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 3940AC54FB3 for ; Mon, 26 May 2025 09:11:06 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uJTrG-00061A-4T; Mon, 26 May 2025 05:10:58 -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 1uJTrD-0005x7-IK for qemu-devel@nongnu.org; Mon, 26 May 2025 05:10:56 -0400 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 1uJTrA-0001Ka-VN for qemu-devel@nongnu.org; Mon, 26 May 2025 05:10:55 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1748250651; 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=bKffX4HPzxGdQ26b5rEZ+t2Ey5x4Zaw5tmG6EaQlZy4=; b=bx7Ed61m8OJyNBGYvjfLqhvF0rl4aollPmeCWpVT33FwRO+bPmAuv6tscNESnIRY7rtQLj fGIdRB9JCWg8HuPp21oiQrqPB+KBG+AFIiDi9mwGpwymyxK/YP3Vcm4mAJ+E68WB0E6X8K 1u28CxxXmo74au/TrHVYG4KVizHWgC0= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-523-zrus-ApNNgiD_pDp2xFa7A-1; Mon, 26 May 2025 05:10:46 -0400 X-MC-Unique: zrus-ApNNgiD_pDp2xFa7A-1 X-Mimecast-MFC-AGG-ID: zrus-ApNNgiD_pDp2xFa7A_1748250645 Received: from mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.40]) (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-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id B3AB5195609F; Mon, 26 May 2025 09:10:44 +0000 (UTC) Received: from redhat.com (unknown [10.44.33.150]) by mx-prod-int-04.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id A7C5119560AF; Mon, 26 May 2025 09:10:39 +0000 (UTC) Date: Mon, 26 May 2025 11:10:37 +0200 From: Kevin Wolf To: Andrey Drobyshev Cc: Fiona Ebner , 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4fdff680-5e77-40f2-812b-70697ad8ae64@virtuozzo.com> X-Scanned-By: MIMEDefang 3.0 on 10.30.177.40 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: -59 X-Spam_score: -6.0 X-Spam_bar: ------ X-Spam_report: (-6.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.904, 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=-1, RCVD_IN_MSPIKE_WL=-0.01, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 23.05.2025 um 20:12 hat Andrey Drobyshev geschrieben: > On 5/20/25 1:29 PM, Fiona Ebner wrote: > > This is in preparation to mark bdrv_drained_begin() as GRAPH_UNLOCKED. > > > > More granular draining is not trivially possible, because > > bdrv_snapshot_delete() can recursively call itself. > > > > The return value of bdrv_all_delete_snapshot() changes from -1 to > > -errno propagated from failed sub-calls. This is fine for the existing > > callers of bdrv_all_delete_snapshot(). > > > > Signed-off-by: Fiona Ebner > > --- > > > > Changes in v2: > > * Use 'must be drained' instead of 'needs to be drained'. > > * Use goto for cleanup/error handling in bdrv_all_delete_snapshot(). > > * Don't use atomics to access bs->quiesce_counter. > > > > block/snapshot.c | 26 +++++++++++++++----------- > > blockdev.c | 25 +++++++++++++++++-------- > > qemu-img.c | 2 ++ > > 3 files changed, 34 insertions(+), 19 deletions(-) > > > > diff --git a/block/snapshot.c b/block/snapshot.c > > index 22567f1fb9..9f300a78bd 100644 > > --- a/block/snapshot.c > > +++ b/block/snapshot.c > > @@ -327,7 +327,7 @@ int bdrv_snapshot_goto(BlockDriverState *bs, > > > > /** > > * Delete an internal snapshot by @snapshot_id and @name. > > - * @bs: block device used in the operation > > + * @bs: block device used in the operation, must be drained > > * @snapshot_id: unique snapshot ID, or NULL > > * @name: snapshot name, or NULL > > * @errp: location to store error > > @@ -358,6 +358,8 @@ int bdrv_snapshot_delete(BlockDriverState *bs, > > > > GLOBAL_STATE_CODE(); > > > > + assert(bs->quiesce_counter > 0); > > + > > if (!drv) { > > error_setg(errp, "Device '%s' has no medium", > > bdrv_get_device_name(bs)); > > @@ -368,9 +370,6 @@ int bdrv_snapshot_delete(BlockDriverState *bs, > > return -EINVAL; > > } > > > > - /* drain all pending i/o before deleting snapshot */ > > - bdrv_drained_begin(bs); > > - > > if (drv->bdrv_snapshot_delete) { > > ret = drv->bdrv_snapshot_delete(bs, snapshot_id, name, errp); > > } else if (fallback_bs) { > > @@ -382,7 +381,6 @@ int bdrv_snapshot_delete(BlockDriverState *bs, > > ret = -ENOTSUP; > > } > > > > - bdrv_drained_end(bs); > > return ret; > > } > > > > @@ -571,19 +569,22 @@ int bdrv_all_delete_snapshot(const char *name, > > ERRP_GUARD(); > > g_autoptr(GList) bdrvs = NULL; > > GList *iterbdrvs; > > + int ret = 0; > > > > GLOBAL_STATE_CODE(); > > - GRAPH_RDLOCK_GUARD_MAINLOOP(); > > > > - if (bdrv_all_get_snapshot_devices(has_devices, devices, &bdrvs, errp) < 0) { > > - return -1; > > + bdrv_drain_all_begin(); > > + bdrv_graph_rdlock_main_loop(); > > + > > + ret = bdrv_all_get_snapshot_devices(has_devices, devices, &bdrvs, errp); > > + if (ret < 0) { > > + goto out; > > } > > > > iterbdrvs = bdrvs; > > while (iterbdrvs) { > > BlockDriverState *bs = iterbdrvs->data; > > QEMUSnapshotInfo sn1, *snapshot = &sn1; > > - int ret = 0; > > > > if ((devices || bdrv_all_snapshots_includes_bs(bs)) && > > bdrv_snapshot_find(bs, snapshot, name) >= 0) > > @@ -594,13 +595,16 @@ int bdrv_all_delete_snapshot(const char *name, > > if (ret < 0) { > > error_prepend(errp, "Could not delete snapshot '%s' on '%s': ", > > name, bdrv_get_device_or_node_name(bs)); > > - return -1; > > + goto out; > > } > > > > iterbdrvs = iterbdrvs->next; > > } > > > > - return 0; > > +out: > > + bdrv_graph_rdunlock_main_loop(); > > + bdrv_drain_all_end(); > > + return ret; > > } > > > > > > diff --git a/blockdev.c b/blockdev.c > > index 21443b4514..3982f9776b 100644 > > --- a/blockdev.c > > +++ b/blockdev.c > > @@ -1132,39 +1132,41 @@ SnapshotInfo *qmp_blockdev_snapshot_delete_internal_sync(const char *device, > > int ret; > > > > GLOBAL_STATE_CODE(); > > - GRAPH_RDLOCK_GUARD_MAINLOOP(); > > + > > + bdrv_drain_all_begin(); > > + bdrv_graph_rdlock_main_loop(); > > > > bs = qmp_get_root_bs(device, errp); > > if (!bs) { > > - return NULL; > > + goto error; > > } > > > > if (!id && !name) { > > error_setg(errp, "Name or id must be provided"); > > - return NULL; > > + goto error; > > } > > > > if (bdrv_op_is_blocked(bs, BLOCK_OP_TYPE_INTERNAL_SNAPSHOT_DELETE, errp)) { > > - return NULL; > > + goto error; > > } > > > > ret = bdrv_snapshot_find_by_id_and_name(bs, id, name, &sn, &local_err); > > if (local_err) { > > error_propagate(errp, local_err); > > - return NULL; > > + goto error; > > } > > if (!ret) { > > error_setg(errp, > > "Snapshot with id '%s' and name '%s' does not exist on " > > "device '%s'", > > STR_OR_NULL(id), STR_OR_NULL(name), device); > > - return NULL; > > + goto error; > > } > > > > bdrv_snapshot_delete(bs, id, name, &local_err); > > if (local_err) { > > error_propagate(errp, local_err); > > - return NULL; > > + goto error; > > } > > > > info = g_new0(SnapshotInfo, 1); > > @@ -1180,6 +1182,9 @@ SnapshotInfo *qmp_blockdev_snapshot_delete_internal_sync(const char *device, > > info->has_icount = true; > > } > > > > +error: > > + bdrv_graph_rdunlock_main_loop(); > > + bdrv_drain_all_end(); > > return info; > > } > > > > @@ -1295,12 +1300,14 @@ static void internal_snapshot_abort(void *opaque) > > Error *local_error = NULL; > > > > GLOBAL_STATE_CODE(); > > - GRAPH_RDLOCK_GUARD_MAINLOOP(); > > > > if (!state->created) { > > return; > > } > > > > + bdrv_drain_all_begin(); > > + bdrv_graph_rdlock_main_loop(); > > + > > if (bdrv_snapshot_delete(bs, sn->id_str, sn->name, &local_error) < 0) { > > error_reportf_err(local_error, > > "Failed to delete snapshot with id '%s' and " > > @@ -1308,6 +1315,8 @@ static void internal_snapshot_abort(void *opaque) > > sn->id_str, sn->name, > > bdrv_get_device_name(bs)); > > } > > + bdrv_graph_rdunlock_main_loop(); > > + bdrv_drain_all_end(); > > } > > > > 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. Kevin