ACK: [C][PATCH 0/1] LP: #1797314 - fix another fscache crash

[Khaled Elmously]

On 2018-10-12 12:10:34 , Daniel Axtens wrote:
> SRU Justification
> =================
> 
> [Impact]
> A kernel BUG is sometimes observed when using fscache:
>     [4740718.880898] FS-Cache:
>     [4740718.880920] FS-Cache: Assertion failed
>     [4740718.880934] FS-Cache: 0 > 0 is false
>     [4740718.881001] ------------[ cut here ]------------
>     [4740718.881017] kernel BUG at /usr/src/linux-4.4.0/fs/fscache/operation.c:449!
>     [4740718.881040] invalid opcode: 0000 [#1] SMP
> 
>     [4740718.892659] Call Trace:
>     [4740718.893506] [<ffffffffc1464cf9>] cachefiles_read_copier+0x3a9/0x410 [cachefiles]
>     [4740718.894374] [<ffffffffc037e272>] fscache_op_work_func+0x22/0x50 [fscache]
>     [4740718.895180] [<ffffffff81096da0>] process_one_work+0x150/0x3f0
>     [4740718.895966] [<ffffffff8109751a>] worker_thread+0x11a/0x470
>     [4740718.896753] [<ffffffff81808e59>] ? __schedule+0x359/0x980
>     [4740718.897783] [<ffffffff81097400>] ? rescuer_thread+0x310/0x310
>     [4740718.898581] [<ffffffff8109cdd6>] kthread+0xd6/0xf0
>     [4740718.899469] [<ffffffff8109cd00>] ? kthread_park+0x60/0x60
>     [4740718.900477] [<ffffffff8180d0cf>] ret_from_fork+0x3f/0x70
>     [4740718.901514] [<ffffffff8109cd00>] ? kthread_park+0x60/0x60
> 
> [Problem]
> In include/linux/fscache-cache.h, fscache_retrieval_complete reads, in
> part:
> 
>             atomic_sub(n_pages, &op->n_pages);
>             if (atomic_read(&op->n_pages) <= 0)
>                     fscache_op_complete(&op->op, true);
> 
> The code is using atomic_sub followed by an atomic_read. This causes
> two threads doing a decrement of pages to race with each other seeing
> the op->refcount <= 0 at same time, and end up calling
> fscache_op_complete in both the threads leading to the OOPS.
> 
> [Fix]
> The fix is trivial to use atomic_sub_return instead of two calls.
> 
> [Testcase]
> I believe the user has tested the patch successfully on their fscache/cachefiles setup.
> 
> [Regression Potential]
> Limited to fscache. Small, comprehensible change.
> 
> Kiran Kumar Modukuri (1):
>   UBUNTU: SAUCE: fscache: Fix race in decrementing refcount of
>     op->npages
> 
>  include/linux/fscache-cache.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 

Acked-by: Khalid Elmously <khalid.elmously at canonical.com>