Hi.

On Tue, 15 Feb 2011 18:29:54 +0100
Jan Kara [off-list ref] wrote:

On Tue 15-02-11 12:03:52, Ted Ts'o wrote:

quoted

On Tue, Feb 15, 2011 at 05:06:30PM +0100, Jan Kara wrote:

quoted

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