From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161527AbcIZQLa (ORCPT ); Mon, 26 Sep 2016 12:11:30 -0400 Received: from mx2.suse.de ([195.135.220.15]:48138 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965069AbcIZQL3 (ORCPT ); Mon, 26 Sep 2016 12:11:29 -0400 Date: Mon, 26 Sep 2016 18:11:26 +0200 From: Jan Kara To: Oleg Nesterov Cc: Al Viro , Dave Chinner , Jan Kara , Nikolay Borisov , "Paul E. McKenney" , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] fs/super.c: fix race between freeze_super() and thaw_super() Message-ID: <20160926161126.GA32458@quack2.suse.cz> References: <20160926160724.GA6739@redhat.com> <20160926160748.GA6748@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160926160748.GA6748@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon 26-09-16 18:07:48, Oleg Nesterov wrote: > 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 The patch looks good. Thanks! Reviewed-by: Jan Kara Honza > --- > 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 > > -- Jan Kara SUSE Labs, CR