[Devel] [PATCH rh7 3/3] dcache: zap dentry_lru_del()

[Vladimir Davydov]

dentry_lru_del() is buggy - it tries to remove a dentry from a shrink
list, which is not protected by any locks and can only be safely
modified by a caller of dcache shrinker. Previous two patches replaced
this function calls with d_lru_del() and corresponding checks in two
places where it could result in corrupting shrink lists - dentry_kill()
and select_collect(). The only place left, shrink_dcache_for_umount(),
should be safe, because shrinker should be already unregistered there.

Still, to make sure that it doesn't race with a shrinker, let's replace
dentry_lru_del() with d_lru_del() there and make it issue a warning if
it encounters a dentry which is on a shrink list. After that
dentry_lru_del() is not used anywhere and can be removed. Note, it was
removed upstream either, but there are quite a few patches to be
backported.

Signed-off-by: Vladimir Davydov <vdavydov at virtuozzo.com>
---
 fs/dcache.c | 20 +++-----------------
 1 file changed, 3 insertions(+), 17 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index cba54cf702f5..518c519010e3 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -427,22 +427,6 @@ static void dentry_lru_add(struct dentry *dentry)
 		d_lru_add(dentry);
 }
 
-/*
- * Remove a dentry with references from the LRU.
- *
- * If we are on the shrink list, then we can get to try_prune_one_dentry() and
- * lose our last reference through the parent walk. In this case, we need to
- * remove ourselves from the shrink list, not the LRU.
- */
-static void dentry_lru_del(struct dentry *dentry)
-{
-	if (dentry->d_flags & DCACHE_LRU_LIST) {
-		if (dentry->d_flags & DCACHE_SHRINK_LIST)
-			return d_shrink_del(dentry);
-		d_lru_del(dentry);
-	}
-}
-
 /**
  * d_kill - kill dentry and return parent
  * @dentry: dentry to kill
@@ -1109,7 +1093,9 @@ static void shrink_dcache_for_umount_subtree(struct dentry *dentry)
 			    !d_unhashed(dentry))
 				dentry->d_op->d_prune(dentry);
 
-			dentry_lru_del(dentry);
+			WARN_ON_ONCE(dentry->d_flags & DCACHE_SHRINK_LIST);
+			if (dentry->d_flags & DCACHE_LRU_LIST)
+				d_lru_del(dentry);
 			__d_shrink(dentry);
 
 			if (dentry->d_lockref.count != 0) {
-- 
2.1.4