poll_wait: add mb() to fix theoretical race between waitqueue_active() and .poll()

As the comment above waitqueue_active() explains, it can only be used
if both waker and waiter have mb()'s that pair with each other. However
__pollwait() is broken in this respect.

This is not pipe-specific, but let's look at pipe_poll() for example:

	poll_wait(...); // -> __pollwait() -> add_wait_queue()

	LOAD(pipe->head);
	LOAD(pipe->head);

In theory these LOAD()'s can leak into the critical section inside
add_wait_queue() and can happen before list_add(entry, wq_head), in this
case pipe_poll() can race with wakeup_pipe_readers/writers which do

	smp_mb();
	if (waitqueue_active(wq_head))
		wake_up_interruptible(wq_head);

There are more __pollwait()-like functions (grep init_poll_funcptr), and
it seems that at least ep_ptable_queue_proc() has the same problem, so the
patch adds smp_mb() into poll_wait().

Link: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Oleg Nesterov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>

Author: oleg-nesterov

include/linux/poll.h

@@ -41,8 +41,16 @@ typedef struct poll_table_struct {
 
 static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
 {
-	if (p && p->_qproc && wait_address)
+	if (p && p->_qproc && wait_address) {
 		p->_qproc(filp, wait_address, p);
+		/*
+		 * This memory barrier is paired in the wq_has_sleeper().
+		 * See the comment above prepare_to_wait(), we need to
+		 * ensure that subsequent tests in this thread can't be
+		 * reordered with __add_wait_queue() in _qproc() paths.
+		 */
+		smp_mb();
+	}
 }
 
 /*