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 85714C61CE8 for ; Wed, 4 Jun 2025 18:03:49 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uMsMN-0002TO-Eg; Wed, 04 Jun 2025 13:57:07 -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 1uMsMA-0002Hn-0n for qemu-devel@nongnu.org; Wed, 04 Jun 2025 13:56:55 -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 1uMsM3-0000RK-To for qemu-devel@nongnu.org; Wed, 04 Jun 2025 13:56:53 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1749059798; 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=3FrDWqL/C3nMnJ3mprtXMtzqqjkfJ3qI3BhyNJ7QnyM=; b=Z+6KpxaJoyGXcytAa90cvzVRNLNioFNH6UF8TXoqCj5g6ek4VRy7/GjzJd/E3a1Psjktk3 wAvig1yZcG90QUhHg/+oqEkMr2FfKKH28jvZ2M2zusereYqo6CGxnAAdweI8SptZDs0xk+ 3WbPLnu7uvQpVJhE+wK6+ZGKOj+y7Qo= 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-362--FomXQ--PI2PAcRJu0CauQ-1; Wed, 04 Jun 2025 13:56:35 -0400 X-MC-Unique: -FomXQ--PI2PAcRJu0CauQ-1 X-Mimecast-MFC-AGG-ID: -FomXQ--PI2PAcRJu0CauQ_1749059794 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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 350A7195608E; Wed, 4 Jun 2025 17:56:34 +0000 (UTC) Received: from merkur.fritz.box (unknown [10.44.34.43]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id E644A18002B1; Wed, 4 Jun 2025 17:56:32 +0000 (UTC) From: Kevin Wolf To: qemu-block@nongnu.org Cc: kwolf@redhat.com, qemu-devel@nongnu.org Subject: [PULL 08/24] block: move drain outside of bdrv_change_aio_context() and mark GRAPH_RDLOCK Date: Wed, 4 Jun 2025 19:55:57 +0200 Message-ID: <20250604175613.344113-9-kwolf@redhat.com> In-Reply-To: <20250604175613.344113-1-kwolf@redhat.com> References: <20250604175613.344113-1-kwolf@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.128, 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_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 From: Fiona Ebner This is in preparation to mark bdrv_drained_begin() as GRAPH_UNLOCKED. Note that even if bdrv_drained_begin() were already marked as GRAPH_UNLOCKED, TSA would not complain about the instance in bdrv_change_aio_context() before this change, because it is preceded by a bdrv_graph_rdunlock_main_loop() call. It is not correct to release the lock here, and in case the caller holds a write lock, it wouldn't actually release the lock. In combination with block-stream, there is a deadlock that can happen because of this [0]. In particular, it can happen that main thread IO thread 1. acquires write lock in blk_co_do_preadv_part(): 2. have non-zero blk->in_flight 3. try to acquire read lock 4. begin drain Steps 3 and 4 might be switched. Draining will poll and get stuck, because it will see the non-zero in_flight counter. But the IO thread will not make any progress either, because it cannot acquire the read lock. After this change, all paths to bdrv_change_aio_context() drain: bdrv_change_aio_context() is called by: 1. bdrv_child_cb_change_aio_ctx() which is only called via the change_aio_ctx() callback, see below. 2. bdrv_child_change_aio_context(), see below. 3. bdrv_try_change_aio_context(), where a drained section is introduced. The change_aio_ctx() callback is called by: 1. bdrv_attach_child_common_abort(), where a drained section is introduced. 2. bdrv_attach_child_common(), where a drained section is introduced. 3. bdrv_parent_change_aio_context(), see below. bdrv_child_change_aio_context() is called by: 1. bdrv_change_aio_context(), i.e. recursive, so being in a drained section is invariant. 2. child_job_change_aio_ctx(), which is only called via the change_aio_ctx() callback, see above. bdrv_parent_change_aio_context() is called by: 1. bdrv_change_aio_context(), i.e. recursive, so being in a drained section is invariant. This resolves all code paths. Note that bdrv_attach_child_common() and bdrv_attach_child_common_abort() hold the graph write lock and callers of bdrv_try_change_aio_context() might too, so they are not actually allowed to drain either. This will be addressed in the following commits. More granular draining is not trivially possible, because bdrv_change_aio_context() can recursively call itself e.g. via bdrv_child_change_aio_context(). [0]: https://lore.kernel.org/qemu-devel/73839c04-7616-407e-b057-80ca69e63f51@virtuozzo.com/ Reported-by: Andrey Drobyshev Signed-off-by: Fiona Ebner Reviewed-by: Kevin Wolf Message-ID: <20250530151125.955508-9-f.ebner@proxmox.com> Signed-off-by: Kevin Wolf --- include/block/block_int-common.h | 12 +++++++ block.c | 57 +++++++++++++++++++++++--------- 2 files changed, 53 insertions(+), 16 deletions(-) diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h index 37466c7841..168f703fa1 100644 --- a/include/block/block_int-common.h +++ b/include/block/block_int-common.h @@ -983,6 +983,18 @@ struct BdrvChildClass { bool backing_mask_protocol, Error **errp); + /* + * Notifies the parent that the child is trying to change its AioContext. + * The parent may in turn change the AioContext of other nodes in the same + * transaction. Returns true if the change is possible and the transaction + * can be continued. Returns false and sets @errp if not and the transaction + * must be aborted. + * + * @visited will accumulate all visited BdrvChild objects. The caller is + * responsible for freeing the list afterwards. + * + * Must