From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 55C19C54FB3 for ; Mon, 26 May 2025 11:44:00 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uJWEs-0004wy-JP; Mon, 26 May 2025 07:43:30 -0400 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 1uJWEn-0004wl-HW for qemu-devel@nongnu.org; Mon, 26 May 2025 07:43:26 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1uJWEh-00047p-QE for qemu-devel@nongnu.org; Mon, 26 May 2025 07:43:23 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1748259796; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=3Nf1EwpaMQJ9eurpSk4Y2/D/g648qq3m1SyR1llBZHw=; b=e+yX+gP+z7ld9EU35Gxkorvv+itLrRSBNQ8Kb+aW/ACvmKJWc+aQ6BxzdZHDuu2KVeWCtu q4mtxMYkO1S0lpU+QecfekfkJ0Ntd0Ps0QuFB65d1WdaXh8qVynSkavbJ/apkDwKxavAB5 Spwcyv9XEmgqq+N2yr+wP3YRmc/5XSM= Received: from mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-592-3ovvTQb0PZagNXDoOKev2g-1; Mon, 26 May 2025 07:43:15 -0400 X-MC-Unique: 3ovvTQb0PZagNXDoOKev2g-1 X-Mimecast-MFC-AGG-ID: 3ovvTQb0PZagNXDoOKev2g_1748259793 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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 mx-prod-mc-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 7CB2C1800256; Mon, 26 May 2025 11:43:12 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.150]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 07E7219560AB; Mon, 26 May 2025 11:43:01 +0000 (UTC) Date: Mon, 26 May 2025 13:42:59 +0200 From: Kevin Wolf To: Fiona Ebner Cc: Andrey Drobyshev , qemu-block@nongnu.org, qemu-devel@nongnu.org, den@virtuozzo.com, hreitz@redhat.com, stefanha@redhat.com, eblake@redhat.com, jsnow@redhat.com, vsementsov@yandex-team.ru, xiechanglong.d@gmail.com, wencongyang2@huawei.com, berto@igalia.com, fam@euphon.net, ari@tuxera.com Subject: Re: [PATCH v2 03/24] block/snapshot: move drain outside of read-locked bdrv_snapshot_delete() Message-ID: References: <20250520103012.424311-1-f.ebner@proxmox.com> <20250520103012.424311-4-f.ebner@proxmox.com> <4fdff680-5e77-40f2-812b-70697ad8ae64@virtuozzo.com> <6c640f56-31b8-408d-b747-6f75cdfa7592@proxmox.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6c640f56-31b8-408d-b747-6f75cdfa7592@proxmox.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -49 X-Spam_score: -5.0 X-Spam_bar: ----- X-Spam_report: (-5.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.903, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 26.05.2025 um 12:33 hat Fiona Ebner geschrieben: > Am 26.05.25 um 11:10 schrieb Kevin Wolf: > > Am 23.05.2025 um 20:12 hat Andrey Drobyshev geschrieben: > >> Okay, I've got a very simple and naive question to ask. We've got this > >> pattern recurring throughout the series: > >> > >>> GLOBAL_STATE_CODE(); > >>> bdrv_drain_all_begin(); > >>> bdrv_graph_rdlock_main_loop(); > >>> > >>> ... > >>> > >>> bdrv_graph_rdunlock_main_loop(); > >>> bdrv_drain_all_end(); > >> > >> bdrv_graph_rdlock_main_loop() doesn't actually take any locks, it > >> asserts that we're running in the main thread and not in a coroutine. > >> bdrv_graph_rdunlock_main_loop() does the same. > >> GRAPH_RDLOCK_GUARD_MAINLOOP() does both those calls, in the beginning of > >> a function and when leaving its scope, so essentially it also just does > >> assert(qemu_in_main_thread() && !qemu_in_coroutine()). > >> > >> Therefore: > >> > >> 1. Is there any real benefit from using those > >> {rdlock/rdunlock}_main_loop() constructions, or they're here due to > >> historical reasons only? > > > > It's the price we pay for the compiler to verify our locking rules. > > > >> 2. Would it hurt if we only leave GRAPH_RDLOCK_GUARD_MAINLOOP() in all > >> such occurrences? At least when it's obvious we can't get out of the > >> main thread. That would simply deliver us from performing same checks > >> several times, similar to what's done in commit 22/24 ("block/io: remove > >> duplicate GLOBAL_STATE_CODE() in bdrv_do_drained_end()"). > > > > Once bdrv_drain_all_begin() is marked GRAPH_UNLOCKED, calling it after > > GRAPH_RDLOCK_GUARD_MAINLOOP() would be wrong according to TSA rules > > (which don't know anything about this being only a fake lock) and the > > build would fail. > > Note that I did not mark bdrv_drain_all_begin() as GRAPH_UNLOCKED in the > series yet. The reason is that I wasn't fully sure if that is okay, > given that it also can be called from a coroutine and does > bdrv_co_yield_to_drain() then. But I suppose that doesn't do anything > with the graph lock, so I'll add the GRAPH_UNLOCKED marker in v3. I think it's still GRAPH_UNLOCKED even when called from a coroutine, because otherwise the polling could wait for the calling coroutine to make progress and release the lock, resulting in a deadlock. Kevin