[Mesa-dev] [PATCH] loader/dri3: Improve dri3 thread-safety
Nicolai Hähnle
nhaehnle at gmail.com
Mon Nov 6 11:14:02 UTC 2017
On 03.11.2017 12:02, Thomas Hellstrom wrote:
> It turned out that with recent changes that call into dri3 from glFinish(),
> it appears like different thread end up waiting for X events simultaneously,
> causing deadlocks since they steal events from eachoter and update the dri3
> counters behind eachothers backs.
>
> This patch intends to improve on that. It allows at most one thread at a
> time to wait on events for a single drawable. If another thread intends to
> do the same, it's put to sleep until the first thread finishes waiting, and
> then it rechecks counters and optionally retries the waiting. Threads that
> poll for X events never pulls X events off the event queue if there are
> other threads waiting for events on that drawable. Counters in the
> dri3 drawable structure are protected by a mutex. Finally, the mutex we
> introduce is never held while waiting for the X server to avoid
> unnecessary stalls.
>
> This does not make dri3 drawables completely thread-safe but at least it's a
> first step.
>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102358
> Fixes: d5ba75f8881 "st/dri2 Plumb the flush_swapbuffer functionality through to dri3"
> Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>
> ---
> src/loader/loader_dri3_helper.c | 77 +++++++++++++++++++++++++++++++----------
> src/loader/loader_dri3_helper.h | 10 ++++++
> 2 files changed, 69 insertions(+), 18 deletions(-)
>
> diff --git a/src/loader/loader_dri3_helper.c b/src/loader/loader_dri3_helper.c
> index 19ab581..7e6b8b2 100644
> --- a/src/loader/loader_dri3_helper.c
> +++ b/src/loader/loader_dri3_helper.c
> @@ -32,7 +32,6 @@
>
> #include <X11/Xlib-xcb.h>
>
> -#include <c11/threads.h>
> #include "loader_dri3_helper.h"
>
> /* From xmlpool/options.h, user exposed so should be stable */
> @@ -186,8 +185,11 @@ dri3_fence_await(xcb_connection_t *c, struct loader_dri3_drawable *draw,
> {
> xcb_flush(c);
> xshmfence_await(buffer->shm_fence);
> - if (draw)
> + if (draw) {
> + mtx_lock(&draw->mtx);
> dri3_flush_present_events(draw);
> + mtx_unlock(&draw->mtx);
> + }
> }
>
> static void
> @@ -245,6 +247,9 @@ loader_dri3_drawable_fini(struct loader_dri3_drawable *draw)
> xcb_discard_reply(draw->conn, cookie.sequence);
> xcb_unregister_for_special_event(draw->conn, draw->special_event);
> }
> +
> + cnd_destroy(&draw->event_cnd);
> + mtx_destroy(&draw->mtx);
> }
>
> int
> @@ -276,6 +281,8 @@ loader_dri3_drawable_init(xcb_connection_t *conn,
>
> draw->cur_blit_source = -1;
> draw->back_format = __DRI_IMAGE_FORMAT_NONE;
> + mtx_init(&draw->mtx, mtx_plain);
> + cnd_init(&draw->event_cnd);
>
> if (draw->ext->config)
> draw->ext->config->configQueryi(draw->dri_screen,
> @@ -407,13 +414,27 @@ dri3_handle_present_event(struct loader_dri3_drawable *draw,
> }
>
> static bool
> -dri3_wait_for_event(struct loader_dri3_drawable *draw)
> +dri3_wait_for_event_locked(struct loader_dri3_drawable *draw)
> {
> xcb_generic_event_t *ev;
> xcb_present_generic_event_t *ge;
>
> xcb_flush(draw->conn);
(Why) Doesn't the flush need to be protected as well? Can it be removed
entirely given that it's already called from loader_dri3_wait_for_msc?
Though dri3_find_back is different - why?
Apart from that, the patch does indeed look like a good first step,
though Keep in mind that I'm not too familiar with this stuff...
