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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id D1AC0C7EE25 for ; Wed, 7 Jun 2023 12:37:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235545AbjFGMgt (ORCPT ); Wed, 7 Jun 2023 08:36:49 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46420 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235243AbjFGMgq (ORCPT ); Wed, 7 Jun 2023 08:36:46 -0400 Received: from smtp-out2.suse.de (smtp-out2.suse.de [195.135.220.29]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 7E63310F8 for ; Wed, 7 Jun 2023 05:36:44 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id 3DFF21FDB0; Wed, 7 Jun 2023 12:36:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1686141403; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=PGvn6ZdohOOtVUUdOvyiYpScXO3VVCt1a9Kf6pUq/oo=; b=c+AMWVzZLkboDs3i9HYvNVea+80TZY8OLFt31HnDAQD8RVVGlB/WU3txkylDnJV+TljaBf Uuxyhud9XLrbu6MzRg6uAjrkRJnWOHLj9hpjrtFfE4ZBWBMvoFRH7OoVldnlUA0WhSjxoI Onf1if4shMM1/GQMrW0D95Yx3gFqGFw= Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 19A4713776; Wed, 7 Jun 2023 12:36:43 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id ubJ8Bdt5gGQySgAAMHmgww (envelope-from ); Wed, 07 Jun 2023 12:36:43 +0000 From: Petr Pavlu To: jgross@suse.com, sstabellini@kernel.org, oleksandr_tyshchenko@epam.com Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, Petr Pavlu Subject: [PATCH] xen/xenbus: Avoid a lockdep warning when adding a watch Date: Wed, 7 Jun 2023 14:36:24 +0200 Message-Id: <20230607123624.15739-1-petr.pavlu@suse.com> X-Mailer: git-send-email 2.35.3 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following lockdep warning appears during boot on a Xen dom0 system: [ 96.388794] ====================================================== [ 96.388797] WARNING: possible circular locking dependency detected [ 96.388799] 6.4.0-rc5-default+ #8 Tainted: G EL [ 96.388803] ------------------------------------------------------ [ 96.388804] xenconsoled/1330 is trying to acquire lock: [ 96.388808] ffffffff82acdd10 (xs_watch_rwsem){++++}-{3:3}, at: register_xenbus_watch+0x45/0x140 [ 96.388847] but task is already holding lock: [ 96.388849] ffff888100c92068 (&u->msgbuffer_mutex){+.+.}-{3:3}, at: xenbus_file_write+0x2c/0x600 [ 96.388862] which lock already depends on the new lock. [ 96.388864] the existing dependency chain (in reverse order) is: [ 96.388866] -> #2 (&u->msgbuffer_mutex){+.+.}-{3:3}: [ 96.388874] __mutex_lock+0x85/0xb30 [ 96.388885] xenbus_dev_queue_reply+0x48/0x2b0 [ 96.388890] xenbus_thread+0x1d7/0x950 [ 96.388897] kthread+0xe7/0x120 [ 96.388905] ret_from_fork+0x2c/0x50 [ 96.388914] -> #1 (xs_response_mutex){+.+.}-{3:3}: [ 96.388923] __mutex_lock+0x85/0xb30 [ 96.388930] xenbus_backend_ioctl+0x56/0x1c0 [ 96.388935] __x64_sys_ioctl+0x90/0xd0 [ 96.388942] do_syscall_64+0x5c/0x90 [ 96.388950] entry_SYSCALL_64_after_hwframe+0x72/0xdc [ 96.388957] -> #0 (xs_watch_rwsem){++++}-{3:3}: [ 96.388965] __lock_acquire+0x1538/0x2260 [ 96.388972] lock_acquire+0xc6/0x2b0 [ 96.388976] down_read+0x2d/0x160 [ 96.388983] register_xenbus_watch+0x45/0x140 [ 96.388990] xenbus_file_write+0x53d/0x600 [ 96.388994] vfs_write+0xe4/0x490 [ 96.389003] ksys_write+0xb8/0xf0 [ 96.389011] do_syscall_64+0x5c/0x90 [ 96.389017] entry_SYSCALL_64_after_hwframe+0x72/0xdc [ 96.389023] other info that might help us debug this: [ 96.389025] Chain exists of: xs_watch_rwsem --> xs_response_mutex --> &u->msgbuffer_mutex [ 96.413429] Possible unsafe locking scenario: [ 96.413430] CPU0 CPU1 [ 96.413430] ---- ---- [ 96.413431] lock(&u->msgbuffer_mutex); [ 96.413432] lock(xs_response_mutex); [ 96.413433] lock(&u->msgbuffer_mutex); [ 96.413434] rlock(xs_watch_rwsem); [ 96.413436] *** DEADLOCK *** [ 96.413436] 1 lock held by xenconsoled/1330: [ 96.413438] #0: ffff888100c92068 (&u->msgbuffer_mutex){+.+.}-{3:3}, at: xenbus_file_write+0x2c/0x600 [ 96.413446] An ioctl call IOCTL_XENBUS_BACKEND_SETUP (record #1 in the report) results in calling xenbus_alloc() -> xs_suspend() which introduces ordering xs_watch_rwsem --> xs_response_mutex. The xenbus_thread() operation (record #2) creates xs_response_mutex --> &u->msgbuffer_mutex. An XS_WATCH write to the xenbus file then results in a complain about the opposite lock order &u->msgbuffer_mutex --> xs_watch_rwsem. The dependency xs_watch_rwsem --> xs_response_mutex is spurious. Avoid it and the warning by changing the ordering in xs_suspend(), first acquire xs_response_mutex and then xs_watch_rwsem. Reverse also the unlocking order in xs_suspend_cancel() for consistency, but keep xs_resume() as is because it needs to have xs_watch_rwsem unlocked only after exiting xs suspend and re-adding all watches. Signed-off-by: Petr Pavlu --- drivers/xen/xenbus/xenbus_xs.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/drivers/xen/xenbus/xenbus_xs.c b/drivers/xen/xenbus/xenbus_xs.c index 12e02eb01f59..028a182bcc9e 100644 --- a/drivers/xen/xenbus/xenbus_xs.c +++ b/drivers/xen/xenbus/xenbus_xs.c @@ -840,8 +840,8 @@ void xs_suspend(void) { xs_suspend_enter(); - down_write(&xs_watch_rwsem); mutex_lock(&xs_response_mutex); + down_write(&xs_watch_rwsem); } void xs_resume(void) @@ -866,8 +866,8 @@ void xs_resume(void) void xs_suspend_cancel(void) { - mutex_unlock(&xs_response_mutex); up_write(&xs_watch_rwsem); + mutex_unlock(&xs_response_mutex); xs_suspend_exit(); } -- 2.35.3