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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