[SRU B][PATCH 5/6] cachefiles: Fix missing clear of the CACHEFILES_OBJECT_ACTIVE flag

[Daniel Axtens]

From: Kiran Kumar Modukuri <kiran.modukuri at gmail.com>

BugLink: https://bugs.launchpad.net/bugs/1776254

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 at gmail.com>
Signed-off-by: David Howells <dhowells at redhat.com>
(backported from commit 5ce83d4bb7d8e11e8c1c687d09f4b5ae67ef3ce3)
Signed-off-by: Daniel Axtens <daniel.axtens at canonical.com>
---
 fs/cachefiles/namei.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/cachefiles/namei.c b/fs/cachefiles/namei.c
index 3978b324cbca..6c9758ec8cef 100644
--- a/fs/cachefiles/namei.c
+++ b/fs/cachefiles/namei.c
@@ -191,6 +191,8 @@ static int cachefiles_mark_object_active(struct cachefiles_cache *cache,
 	/* 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(&xobject->fscache)) {
 		pr_err("\n");
 		pr_err("Error: Unexpected object collision\n");
@@ -252,7 +254,6 @@ static int cachefiles_mark_object_active(struct cachefiles_cache *cache,
 	goto try_again;
 
 requeue:
-	clear_bit(CACHEFILES_OBJECT_ACTIVE, &object->flags);
 	cache->cache.ops->put_object(&xobject->fscache);
 	_leave(" = -ETIMEDOUT");
 	return -ETIMEDOUT;
-- 
2.17.1