On Thu, Dec 01, 2011 at 01:51:28PM -0600, Ben Myers wrote: > Process A reads from the grant reserve head at 2641 (and there currently is enough space) > Process B wakes at either 2646 or 2650, in xlog_reserveq_wait, locks, and reads from the grant reserve head (and currently there is enough space) > Process B removes itself from the list > Process A reads from the reservq list and finds it to be empty > Process A finds that there was enough space at 2646 > Process B returns from xlog_reserveq_wait, unlocks, grants space at 2656, returns > Process A grants log space at 2656, and returns > > AFAICS there is nothing that prevents these guys from granting the same > space when you approach free_bytes >= need_bytes concurrently. > > This lockless stuff is always a mind job for me. I'll take another look at > some of the other aspects of the patch. Even if it doesn't resolve my > question about the lockless issue, it seems to resolve Chandra's race. Indeed, I think we have this race. Then again I I think we had exactly the same one before, too. The only way to fix it would be to do a sort of double cmpxchg that only moves the grant head forward if it's still in available space vs the tails lsn. _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
