From fd3a0a143509a829f1e5ace9322d035406517991 Mon Sep 17 00:00:00 2001
From: Ryan Pavlik <ryan.pavlik@collabora.com>
Date: Fri, 20 May 2022 14:09:37 -0500
Subject: [PATCH] c/multi: Fix condition var waiting for wait thread.

Spurious wakeups happen, have to actually have a condition
to check in a loop with the cond var wait.
---
 src/xrt/compositor/multi/comp_multi_compositor.c | 10 ++++++++--
 src/xrt/compositor/multi/comp_multi_private.h    |  7 ++++++-
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/src/xrt/compositor/multi/comp_multi_compositor.c b/src/xrt/compositor/multi/comp_multi_compositor.c
index 6794e8949..4cecb016c 100644
--- a/src/xrt/compositor/multi/comp_multi_compositor.c
+++ b/src/xrt/compositor/multi/comp_multi_compositor.c
@@ -226,6 +226,7 @@ run_func(void *ptr)
 	while (os_thread_helper_is_running_locked(&mc->wait_thread.oth)) {
 
 		if (mc->wait_thread.xcsem == NULL && mc->wait_thread.xcf == NULL) {
+			//! @todo what condition should we spin on here to handle spurious wakeups?
 			os_thread_helper_wait_locked(&mc->wait_thread.oth);
 			// Fall through here on stopping to clean up and outstanding waits.
 		}
@@ -274,6 +275,8 @@ run_func(void *ptr)
 		mc->wait_thread.waiting = false;
 
 		if (mc->wait_thread.blocked) {
+			// Release one thread
+			mc->wait_thread.blocked = false;
 			os_thread_helper_signal_locked(&mc->wait_thread.oth);
 		}
 	}
@@ -289,9 +292,12 @@ wait_for_wait_thread_locked(struct multi_compositor *mc)
 	// Should we wait for the last frame.
 	if (mc->wait_thread.waiting) {
 		COMP_TRACE_IDENT(blocked);
+
+		// OK, wait until the wait thread releases us by setting blocked to false
 		mc->wait_thread.blocked = true;
-		os_thread_helper_wait_locked(&mc->wait_thread.oth);
-		mc->wait_thread.blocked = false;
+		while (mc->wait_thread.blocked) {
+			os_thread_helper_wait_locked(&mc->wait_thread.oth);
+		}
 	}
 }
 
diff --git a/src/xrt/compositor/multi/comp_multi_private.h b/src/xrt/compositor/multi/comp_multi_private.h
index 2a07cb47d..a974f3c32 100644
--- a/src/xrt/compositor/multi/comp_multi_private.h
+++ b/src/xrt/compositor/multi/comp_multi_private.h
@@ -146,7 +146,12 @@ struct multi_compositor
 		//! Is the thread waiting, if so the client should block.
 		bool waiting;
 
-		//! Is the client thread blocked, if so it should be woken up.
+		/*!
+		 * Is the client thread blocked?
+		 *
+		 * Set to true by the client thread,
+		 * cleared by the wait thread to release the client thread.
+		 */
 		bool blocked;
 	} wait_thread;