be called with the affected block nodes drained. + */ bool GRAPH_RDLOCK_PTR (*change_aio_ctx)(BdrvChild *child, AioContext *ctx, GHashTable *visited, Transaction *tran, Error **errp); diff --git a/block.c b/block.c index 01144c895e..6f42c0f1ab 100644 --- a/block.c +++ b/block.c @@ -106,9 +106,9 @@ static void bdrv_reopen_abort(BDRVReopenState *reopen_state); static bool bdrv_backing_overridden(BlockDriverState *bs); -static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx, - GHashTable *visited, Transaction *tran, - Error **errp); +static bool GRAPH_RDLOCK +bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx, + GHashTable *visited, Transaction *tran, Error **errp); /* If non-zero, use only whitelisted block drivers */ static int use_bdrv_whitelist; @@ -3040,8 +3040,10 @@ static void GRAPH_WRLOCK bdrv_attach_child_common_abort(void *opaque) /* No need to visit `child`, because it has been detached already */ visited = g_hash_table_new(NULL, NULL); + bdrv_drain_all_begin(); ret = s->child->klass->change_aio_ctx(s->child, s->old_parent_ctx, visited, tran, &error_abort); + bdrv_drain_all_end(); g_hash_table_destroy(visited); /* transaction is supposed to always succeed */ @@ -3122,9 +3124,11 @@ bdrv_attach_child_common(BlockDriverState *child_bs, bool ret_child; g_hash_table_add(visited, new_child); + bdrv_drain_all_begin(); ret_child = child_class->change_aio_ctx(new_child, child_ctx, visited, aio_ctx_tran, NULL); + bdrv_drain_all_end(); if (ret_child == true) { error_free(local_err); ret = 0; @@ -7576,6 +7580,17 @@ typedef struct BdrvStateSetAioContext { BlockDriverState *bs; } BdrvStateSetAioContext; +/* + * Changes the AioContext of @child to @ctx and recursively for the associated + * block nodes and all their children and parents. Returns true if the change is + * possible and the transaction @tran can be continued. Returns false and sets + * @errp if not and the transaction must be aborted. + * + * @visited will accumulate all visited BdrvChild objects. The caller is + * responsible for freeing the list afterwards. + * + * Must be called with the affected block nodes drained. + */ static bool GRAPH_RDLOCK bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx, GHashTable *visited, Transaction *tran, @@ -7604,6 +7619,17 @@ bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx, return true; } +/* + * Changes the AioContext of @c->bs to @ctx and recursively for all its children + * and parents. Returns true if the change is possible and the transaction @tran + * can be continued. Returns false and sets @errp if not and the transaction + * must be aborted. + * + * @visited will accumulate all visited BdrvChild objects. The caller is + * responsible for freeing the list afterwards. + * + * Must be called with the affected block nodes drained. + */ bool bdrv_child_change_aio_context(BdrvChild *c, AioContext *ctx, GHashTable *visited, Transaction *tran, Error **errp) @@ -7619,10 +7645,6 @@ bool bdrv_child_change_aio_context(BdrvChild *c, AioContext *ctx, static void bdrv_set_aio_context_clean(void *opaque) { BdrvStateSetAioContext *state = (BdrvStateSetAioContext *) opaque; - BlockDriverState *bs = (BlockDriverState *) state->bs; - - /* Paired with bdrv_drained_begin in bdrv_change_aio_context() */ - bdrv_drained_end(bs); g_free(state); } @@ -7650,10 +7672,12 @@ static TransactionActionDrv set_aio_context = { * * @visited will accumulate all visited BdrvChild objects. The caller is * responsible for freeing the list afterwards. + * + * @bs must be drained. */ -static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx, - GHashTable *visited, Transaction *tran, - Error **errp) +static bool GRAPH_RDLOCK +bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx, + GHashTable *visited, Transaction *tran, Error **errp) { BdrvChild *c; BdrvStateSetAioContext *state; @@ -7664,21 +7688,17 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx, return true; } - bdrv_graph_rdlock_main_loop(); QLIST_FOREACH(c, &bs->parents, next_parent) { if (!bdrv_parent_change_aio_context(c, ctx, visited, tran, errp)) { - bdrv_graph_rdunlock_main_loop(); return false; } } QLIST_FOREACH(c, &bs->children, next) { if (!bdrv_child_change_aio_context(c, ctx, visited, tran, errp)) { - bdrv_graph_rdunlock_main_loop(); return false; } } - bdrv_graph_rdunlock_main_loop(); state = g_new(BdrvStateSetAioContext, 1); *state = (BdrvStateSetAioContext) { @@ -7686,8 +7706,7 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx, .bs = bs, }; - /* Paired with bdrv_drained_end in bdrv_set_aio_context_clean() */ - bdrv_drained_begin(bs); + assert(bs->quiesce_counter > 0); tran_add(tran, &set_aio_context, state); @@ -7720,6 +7739,8 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx, if (ignore_child) { g_hash_table_add(visited, ignore_child); } + bdrv_drain_all_begin(); + bdrv_graph_rdlock_main_loop(); ret = bdrv_change_aio_context(bs, ctx, visited, tran, errp); g_hash_table_destroy(visited); @@ -7733,10 +7754,14 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx, if (!ret) { /* Just run clean() callbacks. No AioContext changed. */ tran_abort(tran); + bdrv_graph_rdunlock_main_loop(); + bdrv_drain_all_end(); return -EPERM; } tran_commit(tran); + bdrv_graph_rdunlock_main_loop(); + bdrv_drain_all_end(); return 0; } -- 2.49.0