Messages in this thread Patch in this message | | | From | Jiri Slaby <> | Subject | [PATCH 3.12 001/142] dcache: use IS_ROOT to decide where dentry is hashed | Date | Wed, 24 Feb 2016 11:03:16 +0100 |
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From: "J. Bruce Fields" <bfields@redhat.com>
3.12-stable review patch. If anyone has any objections, please let me know.
===============
commit 7632e465feb182cadc3c9aa1282a057201818a8c upstream.
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 <npiggin@kernel.dk> Acked-by: Christoph Hellwig <hch@infradead.org> Reviewed-by: NeilBrown <neilb@suse.de> Signed-off-by: J. Bruce Fields <bfields@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Jiri Slaby <jslaby@suse.cz> --- fs/dcache.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/fs/dcache.c b/fs/dcache.c index 0fa3b3dba96f..40bf046884b1 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -441,7 +441,12 @@ 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); -- 2.7.1
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