[PATCH wayland] client: update documentation about threading
Marek Chalupa
mchqwerty at gmail.com
Fri Nov 28 03:17:30 PST 2014
On 26 November 2014 at 13:48, Pekka Paalanen <ppaalanen at gmail.com> wrote:
> On Fri, 21 Nov 2014 11:12:35 +0100
> Marek Chalupa <mchqwerty at gmail.com> wrote:
>
> > Remove out-dated documentation and add few more words
> > about this topic.
> >
> > Signed-off-by: Marek Chalupa <mchqwerty at gmail.com>
> > ---
> > src/wayland-client.c | 106
> ++++++++++++++++++++++++++++++++++++++++++---------
> > 1 file changed, 87 insertions(+), 19 deletions(-)
> >
> > diff --git a/src/wayland-client.c b/src/wayland-client.c
> > index 41c49e9..b1a1fa0 100644
> > --- a/src/wayland-client.c
> > +++ b/src/wayland-client.c
> > @@ -909,6 +909,12 @@ static const struct wl_callback_listener
> sync_listener = {
> > * Blocks until the server process all currently issued requests and
> > * sends out pending events on the event queue.
> > *
> > + * \note This function uses wl_display_dispatch_queue() internally. If
> you
> > + * are using wl_display_read_events() from more threads, don't use this
> function
> > + * (or make sure that calling wl_display_roundtrip_queue() doesn't
> interfere
> > + * with calling wl_display_prepare_read() and wl_display_read_events())
> > + *
> > + * \sa wl_display_roundtrip()
> > * \memberof wl_display
> > */
> > WL_EXPORT int
> > @@ -940,6 +946,11 @@ wl_display_roundtrip_queue(struct wl_display
> *display, struct wl_event_queue *qu
> > * Blocks until the server process all currently issued requests and
> > * sends out pending events on the default event queue.
> > *
> > + * \note This function uses wl_display_dispatch_queue() internally. If
> you
> > + * are using wl_display_read_events() from more threads, don't use this
> function
> > + * (or make sure that calling wl_display_roundtrip() doesn't interfere
> > + * with calling wl_display_prepare_read() and wl_display_read_events())
> > + *
> > * \memberof wl_display
> > */
> > WL_EXPORT int
> > @@ -1211,14 +1222,55 @@ cancel_read(struct wl_display *display)
> > *
> > * This will read events from the file descriptor for the display.
> > * This function does not dispatch events, it only reads and queues
> > - * events into their corresponding event queues. If no data is
> > + * events into their corresponding event queues. If no data is
> > * avilable on the file descriptor, wl_display_read_events() returns
> > - * immediately. To dispatch events that may have been queued, call
> > - * wl_display_dispatch_pending() or
> > - * wl_display_dispatch_queue_pending().
> > + * immediately. To dispatch events that may have been queued, call
> > + * wl_display_dispatch_pending() or wl_display_dispatch_queue_pending().
> > *
> > * Before calling this function, wl_display_prepare_read() must be
> > - * called first.
> > + * called first. When running in more threads (which is the usual
> > + * case, since we'd use wl_display_dispatch() otherwise), every thread
> > + * must call wl_display_prepare_read() before calling this function.
> > + *
> > + * After calling wl_display_prepare_read() there can be some extra code
> > + * before calling wl_display_read_events(), for example poll() or alike.
> > + * Example of code in a thread:
> > + *
> > + * \code
> > + *
> > + * while (wl_display_prepare_read(display) < 0)
> > + * wl_display_dispatch_pending(display);
> > + * wl_display_flush(display);
> > + *
> > + * ... some code ...
> > + *
> > + * fds[0].fd = wl_display_get_fd(display);
> > + * fds[0].event = POLLIN | POLLHUP | POLLERR;
> > + * poll(fds, 1, -1);
> > + *
> > + * if (!everything_ok()) {
> > + * wl_display_cancel_read(display);
> > + * handle_error();
> > + * }
> > + *
> > + * if (wl_display_read_events(display) < 0)
> > + * handle_error();
> > + *
> > + * ...
> > + * \endcode
> > + *
> > + * The code should be as short as possible since other threads may
> > + * get sleeping until all threads called wl_display_read_events()
> > + * or wl_display_cancel_read().
>
> Hi,
>
> this paragraph is a bit confusing, "short code" is not a clear term.
> There is also the poll() which may take an arbitary amount of time to
> return. I probably see what you are getting at, but it should be
> rephrased.
>
> How about something like:
>
> After wl_display_prepare_read() succeeds, other threads that enter
> wl_display_read_events() will sleep until the very last thread enters
> it too or cancels. Therefore when the display fd becomes (or already
> is) readable, wl_display_read_events() should be called as soon as
> possible to unblock all threads. If wl_display_read_events() will not
> be called, then wl_display_cancel_read() must be called
> posthaste instead to let the other threads continue.
>
Yes, that's clearer :)
>
> > + *
> > + * This function must not be called simultaneously with
> wl_display_dispatch().
> > + * It may lead to deadlock. If programmer wants, for some reason, use
> > + * wl_display_dispatch() in one thread and wl_display_read_events() in
> another,
> > + * extra care must be taken to serialize these calls, i. e. use mutexes
> or
> > + * similar.
>
> Hmm, that mutex should protect not only wl_display_read_events() call,
> but the whole sequence between prepare_read and
> read_events/cancel_read, should it not?
