From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:57314 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753556AbcIZQH4 (ORCPT ); Mon, 26 Sep 2016 12:07:56 -0400 Date: Mon, 26 Sep 2016 18:07:48 +0200 From: Oleg Nesterov To: Al Viro Cc: Dave Chinner , Jan Kara , Nikolay Borisov , "Paul E. McKenney" , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH 1/2] fs/super.c: fix race between freeze_super() and thaw_super() Message-ID: <20160926160748.GA6748@redhat.com> References: <20160926160724.GA6739@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160926160724.GA6739@redhat.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Change thaw_super() to check frozen != SB_FREEZE_COMPLETE rather than frozen == SB_UNFROZEN, otherwise it can race with freeze_super() which drops sb->s_umount after SB_FREEZE_WRITE to preserve the lock ordering. In this case thaw_super() will wrongly call s_op->unfreeze_fs() before it was actually frozen, and call sb_freeze_unlock() which leads to the unbalanced percpu_up_write(). Unfortunately lockdep can't detect this, so this triggers misc BUG_ON()'s in kernel/rcu/sync.c. Reported-and-tested-by: Nikolay Borisov Signed-off-by: Oleg Nesterov Cc: stable@vger.kernel.org --- fs/super.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/fs/super.c b/fs/super.c index d78b984..2549896c 100644 --- a/fs/super.c +++ b/fs/super.c @@ -1324,8 +1324,8 @@ int freeze_super(struct super_block *sb) } } /* - * This is just for debugging purposes so that fs can warn if it - * sees write activity when frozen is set to SB_FREEZE_COMPLETE. + * For debugging purposes so that fs can warn if it sees write activity + * when frozen is set to SB_FREEZE_COMPLETE, and for thaw_super(). */ sb->s_writers.frozen = SB_FREEZE_COMPLETE; up_write(&sb->s_umount); @@ -1344,7 +1344,7 @@ int thaw_super(struct super_block *sb) int error; down_write(&sb->s_umount); - if (sb->s_writers.frozen == SB_UNFROZEN) { + if (sb->s_writers.frozen != SB_FREEZE_COMPLETE) { up_write(&sb->s_umount); return -EINVAL; } -- 2.5.0