lib/thread: Allow nesting thread_cooperate and thread_prevent_coop

This change allows nesting critical sections, and frees the caller from
having to keep track of whether the thread has coop enabled.


BUG=b:179699789
TEST=Boot guybrush with SPI DMA

Suggested-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: I325ab6181b17c5c084ca1e2c181b4df235020557
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56350
Reviewed-by: Karthik Ramasubramanian <kramasub@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/include/thread.h

@@ -43,8 +43,13 @@ int thread_yield_microseconds(unsigned int microsecs);
 
 /* Allow and prevent thread cooperation on current running thread. By default
  * all threads are marked to be cooperative. That means a thread can yield
- * to another thread at a pre-determined switch point. Current there is
- * only a single place where switching may occur: a call to udelay(). */
+ * to another thread at a pre-determined switch point. i.e., udelay,
+ * thread_yield, or thread_yield_microseconds.
+ *
+ * These methods should be used to guard critical sections so a dead lock does
+ * not occur. The critical sections can be nested. Just make sure the methods
+ * are used in pairs.
+ */
 void thread_cooperate(void);
 void thread_prevent_coop(void);
 

src/lib/thread.c

@@ -31,7 +31,7 @@ static inline struct cpu_info *thread_cpu_info(const struct thread *t)
 
 static inline int thread_can_yield(const struct thread *t)
 {
-	return (t != NULL && t->can_yield);
+	return (t != NULL && t->can_yield > 0);
 }
 
 /* Assumes current CPU info can switch. */
@@ -358,8 +358,12 @@ void thread_cooperate(void)
 
 	current = current_thread();
 
-	if (current != NULL)
-		current->can_yield = 1;
+	if (current == NULL)
+		return;
+
+	assert(current->can_yield <= 0);
+
+	current->can_yield++;
 }
 
 void thread_prevent_coop(void)
@@ -368,6 +372,8 @@ void thread_prevent_coop(void)
 
 	current = current_thread();
 
-	if (current != NULL)
-		current->can_yield = 0;
+	if (current == NULL)
+		return;
+
+	current->can_yield--;
 }