Re: [RFC PATCH RESEND] fs: fix a hungtask problem when freeze/unfreeze fs

On Mon 28-12-20 10:15:16, Shijie Luo wrote:

On 2020/12/26 23:55, Al Viro wrote:

quoted

On Sat, Dec 26, 2020 at 04:56:41AM -0500, Shijie Luo wrote:

quoted

The root cause is that when offline/onlines disks, the filesystem can easily get into
a error state and this makes it change to be read-only. Function freeze_super() will hold
all sb_writers rwsems including rwsem of SB_FREEZE_WRITE when filesystem not read-only,
but thaw_super_locked() cannot release these while the filesystem suddenly become read-only,
because the logic will go to out.

freeze_super
     hold sb_writers rwsems
         sb->s_writers.frozen = SB_FREEZE_COMPLETE
                                                  thaw_super_locked
                                                      sb_rdonly
                                                         sb->s_writers.frozen = SB_UNFROZEN;
                                                             goto out // not release rwsems

And at this time, if we call mnt_want_write(), the process will be blocked.

This patch fixes this problem, when filesystem is read-only, just not to set sb_writers.frozen
be SB_FREEZE_COMPLETE in freeze_super() and then release all rwsems in thaw_super_locked.

I really don't like that - you end up with a case when freeze_super() returns 0 *and*
consumes the reference it had been give.

Consuming the reference here because we won't "set frozen =
SB_FREEZE_COMPLETE" in thaw_super_locked() now.

If don't do that, we never have a chance to consume it, thaw_super_locked()
will directly return "-EINVAL". But I do

agree that it's not a good idea to return 0. How about returning "-EINVAL or
-ENOTSUPP" ?

And, If we keep "frozen = SB_FREEZE_COMPLETE" in freeze_super() here, it
will cause another problem, thaw_super_locked()

will always release rwsems in my logic, but freeze_super() won't acquire the
rwsems when filesystem is read-only.

I was thinking about this for a while. I think the best solution would be
to track whether the fs was read only (and thus frozen without locking
percpu semaphores) at the time of freezing. I'm attaching that patch. Does
it fix your problem?

									Honza

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