ring-buffer: Reuse rb_watermark_hit() for the poll logic

The check for knowing if the poll should wait or not is basically the
exact same logic as rb_watermark_hit(). The only difference is that
rb_watermark_hit() also handles the !full case. But for the full case, the
logic is the same. Just call that instead of duplicating the code in
ring_buffer_poll_wait().

Link: https://lore.kernel.org/linux-trace-kernel/20240312131952.802267543@goodmis.org

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/ring_buffer.c

@@ -960,25 +960,18 @@ __poll_t ring_buffer_poll_wait(struct trace_buffer *buffer, int cpu,
 	}
 
 	if (full) {
-		unsigned long flags;
-
 		poll_wait(filp, &rbwork->full_waiters, poll_table);
 
-		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
-		if (!cpu_buffer->shortest_full ||
-		    cpu_buffer->shortest_full > full)
-			cpu_buffer->shortest_full = full;
-		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
-		if (full_hit(buffer, cpu, full))
+		if (rb_watermark_hit(buffer, cpu, full))
 			return EPOLLIN | EPOLLRDNORM;
 		/*
 		 * Only allow full_waiters_pending update to be seen after
-		 * the shortest_full is set. If the writer sees the
-		 * full_waiters_pending flag set, it will compare the
-		 * amount in the ring buffer to shortest_full. If the amount
-		 * in the ring buffer is greater than the shortest_full
-		 * percent, it will call the irq_work handler to wake up
-		 * this list. The irq_handler will reset shortest_full
+		 * the shortest_full is set (in rb_watermark_hit). If the
+		 * writer sees the full_waiters_pending flag set, it will
+		 * compare the amount in the ring buffer to shortest_full.
+		 * If the amount in the ring buffer is greater than the
+		 * shortest_full percent, it will call the irq_work handler
+		 * to wake up this list. The irq_handler will reset shortest_full
 		 * back to zero. That's done under the reader_lock, but
 		 * the below smp_mb() makes sure that the update to
 		 * full_waiters_pending doesn't leak up into the above.