Re: [PATCH 2/3] fs: wait for partially frozen filesystems

["Darrick J. Wong" <djwong@kernel.org>]

On Sun, Jun 11, 2023 at 09:01:48PM -0700, Christoph Hellwig wrote:
> On Sun, Jun 11, 2023 at 08:15:28PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <djwong@kernel.org>
> > 
> > Jan Kara suggested that when one thread is in the middle of freezing a
> > filesystem, another thread trying to freeze the same fs but with a
> > different freeze_holder should wait until the freezer reaches either end
> > state (UNFROZEN or COMPLETE) instead of returning EBUSY immediately.
> > 
> > Plumb in the extra coded needed to wait for the fs freezer to reach an
> > end state and try the freeze again.
> > 
> > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > ---
> >  fs/super.c |   27 +++++++++++++++++++++++++--
> >  1 file changed, 25 insertions(+), 2 deletions(-)
> > 
> > 
> > diff --git a/fs/super.c b/fs/super.c
> > index 36adccecc828..151e0eeff2c2 100644
> > --- a/fs/super.c
> > +++ b/fs/super.c
> > @@ -1647,6 +1647,15 @@ static int freeze_frozen_super(struct super_block *sb, enum freeze_holder who)
> >  	return 0;
> >  }
> >  
> > +static void wait_for_partially_frozen(struct super_block *sb)
> > +{
> > +	up_write(&sb->s_umount);
> > +	wait_var_event(&sb->s_writers.frozen,
> > +			sb->s_writers.frozen == SB_UNFROZEN ||
> > +			sb->s_writers.frozen == SB_FREEZE_COMPLETE);
> > +	down_write(&sb->s_umount);
> 
> Does sb->s_writers.frozen need WRITE_ONCE/READ_ONCE treatment if we want
> to check it outside of s_umount?  Or should we maybe just open code
> wait_var_event and only drop the lock after checking the variable?

How about something like:

	do {
		up_write(&sb->s_umount);
		down_write(&sb->s_umount);
	} while (sb->s_writers.frozen != SB_UNFROZEN &&
		 sb->s_writers.frozen != SB_FREEZE_COMPLETE);

so that we always return in either end state of a freezer transition?

> >  	if (sb->s_writers.frozen != SB_UNFROZEN) {
> > -		deactivate_locked_super(sb);
> > -		return -EBUSY;
> > +		if (!try_again) {
> > +			deactivate_locked_super(sb);
> > +			return -EBUSY;
> > +		}
> > +
> > +		wait_for_partially_frozen(sb);
> > +		try_again = false;
> > +		goto retry;
> 
> Can you throw in a comment on wait we're only waiting for a partial
> freeze one here?

I didn't want a thread to get stuck in the retry forever if it always
loses the race.  However, I think any other threads running freeze_super
will always end at UNFROZEN or COMPLETE; and thaw_super always goes
straight froM COMPLETE to UNFROZEN, so I think I'll get rid of the retry
flag logic entirely.

--D