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From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: yabinc@google.com, acme@kernel.org,
	alexander.shishkin@linux.intel.com, jolsa@redhat.com,
	namhyung@kernel.org, mark.rutland@arm.com,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>
Subject: Re: [PATCH 1/4] perf/ring_buffer: Fix exposing a temporarily decreased data_head.
Date: Fri, 17 May 2019 16:26:48 +0200	[thread overview]
Message-ID: <20190517142648.GC2589@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20190517130509.GA90824@gmail.com>

On Fri, May 17, 2019 at 03:05:09PM +0200, Ingo Molnar wrote:
> 
> * Peter Zijlstra <peterz@infradead.org> 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.
> > 
> > This can be fixed by moving dec(&rb->nest) to after updating data_head,
> > which prevents the IRQ/NMI above from updating data_head.
> > 
> > Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Jiri Olsa <jolsa@redhat.com>
> > Cc: Namhyung Kim <namhyung@kernel.org>
> > Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> > Fixes: ef60777c9abd ("perf: Optimize the perf_output() path by removing IRQ-disables")
> > Signed-off-by: Yabin Cui <yabinc@google.com>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Link: https://lkml.kernel.org/r/20190516184010.167903-1-yabinc@google.com
> 
> So these are missing a bunch of:
> 
>   From: Yabin Cui <yabinc@google.com>
> 
> lines, right?
> 

The first. certainly, the rest, while inspired by his patch, is more
complete than what he did.

  reply	other threads:[~2019-05-17 14:27 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-17 11:52 [PATCH 0/4] perf ring-buffer fixes Peter Zijlstra
2019-05-17 11:52 ` [PATCH 1/4] perf/ring_buffer: Fix exposing a temporarily decreased data_head Peter Zijlstra
2019-05-17 13:05   ` Ingo Molnar
2019-05-17 14:26     ` Peter Zijlstra [this message]
2019-05-24  7:01       ` Ingo Molnar
2019-05-24  8:08   ` [tip:perf/urgent] " tip-bot for Yabin Cui
2019-05-17 11:52 ` [PATCH 2/4] perf/ring_buffer: Add ordering to rb->nest increment Peter Zijlstra
2019-05-24  8:08   ` [tip:perf/urgent] " tip-bot for Peter Zijlstra
2019-05-17 11:52 ` [PATCH 3/4] perf/ring-buffer: Always use {READ,WRITE}_ONCE() for rb->user_page data Peter Zijlstra
2019-05-24  8:09   ` [tip:perf/urgent] " tip-bot for Peter Zijlstra
2019-05-17 11:52 ` [PATCH 4/4] perf/ring-buffer: Use regular variables for nesting Peter Zijlstra
2019-05-24  8:10   ` [tip:perf/urgent] " tip-bot for Peter Zijlstra

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