From: J. Bruce Fields Date: Thu, 28 Jun 2012 16:10:55 +0000 (-0400) Subject: dcache: use IS_ROOT to decide where dentry is hashed X-Git-Tag: upstream/snapshot3+hdmi~3996^2~24 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=7632e465feb182cadc3c9aa1282a057201818a8c;p=platform%2Fadaptation%2Frenesas_rcar%2Frenesas_kernel.git dcache: use IS_ROOT to decide where dentry is hashed Every hashed dentry is either hashed in the dentry_hashtable, or a superblock's s_anon list. __d_drop() assumes it can determine which is the case by checking DCACHE_DISCONNECTED; this is not true. It is true that when DCACHE_DISCONNECTED is cleared, the dentry is not only hashed on dentry_hashtable, but is fully connected to its parents back to the root. But the converse is *not* true: fs/exportfs/expfs.c:reconnect_path() attempts to connect a directory (found by filehandle lookup) back to root by ascending to parents and performing lookups one at a time. It does not clear DCACHE_DISCONNECTED until it's done, and that is not at all an atomic process. In particular, it is possible for DCACHE_DISCONNECTED to be set on a dentry which is hashed on the dentry_hashtable. Instead, use IS_ROOT() to check which hash chain a dentry is on. This *does* work: Dentries are hashed only by: - d_obtain_alias, which adds an IS_ROOT() dentry to sb_anon. - __d_rehash, called by _d_rehash: hashes to the dentry's parent, and all callers of _d_rehash appear to have d_parent set to a "real" parent. - __d_rehash, called by __d_move: rehashes the moved dentry to hash chain determined by target, and assigns target's d_parent to its d_parent, before dropping the dentry's d_lock. Therefore I believe it's safe for a holder of a dentry's d_lock to assume that it is hashed on sb_anon if and only if IS_ROOT(dentry) is true. I believe the incorrect assumption about DCACHE_DISCONNECTED was originally introduced by ceb5bdc2d246 "fs: dcache per-bucket dcache hash locking". Also add a comment while we're here. Cc: Nick Piggin Acked-by: Christoph Hellwig Reviewed-by: NeilBrown Signed-off-by: J. Bruce Fields Signed-off-by: Al Viro --- diff --git a/fs/dcache.c b/fs/dcache.c index 525770e..c2add37 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -503,7 +503,12 @@ void __d_drop(struct dentry *dentry) { if (!d_unhashed(dentry)) { struct hlist_bl_head *b; - if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED)) + /* + * Hashed dentries are normally on the dentry hashtable, + * with the exception of those newly allocated by + * d_obtain_alias, which are always IS_ROOT: + */ + if (unlikely(IS_ROOT(dentry))) b = &dentry->d_sb->s_anon; else b = d_hash(dentry->d_parent, dentry->d_name.hash);