Re: [PATCH 1/1] perf: Prevent race in PERF_SAMPLE_READ group format sample output

[Peter Zijlstra <peterz@infradead.org>]

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On Wed, Jun 25, 2014 at 08:44:35PM +0200, Jiri Olsa wrote:
> From: Jiri Olsa <jolsa@redhat.com>
> 
> While iterating siblings in perf_output_read_group we could
> race with addition and removal of sibling in perf_group_attach
> and perf_group_detach respective.

So why would anybody do this?

> While in perf_output_read_group we are under active context,
> so the only sibling_list modification could come via IPI in:
>   perf_install_in_context or perf_remove_from_context
> 
> Disable interrupts before iterating siblings to prevent
> this race.
> 
> Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
> Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  kernel/events/core.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a33d9a2b..66649d3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4509,6 +4509,7 @@ static void perf_output_read_group(struct perf_output_handle *handle,
>  {
>  	struct perf_event *leader = event->group_leader, *sub;
>  	u64 read_format = event->attr.read_format;
> +	unsigned long flags;
>  	u64 values[5];
>  	int n = 0;
>  
> @@ -4529,6 +4530,15 @@ static void perf_output_read_group(struct perf_output_handle *handle,
>  
>  	__output_copy(handle, values, n * sizeof(u64));
>  
> +	/*
> +	 * We are now under active context, so the only sibling_list
> +	 * modification could come via IPI in:
> +	 *   perf_install_in_context and perf_remove_from_context
> +	 *
> +	 * Disable interrupts to prevent this race.
> +	 */
> +	local_irq_save(flags);

I think this is too late; you want it right at the beginning, before we
read ->nr_siblings, as that is also changed by
add_event_to_ctx()->perf_group_attach().

That said; it would be nice not to have to poke at the interrupt flag,
its expensive.

So is this really a problem, or just a case of: if you do silly things,
you get silly results?

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