On Tue, 12 Mar 2024 09:19:20 -0400
Steven Rostedt <rost...@goodmis.org> wrote:
> From: "Steven Rostedt (Google)" <rost...@goodmis.org>
>
> 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 ]
Oh, so `[ buffer percent ] > shortest_full` does not work because
if this happens in this order, shortest_full may be 0.
>
> 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.
>
Looks good to me.
Reviewed-by: Masami Hiramatsu (Google) <mhira...@kernel.org>
Thank you,
> Cc: sta...@vger.kernel.org
> Fixes: 42fb0a1e84ff5 ("tracing/ring-buffer: Have polling block on watermark")
> Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
> ---
> kernel/trace/ring_buffer.c | 27 ++++++++++++++++++++-------
> 1 file changed, 20 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index aa332ace108b..adfe603a769b 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -964,16 +964,32 @@ __poll_t ring_buffer_poll_wait(struct trace_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 {
> - poll_wait(filp, &rbwork->waiters, poll_table);
> - rbwork->waiters_pending = true;
> + 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
> * checking if the ring buffer is empty. Once the waiters_pending bit
> @@ -989,9 +1005,6 @@ __poll_t ring_buffer_poll_wait(struct trace_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;
> --
> 2.43.0
>
>
--
Masami Hiramatsu (Google) <mhira...@kernel.org>
