lib: os: mpsc_pbuf: do not wait when spinlock is held

Check if the spinlock is held before attempting to wait by
taking the semaphore, as that would cause a context switch which
isn't allowed and will trigger an assertion error when
`CONFIG_SPIN_VALIDATE` is enabled.

Logging in spinlock-held context when the log buffer is full can lead
to an infinite assertion error loop, as the logging subsys attempts to
allocate buffer when there's none available, it will try to wait for
one and thus triggers the assertion error, the error message will be
printed through the logging sybsys but there's no buffer available,
so it will try to wait for one and triggers another assertion error..
This loop just goes on and on forever, and nothing gets printed to
the terminal.

Added a test to validate the fix.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>
Signed-off-by: Maxim Adelman <imax@meta.com>
(cherry picked from commit 1a578eb72c)

lib/os/mpsc_pbuf.c

@@ -373,7 +373,7 @@ union mpsc_pbuf_generic *mpsc_pbuf_alloc(struct mpsc_pbuf_buffer *buffer,
 			add_skip_item(buffer, free_wlen);
 			cont = true;
 		} else if (IS_ENABLED(CONFIG_MULTITHREADING) && !K_TIMEOUT_EQ(timeout, K_NO_WAIT) &&
-			   !k_is_in_isr()) {
+			   !k_is_in_isr() && arch_irq_unlocked(key.key)) {
 			int err;
 
 			k_spin_unlock(&buffer->lock, key);

tests/lib/mpsc_pbuf/src/main.c

@@ -1294,5 +1294,28 @@ ZTEST(log_buffer, test_utilization)
 	zassert_true(packet == NULL);
 }
 
+/* Make sure that `mpsc_pbuf_alloc()` works in spinlock-held context when buf is not available */
+ZTEST(log_buffer, test_alloc_in_spinlock)
+{
+	struct mpsc_pbuf_buffer buffer;
+	struct test_data_var *packet;
+	struct k_spinlock l = {0};
+
+	init(&buffer, 32, false);
+
+	/* Allocate all available buffer */
+	packet = (struct test_data_var *)mpsc_pbuf_alloc(
+		&buffer, 32, K_MSEC(10));
+	zassert_not_null(packet);
+
+	K_SPINLOCK(&l) {
+		/* Try to allocate another buf */
+		packet = (struct test_data_var *)mpsc_pbuf_alloc(
+			&buffer, 32, K_MSEC(10));
+		/* No buf is available this time */
+		zassert_is_null(packet);
+	}
+}
+
 /*test case main entry*/
 ZTEST_SUITE(log_buffer, NULL, NULL, NULL, NULL, NULL);