On Fri, Jan 17, 2014 at 04:17:23AM -0800, Christoph Hellwig wrote: > On Wed, Jan 15, 2014 at 10:17:49AM -0500, J. Bruce Fields wrote: > > From: "J. Bruce Fields" <bfields@xxxxxxxxxx> > > > > d_splice_alias can create duplicate directory aliases (in the !new > > case), or (in the new case) d_move without holding appropriate locks. > > > > d_materialise_unique deals with both of these problems. (The latter > > seems to be dealt by trylocks (see __d_unalias), which look like they > > could cause spurious lookup failures--but that's at least better than > > corrupting the dcache.) > > I'm a bit worried about those spurious failures, maybe we should > retry in that case? Maybe so. I'm not sure how. d_materialise_unique is called from lookup and we'd need to at least drop the parent i_mutex to give a concurrent rename a chance to progress. I think NFS or cluster filesystem clients could hit this case with: host A host B --------- ------------------------- process 1 process 1 process 2 --------- --------- --------- mkdir foo/X mv foo/X bar/ stat bar/X mv baz qux When (B,1) looks up X in bar it finds that X still has an alias in foo, tries to rename that alias to bar/X, but can't because the current baz->qux rename is holding the rename mutex. So __d_unalias and the lookup return -EBUSY. None of those operations are particularly fast, so I'm a bit surprised we haven't already heard complaints. I must be missing some reason this doesn't happen. I guess I should set up a test. > Also looking over the changes I wonder if the explicit cecking for > aliases for every non-directory might have a major performance impact, > all the dcache growling already was a major issues in NFS workloads > years ago and I dumb it's become any better. This only happens on the first (uncached) lookup. So we've already acquired a bunch of locks and probably done a round trip to a disk or a server--is walking a (typically short) list really something to worry about? > Also looking at this area I'd like to suggest that if you end up > merging the two I'd continue using the d_splice_alias name and > calling conventions. OK, I guess I don't care which one we keep. > Also the inode == NULL case really should be split out from > d_materialise_unique into a separate helper. It shares almost no > code, is entirely undocumented to the point that I don't really > understand what the purpose is, and the only caller that can get > there (fuse) already branches around that case in the caller anyway. I think I see what you mean, I can fix that. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
