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Subject: Re: [RFC][PATCH 5/9] perf: Simplify and fix
 __perf_install_in_context
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org, Stephane Eranian <eranian@google.com>
In-Reply-To: <20110409192141.870894224@chello.nl>
References: <20110409191739.813727025@chello.nl>
	 <20110409192141.870894224@chello.nl>
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Date: Sun, 10 Apr 2011 10:13:05 +0200
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On Sat, 2011-04-09 at 21:17 +0200, Peter Zijlstra wrote:
> +       if (task_ctx) {
> +               task_ctx_sched_out(task_ctx);
> +               /*
> +                * If the context we're installing events in is not the
> +                * active task_ctx, flip them.
> +                */
> +               if (ctx->task && task_ctx != ctx) {
> +                       raw_spin_unlock(&cpuctx->ctx.lock);
> +                       raw_spin_lock(&ctx->lock);
> +                       cpuctx->task_ctx = task_ctx = ctx;
> +               }
> +               task = task_ctx->task;
> +       } 

That is actually buggy, it should read something like:

	if (task_ctx)
		task_ctx_sched_out(task_ctx);

	if (ctx->task && task_ctx != ctx) {
		raw_spin_unlock(&task_ctx->lock);
		raw_spin_lock(&ctx->lock);
		cpuctx->task_ctx = task_ctx = ctx;
	}

	if (task_ctx)
		task = task_ctx->task;

Aside from the trivial locking bug fixed, the previous version wouldn't
actually deal with installing a task_ctx where there was none before.