[PATCH 3.16 291/366] cachefiles: Fix missing clear of the CACHEFILES_OBJECT_ACTIVE flag

[Ben Hutchings]

3.16.61-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Kiran Kumar Modukuri <kiran.modukuri@xxxxxxxxx>

commit 5ce83d4bb7d8e11e8c1c687d09f4b5ae67ef3ce3 upstream.

In cachefiles_mark_object_active(), the new object is marked active and
then we try to add it to the active object tree.  If a conflicting object
is already present, we want to wait for that to go away.  After the wait,
we go round again and try to re-mark the object as being active - but it's
already marked active from the first time we went through and a BUG is
issued.

Fix this by clearing the CACHEFILES_OBJECT_ACTIVE flag before we try again.

Analysis from Kiran Kumar Modukuri:

[Impact]
Oops during heavy NFS + FSCache + Cachefiles

CacheFiles: Error: Overlong wait for old active object to go away.

BUG: unable to handle kernel NULL pointer dereference at 0000000000000002

CacheFiles: Error: Object already active kernel BUG at
fs/cachefiles/namei.c:163!

[Cause]
In a heavily loaded system with big files being read and truncated, an
fscache object for a cookie is being dropped and a new object being
looked. The new object being looked for has to wait for the old object
to go away before the new object is moved to active state.

[Fix]
Clear the flag 'CACHEFILES_OBJECT_ACTIVE' for the new object when
retrying the object lookup.

[Testcase]
Have run ~100 hours of NFS stress tests and have not seen this bug recur.

[Regression Potential]
 - Limited to fscache/cachefiles.

Fixes: 9ae326a69004 ("CacheFiles: A cache that backs onto a mounted filesystem")
Signed-off-by: Kiran Kumar Modukuri <kiran.modukuri@xxxxxxxxx>
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 fs/cachefiles/namei.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -189,6 +189,7 @@ try_again:
 	/* an old object from a previous incarnation is hogging the slot - we
 	 * need to wait for it to be destroyed */
 wait_for_old_object:
+	clear_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags);
 	if (fscache_object_is_live(&object->fscache)) {
 		pr_err("\n");
 		pr_err("Error: Unexpected object collision\n");
@@ -250,7 +251,6 @@ wait_for_old_object:
 	goto try_again;
 
 requeue:
-	clear_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags);
 	cache->cache.ops->put_object(&xobject->fscache);
 	_leave(" = -ETIMEDOUT");
 	return -ETIMEDOUT;