[SAUCE][XENIAL][PATCH 1/1] [FSCACHE] Fix to handle Oops in fscache module during cookie cleanup

[Kiran Kumar Modukuri]

BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1776277

fscache cookie ref count updated incorrectly during fscache object allocation resulting in following Oops.

kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/internal.h:321!
kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/cookie.c:639!

[Cause]
1)Two threads are trying to do operate on a cookie and two objects.
2a)One thread tries to unmount the filesystem and in process goes over
   a huge list of objects marking them dead and deleting the objects.
   cookie->usage is also decremented in following path
      nfs_fscache_release_super_cookie
       -> __fscache_relinquish_cookie
        ->__fscache_cookie_put
        ->BUG_ON(atomic_read(&cookie->usage) <= 0);

2b)second thread tries to lookup an object for reading data in
   following path

   fscache_alloc_object
   1) cachefiles_alloc_object
       -> fscache_object_init
          -> assign cookie, but usage not bumped.
   2) fscache_attach_object -> fails in cant_attach_object because the
        cookie's backing object or cookie's->parent object are going away
   3)fscache_put_object
       -> cachefiles_put_object
         ->fscache_object_destroy
           ->fscache_cookie_put
              ->BUG_ON(atomic_read(&cookie->usage) <= 0);
[Fix]
 Bump up the cookie usage in fscache_object_init,
 when it is first being assigned a cookie atomically such that the cookie
 is added and bumped up if its refcount is not zero.
 remove the assignment in the attach_object.

[Testcase]
A user has run ~100 hours of NFS stress tests and not seen this bug recur.

[Regression Potential]
 - Limited to fscache/cachefiles.

Signed-off-by: kmodukuri <kmodukuri at nvidia.com>
---
 fs/fscache/cookie.c | 6 ++----
 fs/fscache/object.c | 6 +++++-
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/fscache/cookie.c b/fs/fscache/cookie.c
index 40d6107..8e2c9ad 100644
--- a/fs/fscache/cookie.c
+++ b/fs/fscache/cookie.c
@@ -302,6 +302,7 @@ static int fscache_alloc_object(struct fscache_cache *cache,
                goto error;
        }

+       ASSERTCMP(object->cookie, ==, cookie);
        fscache_stat(&fscache_n_object_alloc);

        object->debug_id = atomic_inc_return(&fscache_object_debug_id);
@@ -358,7 +359,7 @@ static int fscache_attach_object(struct fscache_cookie *cookie,
        _enter("{%s},{OBJ%x}", cookie->def->name, object->debug_id);

        spin_lock(&cookie->lock);
-
+       ASSERTCMP(object->cookie, ==, cookie);
        /* there may be multiple initial creations of this object, but we only
         * want one */
        ret = -EEXIST;
@@ -396,9 +397,6 @@ static int fscache_attach_object(struct fscache_cookie *cookie,
                spin_unlock(&cache->object_list_lock);
        }

-       /* attach to the cookie */
-       object->cookie = cookie;
-       atomic_inc(&cookie->usage);
        hlist_add_head(&object->cookie_link, &cookie->backing_objects);

        fscache_objlist_add(object);
diff --git a/fs/fscache/object.c b/fs/fscache/object.c
index 7a182c8..cfc437d 100644
--- a/fs/fscache/object.c
+++ b/fs/fscache/object.c
@@ -317,7 +317,11 @@ void fscache_object_init(struct fscache_object *object,
        object->store_limit = 0;
        object->store_limit_l = 0;
        object->cache = cache;
-       object->cookie = cookie;
+       if (cookie) {
+               if (atomic_inc_not_zero(&cookie->usage)) {
+                       object->cookie = cookie;
+               }
+       }
        object->parent = NULL;
 #ifdef CONFIG_FSCACHE_OBJECT_LIST
        RB_CLEAR_NODE(&object->objlist_link);
--
2.7.4
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