linux-kernel - Re: [PATCH 3/7] perf: kill perf_event_context

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Date:	Tue, 25 Feb 2014 12:38:48 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mark Rutland <mark.rutland@....com>
Cc:	linux-kernel@...r.kernel.org, will.deacon@....com,
	dave.martin@....com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 3/7] perf: kill perf_event_context_type

On Mon, Feb 10, 2014 at 05:44:20PM +0000, Mark Rutland wrote:
> Currently perf_event_context::type is used to determine whether a
> context is cpu-bound or task-bound. However perf_event_context::task can
> be used to determine this just as cheaply, and requires no additional
> initialisation.
> 
> This patch removes perf_event_context::type, and modifies existing users
> to check check perf_event_context::task instead. The now unused enum
> perf_event_context_type is removed.

> @@ -7130,7 +7129,7 @@ SYSCALL_DEFINE5(perf_event_open,
>  		 * task or CPU context:
>  		 */
>  		if (move_group) {
> -			if (group_leader->ctx->type != ctx->type)
> +			if (group_leader->ctx->task != ctx->task)
>  				goto err_context;
>  		} else {
>  			if (group_leader->ctx != ctx)

That's not an equivalent statement. ctx->task (t1) != ctx->task (t2)
while they're still both of the same type.

Now I don't think you'll ever end up with different tasks in this case
so it might still work out; but you don't mention this and I'd have to
like think to make sure.
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