LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Peter Zijlstra <peterz@infradead.org>
To: Yabin Cui <yabinc@google.com>
Cc: Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head.
Date: Wed, 15 May 2019 12:48:24 +0200	[thread overview]
Message-ID: <20190515104824.GD2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190515003059.23920-1-yabinc@google.com>

On Tue, May 14, 2019 at 05:30:59PM -0700, Yabin Cui wrote:
> In perf_output_put_handle(), an IRQ/NMI can happen in below location and
> write records to the same ring buffer:
> 	...
> 	local_dec_and_test(&rb->nest)
> 	...                          <-- an IRQ/NMI can happen here
> 	rb->user_page->data_head = head;
> 	...
> 
> In this case, a value A is written to data_head in the IRQ, then a value
> B is written to data_head after the IRQ. And A > B. As a result,
> data_head is temporarily decreased from A to B. And a reader may see
> data_head < data_tail if it read the buffer frequently enough, which
> creates unexpected behaviors.

Indeed, good catch! Have you observed this, or is this patch due to code
inspection?

> This can be fixed by moving dec(&rb->nest) to after updating data_head,
> which prevents the IRQ/NMI above from updating data_head.
> 
> Signed-off-by: Yabin Cui <yabinc@google.com>
> ---
>  kernel/events/ring_buffer.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> index 674b35383491..0b9aefe13b04 100644
> --- a/kernel/events/ring_buffer.c
> +++ b/kernel/events/ring_buffer.c
> @@ -54,8 +54,10 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
>  	 * IRQ/NMI can happen here, which means we can miss a head update.
>  	 */
>  
> -	if (!local_dec_and_test(&rb->nest))
> +	if (local_read(&rb->nest) > 1) {
> +		local_dec(&rb->nest);
>  		goto out;
> +	}
>  
>  	/*
>  	 * Since the mmap() consumer (userspace) can run on a different CPU:
> @@ -86,6 +88,13 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
>  	smp_wmb(); /* B, matches C */
>  	rb->user_page->data_head = head;

At the very least this needs:

	barrier();

> +	/*
> +	 * Clear rb->nest after updating data_head. This prevents IRQ/NMI from
> +	 * updating data_head before us. If that happens, we will expose a
> +	 * temporarily decreased data_head.
> +	 */
> +	local_set(&rb->nest, 0);

Since you rely on the 'decrement' (1->0) to happen after we've written
head.

And similarly, you need:

	barrier();

Because we must re-check the head after we've 'decremented'.

>  	/*
>  	 * Now check if we missed an update -- rely on previous implied
>  	 * compiler barriers to force a re-read.


A more paranoid person might've written:

	WARN_ON_ONCE(local_cmpxchg(&rb->nest, 1, 0) != 1);

which would've also implied both barrier()s.


  parent reply	other threads:[~2019-05-15 10:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-15  0:30 Yabin Cui
2019-05-15  6:51 ` Alexander Shishkin
2019-05-15  9:43   ` Peter Zijlstra
2019-05-15 10:04     ` Alexander Shishkin
2019-05-15 10:51       ` Peter Zijlstra
2019-05-15 10:48 ` Peter Zijlstra [this message]
2019-05-16 18:37   ` Yabin Cui
2019-05-16 18:40 ` [PATCH v2] " Yabin Cui
2019-05-17  8:47   ` Peter Zijlstra
2019-05-17 17:59     ` [PATCH] " Yabin Cui

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190515104824.GD2623@hirez.programming.kicks-ass.net \
    --to=peterz@infradead.org \
    --cc=acme@kernel.org \
    --cc=alexander.shishkin@linux.intel.com \
    --cc=jolsa@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=namhyung@kernel.org \
    --cc=yabinc@google.com \
    --subject='Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head.' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).