linux-kernel - Re: [PATCH 3/3] ipc/mqueue: lockless pipelined wakeups

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Date:	1 May 2015 17:52:07 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	dave@...olabs.net, mingo@...hat.com, peterz@...radead.org,
	tglx@...utronix.de
Cc:	bigeasy@...utronix.de, clm@...com, dbueso@...e.de,
	linux-kernel@...r.kernel.org, linux@...izon.com,
	manfred@...orfullife.com, rostedt@...dmis.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 3/3] ipc/mqueue: lockless pipelined wakeups

In general, Acked-by, but you're making me fix all your comments. :-)

This is a nice use of the wake queue, since the code was already handling
the same problem in a similar way with STATE_PENDING.

>  * The receiver accepts the message and returns without grabbing the queue
>+ * spinlock. The used algorithm is different from sysv semaphores (ipc/sem.c):

Is that last sentence even wanted?

>+ *
>+ * - Set pointer to message.
>+ * - Queue the receiver task's for later wakeup (without the info->lock).

It's "task" singular, and the apostrophe would be wrong if it were plural.

>+ * - Update its state to STATE_READY. Now the receiver can continue.
>+ * - Wake up the process after the lock is dropped. Should the process wake up
>+ *   before this wakeup (due to a timeout or a signal) it will either see
>+ *   STATE_READY and continue or acquire the lock to check the sate again.

"check the sTate again".

>+	wake_q_add(wake_q, receiver->task);
>+	/*
>+	 * Rely on the implicit cmpxchg barrier from wake_q_add such
>+	 * that we can ensure that updating receiver->state is the last
>+	 * write operation: As once set, the receiver can continue,
>+	 * and if we don't have the reference count from the wake_q,
>+	 * yet, at that point we can later have a use-after-free
>+	 * condition and bogus wakeup.
>+	 */
> 	receiver->state = STATE_READY;

How about:
	/*
	 * There must be a write barrier here; setting STATE_READY
	 * lets the receiver proceed without further synchronization.
	 * The cmpxchg inside wake_q_add serves as the barrier here.
	 */

The need for a wake queue to take a reference to avoid use-after-free
is generic to wake queues, and handled in generic code; I don't see why
it needs a comment here.

 
>@@ -1084,6 +1094,7 @@ SYSCALL_DEFINE5(mq_timedreceive, mqd_t, mqdes, char __user *, u_msg_ptr,
> 	ktime_t expires, *timeout = NULL;
> 	struct timespec ts;
> 	struct posix_msg_tree_node *new_leaf = NULL;
>+	WAKE_Q(wake_q);
> 
> 	if (u_abs_timeout) {
> 		int res = prepare_timeout(u_abs_timeout, &expires, &ts);
>@@ -1155,8 +1166,9 @@ SYSCALL_DEFINE5(mq_timedreceive, mqd_t, mqdes, char __user *, u_msg_ptr,
> 				CURRENT_TIME;
> 
> 		/* There is now free space in queue. */
>-		pipelined_receive(info);
>+		pipelined_receive(&wake_q, info);
> 		spin_unlock(&info->lock);
>+		wake_up_q(&wake_q);
> 		ret = 0;
> 	}
> 	if (ret == 0) {

Since WAKE_Q actually involves some initialization, would it make sense to
move its declaration to inside the condition that needs it?

(I'm also a fan of declaring variables in the smallest scope possible,
just on general principles.)
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