[Devel] [PATCH RHEL7 COMMIT] ms/lock_parent: don't step on stale ->d_parent of all-but-freed one

[Vasily Averin]

The commit is pushed to "branch-rh7-3.10.0-1160.11.1.vz7.172.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-1160.11.1.vz7.172.13
------>
commit d8fcddcb834f35d3655e41e7ad0c25f93d8b9485
Author: Al Viro <viro at zeniv.linux.org.uk>
Date:   Wed Feb 17 15:36:08 2021 +0300

    ms/lock_parent: don't step on stale ->d_parent of all-but-freed one
    
    Dentry that had been through (or into) __dentry_kill() might be seen
    by shrink_dentry_list(); that's normal, it'll be taken off the shrink
    list and freed if __dentry_kill() has already finished.  The problem
    is, its ->d_parent might be pointing to already freed dentry, so
    lock_parent() needs to be careful.
    
    We need to check that dentry hasn't already gone into __dentry_kill()
    *and* grab rcu_read_lock() before dropping ->d_lock - the latter makes
    sure that whatever we see in ->d_parent after dropping ->d_lock it
    won't be freed until we drop rcu_read_lock().
    
    Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
    (cherry picked from commit c2338f2dc7c1e9f6202f370c64ffd7f44f3d4b51)
    Signed-off-by: Vasily Averin <vvs at virtuozzo.com>
---
 fs/dcache.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 41e8f55..3d6b9b4 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -701,10 +701,12 @@ static inline struct dentry *lock_parent(struct dentry *dentry)
 	struct dentry *parent = dentry->d_parent;
 	if (IS_ROOT(dentry))
 		return NULL;
+	if (unlikely((int)dentry->d_lockref.count < 0))
+		return NULL;
 	if (likely(spin_trylock(&parent->d_lock)))
 		return parent;
-	spin_unlock(&dentry->d_lock);
 	rcu_read_lock();
+	spin_unlock(&dentry->d_lock);
 again:
 	parent = ACCESS_ONCE(dentry->d_parent);
 	spin_lock(&parent->d_lock);