From: Christian König <christian.koe...@amd.com>
libX11 acquires the display lock before calling "xcb_take_socket" and inside
the "return_socket" callback, this can lead to a classic deadlock situation
with the XCB iolock. Releasing the lock while calling "return_socket" only
partially helps because we still try to avoid calling return_socket from
multiple threads by using an extra condition.
This patch removes the unnecessary extra signaling and just documents that
"return_socket" might be called by multiple threads at the same time (which
works fine with how libX11 is using the interface).
Signed-off-by: Christian König <christian.koe...@amd.com>
---
src/xcb_out.c | 36 +++++++++++++++++++-----------------
src/xcbext.h | 6 ++++--
src/xcbint.h | 7 ++++---
3 files changed, 27 insertions(+), 22 deletions(-)
diff --git a/src/xcb_out.c b/src/xcb_out.c
index 405f963..5eddab8 100644
--- a/src/xcb_out.c
+++ b/src/xcb_out.c
@@ -86,20 +86,23 @@ static void send_sync(xcb_connection_t *c)
static void get_socket_back(xcb_connection_t *c)
{
- while(c->out.return_socket && c->out.socket_moving)
- pthread_cond_wait(&c->out.socket_cond, &c->iolock);
- if(!c->out.return_socket)
- return;
-
- c->out.socket_moving = 1;
- pthread_mutex_unlock(&c->iolock);
- c->out.return_socket(c->out.socket_closure);
- pthread_mutex_lock(&c->iolock);
- c->out.socket_moving = 0;
-
- pthread_cond_broadcast(&c->out.socket_cond);
- c->out.return_socket = 0;
- c->out.socket_closure = 0;
+ while (c->out.return_socket) {
+ /* we are about to release the lock,
+ so make a copy of the current status */
+ xcb_return_socket_func_t return_socket = c->out.return_socket;
+ void *socket_closure = c->out.socket_closure;
+ int socket_seq = c->out.socket_seq;
+
+ pthread_mutex_unlock(&c->iolock);
+ return_socket(socket_closure);
+ pthread_mutex_lock(&c->iolock);
+
+ /* make sure nobody else has acquired the socket */
+ if (socket_seq == c->out.socket_seq) {
+ c->out.return_socket = 0;
+ c->out.socket_closure = 0;
+ }
+ }
_xcb_in_replies_done(c);
}
@@ -278,6 +281,7 @@ int xcb_take_socket(xcb_connection_t *c, void
(*return_socket)(void *closure), v
{
c->out.return_socket = return_socket;
c->out.socket_closure = closure;
+ ++c->out.socket_seq;
if(flags)
_xcb_in_expect_reply(c, c->out.request,
WORKAROUND_EXTERNAL_SOCKET_OWNER, flags);
assert(c->out.request == c->out.request_written);
@@ -314,11 +318,9 @@ int xcb_flush(xcb_connection_t *c)
int _xcb_out_init(_xcb_out *out)
{
- if(pthread_cond_init(&out->socket_cond, 0))
- return 0;
out->return_socket = 0;
out->socket_closure = 0;
- out->socket_moving = 0;
+ out->socket_seq = 0;
if(pthread_cond_init(&out->cond, 0))
return 0;
diff --git a/src/xcbext.h b/src/xcbext.h
index 98b3c93..f8520a0 100644
--- a/src/xcbext.h
+++ b/src/xcbext.h
@@ -66,8 +66,10 @@ unsigned int xcb_send_request(xcb_connection_t *c, int
flags, struct iovec *vect
* callback which XCB can call when it wants the write side of the
* socket back to make a request. This callback synchronizes with the
* external socket owner and flushes any output queues if appropriate.
- * If you are sending requests which won't cause a reply, please note the
- * comment for xcb_writev which explains some sequence number wrap issues.
+ * It is called without any locks held and so might be called from different
+ * threads at the same time. If you are sending requests which won't
+ * cause a reply, please note the comment for xcb_writev which explains
+ * some sequence number wrap issues.
* */
int xcb_take_socket(xcb_connection_t *c, void (*return_socket)(void *closure),
void *closure, int flags, uint64_t *sent);
diff --git a/src/xcbint.h b/src/xcbint.h
index f9e5a52..c68b2fa 100644
--- a/src/xcbint.h
+++ b/src/xcbint.h
@@ -79,14 +79,15 @@ void *_xcb_map_remove(_xcb_map *q, unsigned int key);
/* xcb_out.c */
+typedef void (*xcb_return_socket_func_t)(void *closure);
+
typedef struct _xcb_out {
pthread_cond_t cond;
int writing;
- pthread_cond_t socket_cond;
- void (*return_socket)(void *closure);
+ xcb_return_socket_func_t return_socket;
void *socket_closure;
- int socket_moving;
+ int socket_seq;
char queue[XCB_QUEUE_BUFFER_SIZE];
int queue_len;
--
1.7.9.5
_______________________________________________
mesa-dev mailing list
mesa-dev@lists.freedesktop.org
http://lists.freedesktop.org/mailman/listinfo/mesa-dev
