On Wed, Jan 15, 2014 at 06:34:56PM +0100, Miklos Szeredi wrote: > On Wed, 2014-01-15 at 10:17 -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. > > It can d_move, because the dentry is known to be disconnected, i.e. it > doesn't have a parent for which we could obtain the lock. DCACHE_DISCONNECTED doesn't mean that. When you lookup a dentry by filehandle that dentry is initially marked DCACHE_DISCONNECTED. It is cleared only after reconnect_path() has verified that the dentry is reachable all the way from the root. So !DCACHE_DISCONNECTED implies that the dentry is connected all the way up to the root, but the converse is not true. This has been a source of confusion, but it is explained in Documentation/filesystems/nfs/Exporting. Recently I've cleaned up a few odd uses of DCACHE_DISCONNECTED and rewritten reconnect_path(), partly as an attempt to clarify the situation. Let me know if any of that doesn't look right to you.... > > 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.) > > > > Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx> > > --- > > fs/dcache.c | 25 +------------------------ > > 1 file changed, 1 insertion(+), 24 deletions(-) > > > > Only lightly tested.... If this is right, then we can also just ditch > > d_splice_alias completely, and clean up the various d_find_alias's. > > > > I think the only reason we have both d_splice_alias and > > d_materialise_unique is that the former was written for exportable > > filesystems and the latter for distributed filesystems. > > > > But we have at least one exportable filesystem (fuse) using > > d_materialise_unique. And I doubt d_splice_alias was ever completely > > correct even for on-disk filesystems. > > > > Am I missing some subtlety? > > One subtle difference is that for a non-directory d_splice_alias() will > reconnect a DCACHE_DISCONNECTED dentry if one exists, while > d_materialise_unique() will not. Actually until f80de2cde10350b8d146e375ff8b634e72e6a827 "dcache: don't clear DCACHE_DISCONNECTED too early", it was the reverse: d_materialise_unique cleared DISCONNECTED and d_splice_alias (correctly) did not. The only place where it should be cleared is reconnect_path(). > Does this matter in practice? The small number of extra dentries > probably does not matter. Directories are assumed to have unique aliases. When they don't, the kernel can deadlock or crash. --b. -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
