** 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 [Problem] - In include/fscache-cache.h, fscache_retrieval_complete reads, in part: + 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] - The user has tested the patch successfully on their fscache/cachefiles setup. + I believe the user has tested the patch successfully on their fscache/cachefiles setup. [Regression Potential] Limited to fscache. Small, comprehensible change.
-- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1797314 Title: fscache: bad refcounting in fscache_op_complete leads to OOPS To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1797314/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