>
Yup
>
> > + *
> > + * \sa wl_display_prepare_read(), wl_display_cancel_read(),
> > + * wl_display_dispatch_pending(), wl_display_dispatch()
> > *
> > * \memberof wl_display
> > */
> > @@ -1296,17 +1348,17 @@ wl_display_prepare_read_queue(struct wl_display
> *display,
> > return ret;
> > }
> >
> > -/** Prepare to read events after polling file descriptor
> > +/** Prepare to read events from the display's file descriptor
> > *
> > * \param display The display context object
> > * \return 0 on success or -1 if event queue was not empty
> > *
> > * This function must be called before reading from the file
> > - * descriptor using wl_display_read_events(). Calling
> > - * wl_display_prepare_read() announces the calling threads intention
> > + * descriptor using wl_display_read_events(). Calling
> > + * wl_display_prepare_read() announces the calling thread's intention
> > * to read and ensures that until the thread is ready to read and
> > * calls wl_display_read_events(), no other thread will read from the
> > - * file descriptor. This only succeeds if the event queue is empty
> > + * file descriptor. This only succeeds if the event queue is empty
> > * though, and if there are undispatched events in the queue, -1 is
> > * returned and errno set to EAGAIN.
> > *
> > @@ -1314,6 +1366,13 @@ wl_display_prepare_read_queue(struct wl_display
> *display,
> > * either call wl_display_read_events() when it's ready or cancel the
> > * read intention by calling wl_display_cancel_read().
> > *
> > + * This function doesn't acquire exclusive access to the display's fd.
> > + * It only registers that the thread calling this function has
> intention to
> > + * read from fd. When all registered readers call
> wl_display_read_events(),
> > + * only one (at random) eventually reads and queues the events and the
> > + * others are sleeping meanwhile. This way we avoid races and still
> > + * can read from more threads.
> > + *
> > * Use this function before polling on the display fd or to integrate
> > * the fd into a toolkit event loop in a race-free way. Typically, a
> > * toolkit will call wl_display_dispatch_pending() before sleeping, to
> > @@ -1350,9 +1409,10 @@ wl_display_prepare_read_queue(struct wl_display
> *display,
> > * Here we call wl_display_prepare_read(), which ensures that between
> > * returning from that call and eventually calling
> > * wl_display_read_events(), no other thread will read from the fd and
> > - * queue events in our queue. If the call to
> > - * wl_display_prepare_read() fails, we dispatch the pending events and
> > - * try again until we're successful.
> > + * queue events in our queue. If the call to wl_display_prepare_read()
> fails,
> > + * we dispatch the pending events and try again until we're successful.
> > + *
> > + * \sa wl_display_cancel_read(), wl_display_read_events()
> > *
> > * \memberof wl_display
> > */
> > @@ -1362,13 +1422,15 @@ wl_display_prepare_read(struct wl_display
> *display)
> > return wl_display_prepare_read_queue(display,
> &display->default_queue);
> > }
> >
> > -/** Release exclusive access to display file descriptor
> > +/** Cancel read intention on display's fd
> > *
> > * \param display The display context object
> > *
> > - * This releases the exclusive access. Useful for canceling the lock
> > - * when a timed out poll returns fd not readable and we're not going
> > - * to read from the fd anytime soon.
> > + * After a thread successfully called wl_display_preapare_read() it must
>
> Typo: preapare.
>
> > + * either call wl_display_read_events() or wl_display_cancel_read().
> > + * If the threads do not follow this rule it will lead to deadlock.
> > + *
> > + * \sa wl_display_prepare_read(), wl_display_read_events()
> > *
> > * \memberof wl_display
> > */
> > @@ -1396,6 +1458,9 @@ wl_display_cancel_read(struct wl_display *display)
> > * threads this will block until the main thread queues events on the
> queue
> > * passed as argument.
> > *
> > + * For behaviour in multi-threaded environment see
> wl_display_read_events()
> > + * and wl_display_prepare_read()
>
> Shouldn't we have the same warning here for wl_display_dispatch_queue()
> as we have below for wl_display_dispatch()?
>
> Only the *_pending() function are safe in a multithreaded app, right?
>
>
Yes, only the ones that do not attempt to read from the fd.
>
> > + *
> > * \memberof wl_display
> > */
> > WL_EXPORT int
> > @@ -1502,10 +1567,13 @@ wl_display_dispatch_queue_pending(struct
> wl_display *display,
> > * or not. For dispatching main queue events without blocking, see \ref
> > * wl_display_dispatch_pending().
> > *
> > - * \note Calling this will release the display file descriptor if this
> > - * thread acquired it using wl_display_acquire_fd().
> > + * In multi-threaded environment, programmer may want to use
> > + * wl_display_read_events(). However, use of wl_display_read_events()
> > + * must not be mixed with wl_display_dispatch(). See
> wl_display_read_events()
> > + * and wl_display_prepare_read() for more details.
> > *
> > - * \sa wl_display_dispatch_pending(), wl_display_dispatch_queue()
> > + * \sa wl_display_dispatch_pending(), wl_display_dispatch_queue(),
> > + * wl_display_read_events()
> > *
> > * \memberof wl_display
> > */
>
> Meanwhile I have merged some patches that make this one need a rebase.
>
> Otherwise it looks ok to me.
>
>
> Thanks,
> pq
>
Thanks,
Marek
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