[PATCH] cachefiles: Fix assertion "6 == 5 is false" at fs/fscache/operation.c:494

[David Howells]

So something like the attached.  Possibly the changes to operation.c should be
split into a separate patch.

David
---
commit f3d49bdba2756bda41d18280b34f04c80f7f324c
Author: kiran modukuri <kiran.modukuri@xxxxxxxxx>
Date:   Tue Jul 18 16:25:49 2017 -0700

    cachefiles: Fix assertion "6 == 5 is false" at fs/fscache/operation.c:494
    
    There is a potential race in fscache operation enqueuing for reading and
    copying multiple pages from cachefiles to netfs.
    Under some heavy load system, it will happen very often.
    
    If this race occurs, an oops similar to the following is seen:
    
     kernel BUG at fs/fscache/operation.c:69!
     invalid opcode: 0000 [#1] SMP
     ...
     #0 [ffff883fff0838d8] machine_kexec at ffffffff81051beb
     #1 [ffff883fff083938] crash_kexec at ffffffff810f2542
     #2 [ffff883fff083a08] oops_end at ffffffff8163e1a8
     #3 [ffff883fff083a30] die at ffffffff8101859b
     #4 [ffff883fff083a60] do_trap at ffffffff8163d860
     #5 [ffff883fff083ab0] do_invalid_op at ffffffff81015204
     #6 [ffff883fff083b60] invalid_op at ffffffff8164701e
        [exception RIP: fscache_enqueue_operation+246]
        RIP: ffffffffa0b793c6  RSP: ffff883fff083c18  RFLAGS: 00010046
        RAX: 0000000000000019  RBX: ffff8832ed1a9ec0  RCX: 0000000000000006
        RDX: 0000000000000000  RSI: 0000000000000046  RDI: 0000000000000046
        RBP: ffff883fff083c20   R8: 0000000000000086   R9: 000000000000178f
        R10: ffffffff816aeb00  R11: ffff883fff08392e  R12: ffff8802f0525620
        R13: ffff88407ffc01d8  R14: 0000000000000000  R15: 0000000000000003
        ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0000
     #7 [ffff883fff083c10] fscache_enqueue_operation at ffffffffa0b793c6
     #8 [ffff883fff083c28] cachefiles_read_waiter at ffffffffa0b15a48
     #9 [ffff883fff083c48] __wake_up_common at ffffffff810af028
    
    Reported-by: Lei Xue <carmark.dlut@xxxxxxxxx>
    Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxx>
    Reported-by: Anthony DeRobertis <aderobertis@xxxxxxxxxxx>
    Reported-by: NeilBrown <neilb@xxxxxxxx>
    Reported-by: Daniel Axtens <dja@xxxxxxxxxx>
    Reported-by: KiranKumar Modukuri <kiran.modukuri@xxxxxxxxx>
    Signed-off-by: David Howells <dhowells@xxxxxxxxxx>

diff --git a/fs/cachefiles/rdwr.c b/fs/cachefiles/rdwr.c
index 5082c8a49686..b2cb6cdcd87f 100644
--- a/fs/cachefiles/rdwr.c
+++ b/fs/cachefiles/rdwr.c
@@ -27,6 +27,7 @@ static int cachefiles_read_waiter(wait_queue_entry_t *wait, unsigned mode,
 	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_queue_entry_t *wait, unsigned mode,
 	list_del(&wait->entry);
 
 	/* 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);
 	spin_unlock(&object->work_lock);
 
 	fscache_enqueue_retrieval(monitor->op);
+        fscache_put_retrieval(op);
 	return 0;
 }
 
diff --git a/fs/fscache/operation.c b/fs/fscache/operation.c
index e30c5975ea58..28ce646cc32c 100644
--- a/fs/fscache/operation.c
+++ b/fs/fscache/operation.c
@@ -69,8 +69,9 @@ void fscache_enqueue_operation(struct fscache_operation *op)
 	ASSERT(list_empty(&op->pend_link));
 	ASSERT(op->processor != NULL);
 	ASSERT(fscache_object_is_available(op->object));
-	ASSERTCMP(atomic_read(&op->usage), >, 0);
-	ASSERTCMP(op->state, ==, FSCACHE_OP_ST_IN_PROGRESS);
+	ASSERTCMP(atomic_read(&op->usage), >, 1);
+	ASSERTIFCMP(op->state != FSCACHE_OP_ST_IN_PROGRESS,
+		    op->state, ==,  FSCACHE_OP_ST_CANCELLED);
 
 	fscache_stat(&fscache_n_op_enqueue);
 	switch (op->flags & FSCACHE_OP_TYPE) {
@@ -499,7 +500,8 @@ void fscache_put_operation(struct fscache_operation *op)
 	struct fscache_cache *cache;
 
 	_enter("{OBJ%x OP%x,%d}",
-	       op->object->debug_id, op->debug_id, atomic_read(&op->usage));
+	       op->object ? op->object->debug_id : 0,
+	       op->debug_id, atomic_read(&op->usage));
 
 	ASSERTCMP(atomic_read(&op->usage), >, 0);