On Fri, 2015-05-01 at 17:52 -0400, George Spelvin wrote:
> 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?
Yeah, we can probably remove it now.
> >+ *
> >+ * - 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.
You are not wrong, but I'd rather leave the comment as is, as it will
vary from user to user. The comments in the sched wake_q bits are
already pretty clear, and if users cannot see the need for holding
reference and the task disappearing on their own they have no business
using wake_q. Furthermore, I think my comment serves better in mqueues
as the need for it isn't immediately obvious.
> >@@ -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.)
Agreed.
Thanks,
Davidlohr
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