Public bug reported:
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/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]
The user has tested the patch successfully on their fscache/cachefiles setup.
[Regression Potential]
Limited to fscache. Small, comprehensible change.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: Incomplete
** Description changed:
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
- Jun 25 11:32:08 kernel: [4740718.880898] FS-Cache:
- Jun 25 11:32:08 kernel: [4740718.880920] FS-Cache: Assertion failed
- Jun 25 11:32:08 kernel: [4740718.880934] FS-Cache: 0 > 0 is false
- Jun 25 11:32:08 kernel: [4740718.881001] ------------[ cut here
]------------
- Jun 25 11:32:08 kernel: [4740718.881017] kernel BUG at
/usr/src/linux-4.4.0/fs/fscache/operation.c:449!
- Jun 25 11:32:08 kernel: [4740718.881040] invalid opcode: 0000 [#1] SMP
- ...
- Jun 25 11:32:08 kernel: [4740718.892659] Call Trace:
- Jun 25 11:32:08 kernel: [4740718.893506] [<ffffffffc1464cf9>]
cachefiles_read_copier+0x3a9/0x410 [cachefiles]
- Jun 25 11:32:08 kernel: [4740718.894374] [<ffffffffc037e272>]
fscache_op_work_func+0x22/0x50 [fscache]
- Jun 25 11:32:08 kernel: [4740718.895180] [<ffffffff81096da0>]
process_one_work+0x150/0x3f0
- Jun 25 11:32:08 kernel: [4740718.895966] [<ffffffff8109751a>]
worker_thread+0x11a/0x470
- Jun 25 11:32:08 kernel: [4740718.896753] [<ffffffff81808e59>] ?
__schedule+0x359/0x980
- Jun 25 11:32:08 kernel: [4740718.897783] [<ffffffff81097400>] ?
rescuer_thread+0x310/0x310
- Jun 25 11:32:08 kernel: [4740718.898581] [<ffffffff8109cdd6>]
kthread+0xd6/0xf0
- Jun 25 11:32:08 kernel: [4740718.899469] [<ffffffff8109cd00>] ?
kthread_park+0x60/0x60
- Jun 25 11:32:08 kernel: [4740718.900477] [<ffffffff8180d0cf>]
ret_from_fork+0x3f/0x70
- Jun 25 11:32:08 kernel: [4740718.901514] [<ffffffff8109cd00>] ?
kthread_park+0x60/0x60
-
[Problem]
In include/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.
-
+ 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]
The user has tested the patch successfully on their fscache/cachefiles setup.
[Regression Potential]
Limited to fscache. Small, comprehensible change.
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https://bugs.launchpad.net/bugs/1797314
Title:
fscache: bad refcounting in fscache_op_complete leads to OOPS
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