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Date:	Wed, 6 Apr 2011 15:40:05 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Toshiyuki Okajima <toshi.okajima@...fujitsu.com>,
	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: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due
 to a deadlock

On Fri, Apr 01, 2011 at 04:08:56PM +0200, Jan Kara wrote:
> On Fri 01-04-11 10:40:50, Dave Chinner wrote:
> > On Mon, Mar 28, 2011 at 05:06:28PM +0900, Toshiyuki Okajima wrote:
> > > On Thu, 17 Feb 2011 11:45:52 +0100
> > > Jan Kara <jack@...e.cz> wrote:
> > > > On Thu 17-02-11 12:50:51, Toshiyuki Okajima wrote:
> > > > > (2011/02/16 23:56), Jan Kara wrote:
> > > > > >On Wed 16-02-11 08:17:46, Toshiyuki Okajima wrote:
> > > > > >>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);
> > > > > 
> > > > > >   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...
> > > > > OK. I understand.
> > > > > 
> > > > > This code only fixes the case for the following path:
> > > > > writeback_inodes_wb
> > > > > -> ext4_da_writepages
> > > > >    -> ext4_journal_start_sb
> > > > >       -> vfs_check_frozen
> > > > > But, the code doesn't fix the other cases.
> > > > > 
> > > > > We must modify the local filesystem part in order to fix all cases...?
> > > >   Yes, possibly. But most importantly we should first find clear locking
> > > > rules for frozen filesystem that avoid deadlocks like the one above. And
> > > > the freezing / unfreezing code might become subtle for that reason, that's
> > > > fine, but it would be really good to avoid any complicated things for the
> > > > code in the rest of the VFS / filesystems.
> > > I have deeply continued to examined the root cause of this problem, then 
> > > I found it.
> > > 
> > > It is that we can write a memory which is mmaped to a file. Then the memory 
> > > becomes "DIRTY" so then the flusher thread (ex. wb_do_writeback) tries to
> > > "writeback" the memory. 
> > 
> > Then surely the issue is that .page_mkwrite is not checking that the
> > filesystem is frozen before allowing the page fault to continue and
> > dirty the page?
>   And is this a bug? That isn't clear to me...

Given the semantics of a frozen filesystem, letting any object be
dirtied while frozen (be it an inode, a page, a metadata block, etc)
is definitely a bug.

The way the freeze code is architected is that incoming dirtying
events are prevented so that the writeback side does not need to
care about the frozen state of the filesystem at all. The freeze
operation is supposed to block new dirtiers, then flush all dirty
objects resulting in everything being clean in the filesystem.

Hence if no objects are being dirtied, then there should never be
any need to block writeback threads due to the filesytem being
frozen because, by definition, there should be no work for them to
do. Hence if objects are being dirtied while the filesystem is
frozen, then that is a bug.

> > > I think the best approach to fix this problem is to let users not to write
> > > memory which is mapped to a certain file while the filesystem is freezing. 
> > > However, it is very difficult to control users not to write memory which has 
> > > been already mapped to the file.
> > 
> > If you don't allow the page to be dirtied in the fist place, then
> > nothing needs to be done to the writeback path because there is
> > nothing dirty for it to write back.
>   Sure but that's only the problem he was able to hit. But generally,
> there's a problem with needing s_umount for unfreezing because it isn't
> clear there aren't other code paths which can block with s_umount held
> waiting for fs to get unfrozen. And these code paths would cause the same
> deadlock. That's why I chose to get rid of s_umount during thawing.

Holding the s_umount lock while checking if frozen and sleeping
is essentially an ABBA lock inversion bug that can bite in many more
places that just thawing the filesystem. . Any where this is done
should be fixed, so I don't think just removing the s_umount lock
from the thaw path is sufficient to avoid problems.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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