3.16.61-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Kiran Kumar Modukuri kiran.modukuri@gmail.com
commit 934140ab028713a61de8bca58c05332416d037d1 upstream.
cachefiles_read_waiter() has the right to access a 'monitor' object by virtue of being called under the waitqueue lock for one of the pages in its purview. However, it has no ref on that monitor object or on the associated operation.
What it is allowed to do is to move the monitor object to the operation's to_do list, but once it drops the work_lock, it's actually no longer permitted to access that object. However, it is trying to enqueue the retrieval operation for processing - but it can only do this via a pointer in the monitor object, something it shouldn't be doing.
If it doesn't enqueue the operation, the operation may not get processed. If the order is flipped so that the enqueue is first, then it's possible for the work processor to look at the to_do list before the monitor is enqueued upon it.
Fix this by getting a ref on the operation so that we can trust that it will still be there once we've added the monitor to the to_do list and dropped the work_lock. The op can then be enqueued after the lock is dropped.
The bug can manifest in one of a couple of ways. The first manifestation looks like:
FS-Cache: FS-Cache: Assertion failed FS-Cache: 6 == 5 is false ------------[ cut here ]------------ kernel BUG at fs/fscache/operation.c:494! RIP: 0010:fscache_put_operation+0x1e3/0x1f0 ... fscache_op_work_func+0x26/0x50 process_one_work+0x131/0x290 worker_thread+0x45/0x360 kthread+0xf8/0x130 ? create_worker+0x190/0x190 ? kthread_cancel_work_sync+0x10/0x10 ret_from_fork+0x1f/0x30
This is due to the operation being in the DEAD state (6) rather than INITIALISED, COMPLETE or CANCELLED (5) because it's already passed through fscache_put_operation().
The bug can also manifest like the following:
kernel BUG at fs/fscache/operation.c:69! ... [exception RIP: fscache_enqueue_operation+246] ... #7 [ffff883fff083c10] fscache_enqueue_operation at ffffffffa0b793c6 #8 [ffff883fff083c28] cachefiles_read_waiter at ffffffffa0b15a48 #9 [ffff883fff083c48] __wake_up_common at ffffffff810af028
I'm not entirely certain as to which is line 69 in Lei's kernel, so I'm not entirely clear which assertion failed.
Fixes: 9ae326a69004 ("CacheFiles: A cache that backs onto a mounted filesystem") Reported-by: Lei Xue carmark.dlut@gmail.com Reported-by: Vegard Nossum vegard.nossum@gmail.com Reported-by: Anthony DeRobertis aderobertis@metrics.net Reported-by: NeilBrown neilb@suse.com Reported-by: Daniel Axtens dja@axtens.net Reported-by: Kiran Kumar Modukuri kiran.modukuri@gmail.com Signed-off-by: David Howells dhowells@redhat.com Reviewed-by: Daniel Axtens dja@axtens.net Signed-off-by: Ben Hutchings ben@decadent.org.uk --- fs/cachefiles/rdwr.c | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-)
--- a/fs/cachefiles/rdwr.c +++ b/fs/cachefiles/rdwr.c @@ -27,6 +27,7 @@ static int cachefiles_read_waiter(wait_q struct cachefiles_one_read *monitor = container_of(wait, struct cachefiles_one_read, monitor); struct cachefiles_object *object; + struct fscache_retrieval *op = monitor->op; struct wait_bit_key *key = _key; struct page *page = wait->private;
@@ -51,16 +52,22 @@ static int cachefiles_read_waiter(wait_q list_del(&wait->task_list);
/* move onto the action list and queue for FS-Cache thread pool */ - ASSERT(monitor->op); + ASSERT(op);
- object = container_of(monitor->op->op.object, - struct cachefiles_object, fscache); + /* We need to temporarily bump the usage count as we don't own a ref + * here otherwise cachefiles_read_copier() may free the op between the + * monitor being enqueued on the op->to_do list and the op getting + * enqueued on the work queue. + */ + fscache_get_retrieval(op);
+ object = container_of(op->op.object, struct cachefiles_object, fscache); spin_lock(&object->work_lock); - list_add_tail(&monitor->op_link, &monitor->op->to_do); + list_add_tail(&monitor->op_link, &op->to_do); spin_unlock(&object->work_lock);
- fscache_enqueue_retrieval(monitor->op); + fscache_enqueue_retrieval(op); + fscache_put_retrieval(op); return 0; }