Re: [PATCH 10/14] xfs: convert grant head manipulations to lockless algorithm

Dave Chinner
2010-12-02T02:09:50Z

On Wed, Dec 01, 2010 at 08:15:13AM -0500, Christoph Hellwig wrote:
> On Mon, Nov 29, 2010 at 12:38:28PM +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > The only thing that the grant lock remains to protect is the grant head
> > manipulations when adding or removing space from the log. These calculations
> > are already based on atomic variables, so we can already update them safely
> > without locks. However, the grant head manpulations require atomic multi-step
> > calculations to be executed, which the algorithms currently don't allow.
> >
> > To make these multi-step calculations atomic, convert the algorithms to
> > compare-and-exchange loops on the atomic variables. That is, we sample the old
> > value, perform the calculation and use atomic64_cmpxchg() to attempt to update
> > the head with the new value. If the head has not changed since we sampled it,
> > it will succeed and we are done. Otherwise, we rerun the calculation again from
> > a new sample of the head.
> >
> > This allows us to remove the grant lock from around all the grant head space
> > manipulations, and that effectively removes the grant lock from the log
> > completely.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
....
> > @@ -3478,11 +3485,18 @@ xlog_verify_dest_ptr(
> > xlog_panic("xlog_verify_dest_ptr: invalid ptr");
> > }
> >
> > +/*
> > + * XXX: because we cannot atomically sample both the reserve and write heads,
> > + * we cannot verify them reliably as they may be sampled in the middle of
> > + * racing modifications. Hence just taking snapshots of the heads can give an
> > + * incorrect view of the state of log. Hence just disable this check for now.
> > + */
> > STATIC void
> > xlog_verify_grant_head(
>
> I can't see any way to keep this check with the atomic reserve/write
> heads, so we might as well remove it entirely.

Ok, will do.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com