core/msg: fix msg_avail_thread() and msg_avail()

maribu · mguetschow · maribu · commit caa5561667b6 · 2025-04-15T15:07:11.000+02:00
This now ensures race-free access to the CIB tracking the number
of messages queued in the ringbuffer for a given thread.

In addition, `msg_avail_thread()` now checks if the provided pid refers
to a thread that is currently existing.

Co-authored-by: Mikolai Gütschow &lt;mikolai.guetschow@tu-dresden.de&gt;
diff --git a/core/include/msg.h b/core/include/msg.h
@@ -375,16 +375,21 @@ int msg_reply_int(msg_t *m, msg_t *reply);
  *
  * @param[in] pid    a PID
  *
- * @return Number of messages available in queue of @p pid on success
- * @return 0, if no caller's message queue is initialized
+ * @return  Number of messages available in queue of the thread identified by
+ *          PID @p pid
+ * @retval  0       The message queue of the thread identified by PID @p pid is
+ *                  *not* initialized or PID @p pid does not refer to a running
+ *                  thread
  */
 unsigned msg_avail_thread(kernel_pid_t pid);
 
 /**
  * @brief Check how many messages are available (waiting) in the message queue
  *
- * @return Number of messages available in our queue on success
- * @return 0, if no caller's message queue is initialized
+ * @pre     The caller is running in thread context
+ *
+ * @return  Number of messages available in our queue
+ * @retval  0       Caller's message queue is *not* initialized
  */
 unsigned msg_avail(void);
 
diff --git a/core/msg.c b/core/msg.c
@@ -458,12 +458,23 @@ static unsigned _msg_avail(thread_t *thread)
 
 unsigned msg_avail_thread(kernel_pid_t pid)
 {
-    return _msg_avail(thread_get(pid));
+    unsigned irq_state = irq_disable();
+    thread_t *t = thread_get(pid);
+    if (!t) {
+        irq_restore(irq_state);
+        return 0;
+    }
+    unsigned result = _msg_avail(t);
+    irq_restore(irq_state);
+    return result;
 }
 
 unsigned msg_avail(void)
 {
-    return _msg_avail(thread_get_active());
+    unsigned irq_state = irq_disable();
+    unsigned result = _msg_avail(thread_get_active());
+    irq_restore(irq_state);
+    return result;
 }
 
 unsigned msg_queue_capacity(kernel_pid_t pid)

Original file line number	Diff line number	Diff line change
`@@ -458,12 +458,23 @@ static unsigned _msg_avail(thread_t *thread)`
`458`	`458`
`459`	`459`	`unsigned msg_avail_thread(kernel_pid_t pid)`
`460`	`460`	`{`
`461`		`- return _msg_avail(thread_get(pid));`
	`461`	`+ unsigned irq_state = irq_disable();`
	`462`	`+ thread_t *t = thread_get(pid);`
	`463`	`+ if (!t) {`
	`464`	`+ irq_restore(irq_state);`
	`465`	`+ return 0;`
	`466`	`+ }`
	`467`	`+ unsigned result = _msg_avail(t);`
	`468`	`+ irq_restore(irq_state);`
	`469`	`+ return result;`
`462`	`470`	`}`
`463`	`471`
`464`	`472`	`unsigned msg_avail(void)`
`465`	`473`	`{`
`466`		`- return _msg_avail(thread_get_active());`
	`474`	`+ unsigned irq_state = irq_disable();`
	`475`	`+ unsigned result = _msg_avail(thread_get_active());`
	`476`	`+ irq_restore(irq_state);`
	`477`	`+ return result;`
`467`	`478`	`}`
`468`	`479`
`469`	`480`	`unsigned msg_queue_capacity(kernel_pid_t pid)`