOlfbKHhr8M= ARC-Authentication-Results: i=1; mx.zohomail.com; dkim=pass; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer2=patchew.org@nongnu.org; dmarc=pass header.from= (p=none dis=none) Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1701800589584772.387223960739; Tue, 5 Dec 2023 10:23:09 -0800 (PST) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rAa4F-0002HP-SC; Tue, 05 Dec 2023 13:22:47 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rAa2b-0000HG-AP for qemu-devel@nongnu.org; Tue, 05 Dec 2023 13:21:06 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rAa2O-0002oH-OH for qemu-devel@nongnu.org; Tue, 05 Dec 2023 13:21:05 -0500 Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-685-8BIN0AuQO9OJpRD2qHD49w-1; Tue, 05 Dec 2023 13:20:46 -0500 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 780813C2A1C1; Tue, 5 Dec 2023 18:20:45 +0000 (UTC) Received: from localhost (unknown [10.39.194.111]) by smtp.corp.redhat.com (Postfix) with ESMTP id D5A7A40C6EB9; Tue, 5 Dec 2023 18:20:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1701800451; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=8/B7zc0XB6x72qbuhQc1b2o1g2AgjtE1qkTNJwgvUlY=; b=es5UEZFnLlnAER/WQ1VKxtnMjgIKwE0DDM1v837p1/n0jTVeKA0d+O2qZidblkg5bDK/Cp YQzVB2uaRoJMrqNPUZDCEBW2WJIQfYffuWr/E007pFDVIbAFlXRuWuphSZ7KE+h/L5yEK1 b9GUUnFTleR5zaHMhZtyd/DWP7msn4g= X-MC-Unique: 8BIN0AuQO9OJpRD2qHD49w-1 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Cc: =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , Stefan Hajnoczi , Vladimir Sementsov-Ogievskiy , Cleber Rosa , Xie Changlong , Paul Durrant , Ari Sundholm , Jason Wang , Eric Blake , John Snow , Eduardo Habkost , Wen Congyang , Alberto Garcia , Anthony Perard , "Michael S. Tsirkin" , Stefano Stabellini , qemu-block@nongnu.org, Juan Quintela , Paolo Bonzini , Kevin Wolf , Coiby Xu , Fabiano Rosas , Hanna Reitz , Zhang Chen , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Pavel Dovgalyuk , Peter Xu , Emanuele Giuseppe Esposito , Fam Zheng , Leonardo Bras , David Hildenbrand , Li Zhijian , xen-devel@lists.xenproject.org Subject: [PATCH v2 11/14] docs: remove AioContext lock from IOThread docs Date: Tue, 5 Dec 2023 13:20:08 -0500 Message-ID: <20231205182011.1976568-12-stefanha@redhat.com> In-Reply-To: <20231205182011.1976568-1-stefanha@redhat.com> References: <20231205182011.1976568-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.2 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+importer2=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; 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, 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+importer2=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer2=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1701800590095100001 Content-Type: text/plain; charset="utf-8" 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 --- docs/devel/multiple-iothreads.txt | 45 +++++++++++-------------------- 1 file changed, 15 insertions(+), 30 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. +section". =20 -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 +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