Cheers,
Nicolai
> - ev = xcb_wait_for_special_event(draw->conn, draw->special_event);
> +
> + /* Only have one thread waiting for events at a time */
> + if (draw->has_event_waiter) {
> + cnd_wait(&draw->event_cnd, &draw->mtx);
> + /* Another thread has updated the protected info, so retest. */
> + return true;
> + } else {
> + draw->has_event_waiter = true;
> + /* Allow other threads access to the drawable while we're waiting. */
> + mtx_unlock(&draw->mtx);
> + ev = xcb_wait_for_special_event(draw->conn, draw->special_event);
> + mtx_lock(&draw->mtx);
> + draw->has_event_waiter = false;
> + cnd_broadcast(&draw->event_cnd);
> + }
> if (!ev)
> return false;
> ge = (void *) ev;
> @@ -442,19 +463,23 @@ loader_dri3_wait_for_msc(struct loader_dri3_drawable *draw,
> divisor,
> remainder);
>
> + mtx_lock(&draw->mtx);
> xcb_flush(draw->conn);
>
> /* Wait for the event */
> if (draw->special_event) {
> while ((int32_t) (msc_serial - draw->recv_msc_serial) > 0) {
> - if (!dri3_wait_for_event(draw))
> + if (!dri3_wait_for_event_locked(draw)) {
> + mtx_unlock(&draw->mtx);
> return false;
> + }
> }
> }
>
> *ust = draw->notify_ust;
> *msc = draw->notify_msc;
> *sbc = draw->recv_sbc;
> + mtx_unlock(&draw->mtx);
>
> return true;
> }
> @@ -476,17 +501,21 @@ loader_dri3_wait_for_sbc(struct loader_dri3_drawable *draw,
> * swaps requested with glXSwapBuffersMscOML for that window have
> * completed."
> */
> + mtx_lock(&draw->mtx);
> if (!target_sbc)
> target_sbc = draw->send_sbc;
>
> while (draw->recv_sbc < target_sbc) {
> - if (!dri3_wait_for_event(draw))
> + if (!dri3_wait_for_event_locked(draw)) {
> + mtx_unlock(&draw->mtx);
> return 0;
> + }
> }
>
> *ust = draw->ust;
> *msc = draw->msc;
> *sbc = draw->recv_sbc;
> + mtx_unlock(&draw->mtx);
> return 1;
> }
>
> @@ -499,16 +528,16 @@ static int
> dri3_find_back(struct loader_dri3_drawable *draw)
> {
> int b;
> - xcb_generic_event_t *ev;
> - xcb_present_generic_event_t *ge;
> - int num_to_consider = draw->num_back;
> + int num_to_consider;
>
> + mtx_lock(&draw->mtx);
> /* Increase the likelyhood of reusing current buffer */
> dri3_flush_present_events(draw);
>
> /* Check whether we need to reuse the current back buffer as new back.
> * In that case, wait until it's not busy anymore.
