4.19-stable review patch. If anyone has any objections, please let me know.
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[ Upstream commit 3f2b7b9035107d6096ea438ea3d97dcf0481b6d2 ]
The code in fscache_retrieval_complete is using atomic_sub followed by an atomic_read:
atomic_sub(n_pages, &op->n_pages); if (atomic_read(&op->n_pages) <= 0) fscache_op_complete(&op->op, true);
This causes two threads doing a decrement of n_pages to race with each other seeing the op->refcount 0 at same time - and they end up calling fscache_op_complete() in both the threads leading to an assertion failure.
Fix this by using atomic_sub_return_relaxed() instead of two calls. Note that I'm using 'relaxed' rather than, say, 'release' as there aren't multiple variables that appear to need ordering across the release.
The oops looks something like:
FS-Cache: Assertion failed FS-Cache: 0 > 0 is false ... kernel BUG at /usr/src/linux-4.4.0/fs/fscache/operation.c:449! ... Workqueue: fscache_operation fscache_op_work_func [fscache] ... RIP: 0010:[<ffffffffc037eacd>] fscache_op_complete+0x10d/0x180 [fscache] ... Call Trace: [<ffffffffc1464cf9>] cachefiles_read_copier+0x3a9/0x410 [cachefiles] [<ffffffffc037e272>] fscache_op_work_func+0x22/0x50 [fscache] [<ffffffff81096da0>] process_one_work+0x150/0x3f0 [<ffffffff8109751a>] worker_thread+0x11a/0x470 [<ffffffff81808e59>] ? __schedule+0x359/0x980 [<ffffffff81097400>] ? rescuer_thread+0x310/0x310 [<ffffffff8109cdd6>] kthread+0xd6/0xf0 [<ffffffff8109cd00>] ? kthread_park+0x60/0x60 [<ffffffff8180d0cf>] ret_from_fork+0x3f/0x70 [<ffffffff8109cd00>] ? kthread_park+0x60/0x60
This seen this in 4.4.x kernels and the same bug affects fscache in latest upstreams kernels.
Fixes: 1bb4b7f98f36 ("FS-Cache: The retrieval remaining-pages counter needs to be atomic_t") Signed-off-by: Kiran Kumar Modukuri kiran.modukuri@gmail.com Signed-off-by: David Howells dhowells@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/fscache-cache.h | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h index 34cf0fdd7dc7..610815e3f1aa 100644 --- a/include/linux/fscache-cache.h +++ b/include/linux/fscache-cache.h @@ -196,8 +196,7 @@ static inline void fscache_enqueue_retrieval(struct fscache_retrieval *op) static inline void fscache_retrieval_complete(struct fscache_retrieval *op, int n_pages) { - atomic_sub(n_pages, &op->n_pages); - if (atomic_read(&op->n_pages) <= 0) + if (atomic_sub_return_relaxed(n_pages, &op->n_pages) <= 0) fscache_op_complete(&op->op, false); }