lib/thread: Add mutex

We need a way to protect shared resources. Since we are using
cooperative multitasking the mutex implementation is pretty trivial.

BUG=b:179699789
TEST=Verify thread lock and unlock.

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ife1ac95ec064ebcdd00fcaacec37a06ac52885ff
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56230
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/lib/thread.c

@@ -1,5 +1,6 @@
 /* SPDX-License-Identifier: GPL-2.0-only */
 
+#include <assert.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>
@@ -377,3 +378,22 @@ void thread_coop_disable(void)
 
 	current->can_yield--;
 }
+
+void thread_mutex_lock(struct thread_mutex *mutex)
+{
+	struct stopwatch sw;
+
+	stopwatch_init(&sw);
+
+	while (mutex->locked)
+		assert(thread_yield() == 0);
+	mutex->locked = true;
+
+	printk(BIOS_SPEW, "took %lu us to acquire mutex\n", stopwatch_duration_usecs(&sw));
+}
+
+void thread_mutex_unlock(struct thread_mutex *mutex)
+{
+	assert(mutex->locked);
+	mutex->locked = 0;
+}