llext: flush logging before unloading extensions

Extensions could have used logging, when log processing is
deferred, the logging thread can run after the extension has
been unloaded and thereby access invalid memory addresses.
Make sure to flush all logs before unloading extensions.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>

subsys/llext/llext.c

@@ -180,12 +180,59 @@ int llext_load(struct llext_loader *ldr, const char *name, struct llext **ext,
 	return ret;
 }
 
+#include <zephyr/logging/log_ctrl.h>
+
+extern struct k_thread logging_thread;
+
+static void llext_log_flush(void)
+{
+	int cur_prio = k_thread_priority_get(k_current_get());
+	int log_prio = k_thread_priority_get(&logging_thread);
+	int target_prio;
+	bool adjust_cur, adjust_log;
+
+	/*
+	 * Our goal is to raise the logger thread priority above current, but if
+	 * current has the highest possble priority, both need to be adjusted,
+	 * particularly if the logger thread has the lowest possible priority
+	 */
+	if (log_prio < cur_prio) {
+		adjust_cur = false;
+		adjust_log = false;
+		target_prio = 0;
+	} else if (cur_prio == K_HIGHEST_THREAD_PRIO) {
+		adjust_cur = true;
+		adjust_log = true;
+		target_prio = cur_prio;
+		k_thread_priority_set(k_current_get(), cur_prio + 1);
+	} else {
+		adjust_cur = false;
+		adjust_log = true;
+		target_prio = cur_prio - 1;
+	}
+
+	/* adjust logging thread priority if needed */
+	if (adjust_log)
+		k_thread_priority_set(&logging_thread, target_prio);
+
+	log_thread_trigger();
+	k_yield();
+
+	if (adjust_log)
+		k_thread_priority_set(&logging_thread, log_prio);
+	if (adjust_cur)
+		k_thread_priority_set(&logging_thread, cur_prio);
+}
+
 int llext_unload(struct llext **ext)
 {
 	__ASSERT(*ext, "Expected non-null extension");
 	struct llext *tmp = *ext;
 
 	k_mutex_lock(&llext_lock, K_FOREVER);
+
+	llext_log_flush();
+
 	__ASSERT(tmp->use_count, "A valid LLEXT cannot have a zero use-count!");
 
 	if (tmp->use_count-- != 1) {