linux-kernel - Re: [PATCH v2 1/2] ring-buffer: Fix full_waiters

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Date: Wed, 13 Mar 2024 00:22:10 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] ring-buffer: Fix full_waiters_pending in poll

On Tue, 12 Mar 2024 09:19:20 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.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) <mhiramat@...nel.org>

Thank you,


> Cc: stable@...r.kernel.org
> Fixes: 42fb0a1e84ff5 ("tracing/ring-buffer: Have polling block on watermark")
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.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) <mhiramat@...nel.org>