From: Jan Kara Subject: Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock Date: Wed, 16 Feb 2011 15:56:27 +0100 Message-ID: <20110216145627.GB5592@quack.suse.cz> References: <20110207205325.FB6A.61FB500B@jp.fujitsu.com> <20110215160630.GH17313@quack.suse.cz> <20110215170352.GE4255@thunk.org> <20110215172954.GK17313@quack.suse.cz> <20110216081746.54d146d1.toshi.okajima@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , Ted Ts'o , Masayoshi MIZUMA , Andreas Dilger , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Toshiyuki Okajima Return-path: Received: from cantor.suse.de ([195.135.220.2]:33683 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751071Ab1BPO4a (ORCPT ); Wed, 16 Feb 2011 09:56:30 -0500 Content-Disposition: inline In-Reply-To: <20110216081746.54d146d1.toshi.okajima@jp.fujitsu.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Wed 16-02-11 08:17:46, Toshiyuki Okajima wrote: > On Tue, 15 Feb 2011 18:29:54 +0100 > Jan Kara wrote: > > On Tue 15-02-11 12:03:52, Ted Ts'o wrote: > > > On Tue, Feb 15, 2011 at 05:06:30PM +0100, Jan Kara wrote: > > > > Thanks for detailed analysis. Indeed this is a bug. Whenever we do IO > > > > under s_umount semaphore, we are prone to deadlock like the one you > > > > describe above. > > > > > > One of the fundamental problems here is that the freeze and thaw > > > routines are using down_write(&sb->s_umount) for two purposes. The > > > first is to prevent the resume/thaw from racing with a umount (which > > > it could do just as well by taking a read lock), but the second is to > > > prevent the resume/thaw code from racing with itself. That's the core > > > fundamental problem here. > > > > > > So I think we can solve this by introduce a new mutex, s_freeze, and > > > having the the resume/thaw first take the s_freeze mutex and then > > > second take a read lock on the s_umount. > > Sadly this does not quite work because even down_read(&sb->s_umount) > > in thaw_super() can block if there is another process that tries to acquire > > s_umount for writing - a situation like: > > TASK 1 (e.g. flusher) TASK 2 (e.g. remount) TASK 3 (unfreeze) > > down_read(&sb->s_umount) > > block on s_frozen > > down_write(&sb->s_umount) > > -blocked > > down_read(&sb->s_umount) > > -blocked > > behind the write access... > > > > The only working solution I see is to check for frozen filesystem before > > taking s_umount semaphore which seems rather ugly (but might be bearable if > > we did so in some well described wrapper). > I created the patch that you imagine yesterday. > > I got a reproducer from Mizuma-san yesterday, and then I executed it on the kernel > without a fixed patch. After an hour, I confirmed that this deadlock happened. > > However, on the kernel with a fixed patch, this deadlock doesn't still happen > after 12 hours passed. > > The patch for linux-2.6.38-rc4 is as follows: > --- > fs/fs-writeback.c | 2 +- > 1 files changed, 1 insertions(+), 1 deletions(-) > > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c > index 59c6e49..1c9a05e 100644 > --- a/fs/fs-writeback.c > +++ b/fs/fs-writeback.c > @@ -456,7 +456,7 @@ static bool pin_sb_for_writeback(struct super_block *sb) > spin_unlock(&sb_lock); > > if (down_read_trylock(&sb->s_umount)) { > - if (sb->s_root) > + if (sb->s_frozen == SB_UNFROZEN && sb->s_root) > return true; > up_read(&sb->s_umount); So this is something along the lines I thought but it actually won't work for example if sync(1) is run while the filesystem is frozen (that takes s_umount semaphore in a different place). And generally, I'm not convinced there are not other places that try to do IO while holding s_umount semaphore... Honza -- Jan Kara SUSE Labs, CR