lib: os: mpsc_pbuf: fix potential semaphore wait forever

One thread calls mpsc_pbuf_alloc to produce data, which invokes
add_skip_item and steps into k_sem_take.

Another thread calls mpsc_pbuf_claim to consume data. In this condition,
mpsc_pbuf_claim has only small remaining space and needs to call rd_idx_inc
to reserve space, but there is still no data available.

The consumer should call k_sem_give to wake mpsc_pbuf_alloc again,
so the producer can allocate space and continue producing data.

Without this wake-up, the producer thread may wait forever in
k_sem_take, leading to a deadlock situation.

Signed-off-by: Fei Wang <[email protected]>

Author: Fei Wang

lib/os/mpsc_pbuf.c

@@ -537,6 +537,7 @@ const union mpsc_pbuf_generic *mpsc_pbuf_claim(struct mpsc_pbuf_buffer *buffer)
 {
 	union mpsc_pbuf_generic *item;
 	bool cont;
+	bool need_post = false;
 
 	do {
 		uint32_t a;
@@ -562,6 +563,7 @@ const union mpsc_pbuf_generic *mpsc_pbuf_claim(struct mpsc_pbuf_buffer *buffer)
 				      idx_inc(buffer, buffer->tmp_rd_idx, inc);
 				rd_idx_inc(buffer, inc);
 				cont = true;
+				need_post = true;
 			} else {
 				item->hdr.busy = 1;
 				buffer->tmp_rd_idx =
@@ -576,6 +578,10 @@ const union mpsc_pbuf_generic *mpsc_pbuf_claim(struct mpsc_pbuf_buffer *buffer)
 		k_spin_unlock(&buffer->lock, key);
 	} while (cont);
 
+	if (IS_ENABLED(CONFIG_MULTITHREADING) && need_post && item == NULL) {
+		k_sem_give(&buffer->sem);
+	}
+
 	return item;
 }