[PATCH 2/4] fscache: Fix race in fscache_op_complete() due to split atomic_sub & read

[David Howells <dhowells@redhat.com>]

From: kiran.modukuri <kiran.modukuri@gmail.com>

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() instead of two calls.

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>
---

 include/linux/fscache-cache.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/fscache-cache.h b/include/linux/fscache-cache.h
index 34cf0fdd7dc7..bf98ed803af2 100644
--- a/include/linux/fscache-cache.h
+++ b/include/linux/fscache-cache.h
@@ -196,11 +196,11 @@ 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(n_pages, &op->n_pages) <= 0)
 		fscache_op_complete(&op->op, false);
 }
 
+
 /**
  * fscache_put_retrieval - Drop a reference to a retrieval operation
  * @op: The retrieval operation affected