Commit 3f91ba96 authored Mar 12, 2024 by Steven Rostedt (Google) Committed by Greg Kroah-Hartman Apr 13, 2024

ring-buffer: Fix full_waiters_pending in poll

[ Upstream commit 8145f1c35fa648da662078efab299c4467b85ad5 ]

If a reader of the ring buffer is doing a poll, and waiting for the ring
buffer to hit a specific watermark, there could be a case where it gets
into an infinite ping-pong loop.

The poll code has:

  rbwork->full_waiters_pending = true;
  if (!cpu_buffer->shortest_full ||
      cpu_buffer->shortest_full > full)
         cpu_buffer->shortest_full = full;

The writer will see full_waiters_pending and check if the ring buffer is
filled over the percentage of the shortest_full value. If it is, it calls
an irq_work to wake up all the waiters.

But the code could get into a circular loop:

	CPU 0					CPU 1
	-----					-----
 [ Poll ]
   [ shortest_full = 0 ]
   rbwork->full_waiters_pending = true;
					  if (rbwork->full_waiters_pending &&
					      [ buffer percent ] > shortest_full) {
					         rbwork->wakeup_full = true;
					         [ queue_irqwork ]

   cpu_buffer->shortest_full = full;

					  [ IRQ work ]
					  if (rbwork->wakeup_full) {
					        cpu_buffer->shortest_full = 0;
					        wakeup poll waiters;
  [woken]
   if ([ buffer percent ] > full)
      break;
   rbwork->full_waiters_pending = true;
					  if (rbwork->full_waiters_pending &&
					      [ buffer percent ] > shortest_full) {
					         rbwork->wakeup_full = true;
					         [ queue_irqwork ]

   cpu_buffer->shortest_full = full;

					  [ IRQ work ]
					  if (rbwork->wakeup_full) {
					        cpu_buffer->shortest_full = 0;
					        wakeup poll waiters;
  [woken]

 [ Wash, rinse, repeat! ]

In the poll, the shortest_full needs to be set before the
full_pending_waiters, as once that is set, the writer will compare the
current shortest_full (which is incorrect) to decide to call the irq_work,
which will reset the shortest_full (expecting the readers to update it).

Also move the setting of full_waiters_pending after the check if the ring
buffer has the required percentage filled. There's no reason to tell the
writer to wake up waiters if there are no waiters.

Link: https://lore.kernel.org/linux-trace-kernel/20240312131952.630922155@goodmis.org



Cc: stable@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Fixes: 42fb0a1e84ff5 ("tracing/ring-buffer: Have polling block on watermark")
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent f7578a3b

kernel/trace/ring_buffer.c

+20 −7

Original line number	Diff line number	Diff line
		@@ -761,15 +761,31 @@ __poll_t ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu,
		poll_wait(filp, &rbwork->full_waiters, poll_table);

		raw_spin_lock_irqsave(&cpu_buffer->reader_lock, flags);
		rbwork->full_waiters_pending = true;
		if (!cpu_buffer->shortest_full \|\|
		cpu_buffer->shortest_full > full)
		cpu_buffer->shortest_full = full;
		raw_spin_unlock_irqrestore(&cpu_buffer->reader_lock, flags);
		} else {
		if (full_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
		* 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.
		*/
		smp_mb();
		rbwork->full_waiters_pending = true;
		return 0;
		}

		poll_wait(filp, &rbwork->waiters, poll_table);
		rbwork->waiters_pending = true;
		}

		/*
		* There's a tight race between setting the waiters_pending and
		@@ -786,9 +802,6 @@ __poll_t ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu,
		*/
		smp_mb();

		if (full)
		return full_hit(buffer, cpu, full) ? EPOLLIN \| EPOLLRDNORM : 0;

		if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) \|\|
		(cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu)))
		return EPOLLIN \| EPOLLRDNORM;