On Wed 16-02-11 08:17:46, Toshiyuki Okajima wrote:
> On Tue, 15 Feb 2011 18:29:54 +0100
> Jan Kara <jack@xxxxxxx> 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 <jack@xxxxxxx>
SUSE Labs, CR
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