On Wed 15-08-12 20:51:17, Liu Bo wrote:

(CCed Jan, the author of freeze code)
On 08/14/2012 10:12 PM, Marco Stornelli wrote:

quoted

Il 14/08/2012 15:53, Liu Bo ha scritto:

quoted

On 08/14/2012 08:59 PM, Marco Stornelli wrote:

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Il 14/08/2012 07:01, liub.liubo@gmail.com ha scritto:

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From: Liu Bo <redacted>

I found this while testing xfstests 068, the story is

      t1                                            t2
    sys_sync                                    thaw_super
      iterate_supers
        down_read(sb->s_umount)                   down_write(sb->s_umount) --->wait for t1
        sync_fs (with wait mode)
          start_transaction
            sb_start_intwrite --------------------> wait for t2 to set s_writers.frozen to SB_UNFROZEN

In this patch, I add an helper sb_start_intwrite_trylock() and use it before we
start_transaction in sync_fs() with wait mode so that we won't hit the deadlock.

IMHO, we should avoid to call the sync operation on a frozen fs. The freeze operation, indeed, already include a sync operation.
According to man page, no other operation should modify the fs after the freeze.
So for me the modification is inside sync_filesystem (and sync_one_sb).

Do you mean that we should add the trylock check in sync_filesystem?

But it seems to be useless because we already run into down_read(sb->s_umount) before starting sync_one_sb().

thanks,
liubo

I meant that we should check if there are in a "complete" freeze state (according to the "states" of a freeze transaction) and simply skip the sync operation.

I'm ok with it.

What do you think about it, Jan?  Any comments?

  Hum, so what I don't exactly understand is why does btrfs start a
transaction in ->sync_fs(). The idea of freeze code is that when filesystem
is frozen, there is no data / metadata to write and thus we avoid deadlocks
arising from trying to write anything with s_umount held.

So checking whether filesystem is frozen and avoiding transaction start in
that case in btrfs_sync_fs() will work but looks hacky. Rather the check
should be for the thing which is the reason for transaction start-end pair
in btrfs_sync_fs(). My naive guess would be we should check whether there
is any transaction running...

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR