On Thu, May 08, 2014 at 09:14:31PM -0500, Eric Sandeen wrote: > On 5/8/14, 8:17 PM, Dave Chinner wrote: > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > The way discontiguous buffers are currently handled in prefetch is > > by unlocking the prefetch tree and reading them one at a time in > > pf_read_discontig(), inside the normal loop of searching for buffers > > to read in a more optimized fashion. > > > > But by unlocking the tree, we allow other threads to come in and > > find buffers which we've already stashed locally on our bplist[]. > > If 2 threads think they own the same set of buffers, they may both > > try to delete them from the prefetch btree, and the second one to > > arrive will not find it, resulting in: > > > > fatal error -- prefetch corruption > > > > To fix this, simply abort the buffer gathering loop when we come > > across a discontiguous buffer, process the gathered list as per > > normal, and then after running the large optimised read, check to > > see if the last buffer on the list is a discontiguous buffer. > > If is is discontiguous, then issue the discontiguous buffer read > > while the locks are not held. We only ever have one discontiguous > > buffer per read loop, so it is safe just to check the last buffer in > > the list. > > > > The fix is loosely based on a a patch provided by Eric Sandeen, who > > did all the hard work of finding the bug and demonstrating how to > > fix it. > > Ok, this makes sense to me. The comment above the discontig read > seems a bit confusing; you say it's safe to read while unlocked, > but I wouldn't have expected it not to be - the lock is just for > btree manipulation, and that's not being done. So I think the > comment adds a little confusion rather than clarification. Ok, I'll just drop the bit about it being safe to read - the bit about being the last buffer on the list is the important bit... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
