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@xxxxxxxxxx> > > > > 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@xxxxxxxxxx> .... > > @@ -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@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs