linux-ext4 - Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

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Message-Id: <20110216081746.54d146d1.toshi.okajima@jp.fujitsu.com>
Date:	Wed, 16 Feb 2011 08:17:46 +0900
From:	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Ted Ts'o <tytso@....edu>,
	Masayoshi MIZUMA <m.mizuma@...fujitsu.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock

Hi.

On Tue, 15 Feb 2011 18:29:54 +0100
Jan Kara <jack@...e.cz> 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);
        }
-- 

Best Regards,
Toshiyuki Okajima
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