Hi.
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);
}
--
Best Regards,
Toshiyuki Okajima
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