smp: don't warn about csd->flags having CSD_FLAG_LOCK cleared for !wait

blk-mq reuses the request potentially immediately, since the most
cache hot is always given out first. This means that rq->csd could
be reused between csd->func() being called and csd_unlock() being
called. This isn't a problem, since we never use wait == 1 for
the smp call function. Add CSD_FLAG_WAIT to be able to tell the
difference, retaining the warning for other cases.

Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

kernel/smp.c

@@ -18,6 +18,7 @@
 #ifdef CONFIG_USE_GENERIC_SMP_HELPERS
 enum {
 	CSD_FLAG_LOCK		= 0x01,
+	CSD_FLAG_WAIT		= 0x02,
 };
 
 struct call_function_data {
@@ -124,7 +125,7 @@ static void csd_lock(struct call_single_data *csd)
 
 static void csd_unlock(struct call_single_data *csd)
 {
-	WARN_ON(!(csd->flags & CSD_FLAG_LOCK));
+	WARN_ON((csd->flags & CSD_FLAG_WAIT) && !(csd->flags & CSD_FLAG_LOCK));
 
 	/*
 	 * ensure we're all done before releasing data:
@@ -146,6 +147,9 @@ void generic_exec_single(int cpu, struct call_single_data *csd, int wait)
 	unsigned long flags;
 	int ipi;
 
+	if (wait)
+		csd->flags |= CSD_FLAG_WAIT;
+
 	raw_spin_lock_irqsave(&dst->lock, flags);
 	ipi = list_empty(&dst->list);
 	list_add_tail(&csd->list, &dst->list);