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

[Stefan Bader]

On 11.06.2018 23:04, Kiran Kumar Modukuri wrote:
> 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!
> 

Any reason this is not sent upstream? Looking at today's tip it seems to me the
exact same problem still exists in current kernel versions. We rather prefer bug
fixes through upstream/stable because they then will be reviewed by upstream
subsystem maintainer(s). But more importantly this will then more likely come
back to *all* currently supported series (like this one to at least 18.04/bionic
and also into the current development series (cosmic).

-Stefan

> [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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