> */
> + num_to_consider = draw->num_back;
> if (!loader_dri3_have_image_blit(draw) && draw->cur_blit_source != -1) {
> num_to_consider = 1;
> draw->cur_blit_source = -1;
> @@ -521,15 +550,14 @@ dri3_find_back(struct loader_dri3_drawable *draw)
>
> if (!buffer || !buffer->busy) {
> draw->cur_back = id;
> + mtx_unlock(&draw->mtx);
> return id;
> }
> }
> - xcb_flush(draw->conn);
> - ev = xcb_wait_for_special_event(draw->conn, draw->special_event);
> - if (!ev)
> + if (!dri3_wait_for_event_locked(draw)) {
> + mtx_unlock(&draw->mtx);
> return -1;
> - ge = (void *) ev;
> - dri3_handle_present_event(draw, ge);
> + }
> }
> }
>
> @@ -745,6 +773,9 @@ dri3_flush_present_events(struct loader_dri3_drawable *draw)
> /* Check to see if any configuration changes have occurred
> * since we were last invoked
> */
> + if (draw->has_event_waiter)
> + return;
> +
> if (draw->special_event) {
> xcb_generic_event_t *ev;
>
> @@ -774,6 +805,7 @@ loader_dri3_swap_buffers_msc(struct loader_dri3_drawable *draw,
>
> back = dri3_find_back_alloc(draw);
>
> + mtx_lock(&draw->mtx);
> if (draw->is_different_gpu && back) {
> /* Update the linear buffer before presenting the pixmap */
> (void) loader_dri3_blit_image(draw,
> @@ -893,6 +925,7 @@ loader_dri3_swap_buffers_msc(struct loader_dri3_drawable *draw,
> if (draw->stamp)
> ++(*draw->stamp);
> }
> + mtx_unlock(&draw->mtx);
>
> draw->ext->flush->invalidate(draw->dri_drawable);
>
> @@ -903,11 +936,14 @@ int
> loader_dri3_query_buffer_age(struct loader_dri3_drawable *draw)
> {
> struct loader_dri3_buffer *back = dri3_find_back_alloc(draw);
> + int ret;
>
> - if (!back || back->last_swap == 0)
> - return 0;
> + mtx_lock(&draw->mtx);
> + ret = (!back || back->last_swap == 0) ? 0 :
> + draw->send_sbc - back->last_swap + 1;
> + mtx_unlock(&draw->mtx);
>
> - return draw->send_sbc - back->last_swap + 1;
> + return ret;
> }
>
> /** loader_dri3_open
> @@ -1125,6 +1161,7 @@ static int
> dri3_update_drawable(__DRIdrawable *driDrawable,
> struct loader_dri3_drawable *draw)
> {
> + mtx_lock(&draw->mtx);
> if (draw->first_init) {
> xcb_get_geometry_cookie_t geom_cookie;
> xcb_get_geometry_reply_t *geom_reply;
> @@ -1165,8 +1202,10 @@ dri3_update_drawable(__DRIdrawable *driDrawable,
>
> geom_reply = xcb_get_geometry_reply(draw->conn, geom_cookie, NULL);
>
> - if (!geom_reply)
> + if (!geom_reply) {
> + mtx_unlock(&draw->mtx);
> return false;
> + }
>
> draw->width = geom_reply->width;
> draw->height = geom_reply->height;
> @@ -1198,6 +1237,7 @@ dri3_update_drawable(__DRIdrawable *driDrawable,
> if (error) {
> if (error->error_code != BadWindow) {
> free(error);
> + mtx_unlock(&draw->mtx);
> return false;
> }
> draw->is_pixmap = true;
> @@ -1206,6 +1246,7 @@ dri3_update_drawable(__DRIdrawable *driDrawable,
> }
> }
> dri3_flush_present_events(draw);
> + mtx_unlock(&draw->mtx);
> return true;
> }
>
> diff --git a/src/loader/loader_dri3_helper.h b/src/loader/loader_dri3_helper.h
> index d3f4b0c..0dd37e9 100644
> --- a/src/loader/loader_dri3_helper.h
> +++ b/src/loader/loader_dri3_helper.h
> @@ -33,6 +33,7 @@
>
> #include <GL/gl.h>
> #include <GL/internal/dri_interface.h>
> +#include <c11/threads.h>
>
> enum loader_dri3_buffer_type {
> loader_dri3_buffer_back = 0,
> @@ -159,6 +160,15 @@ struct loader_dri3_drawable {
>
> unsigned int swap_method;
> unsigned int back_format;
> +
> + /* Currently protects the following fields:
> + * event_cnd, has_event_waiter,
> + * recv_sbc, ust, msc, recv_msc_serial,
> + * notify_ust, notify_msc
> + */
> + mtx_t mtx;
> + cnd_t event_cnd;
> + bool has_event_waiter;
> };
>
> void
>
--
Lerne, wie die Welt wirklich ist,
Aber vergiss niemals, wie sie sein sollte.
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