linux-kernel - Re: [RFC][PATCH 5/9] perf: Simplify and fix __perf_install_in

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Date:	Mon, 11 Apr 2011 10:50:28 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC][PATCH 5/9] perf: Simplify and fix
 __perf_install_in_context

On Mon, 2011-04-11 at 16:44 +0800, Lin Ming wrote:
> On Sun, 2011-04-10 at 10:13 +0200, Peter Zijlstra wrote:
> > 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) {
		if (task_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.

Let me place your comment with the new version, as the old one is
borken ;-)

> In which case will this happen?
> 
> For task event, we have:
> 
> perf_install_in_context
>    task_function_call(task, __perf_install_in_context, event)
>       __perf_install_in_context
> 
> Doesn't this ensure that the context we're installing events is same
> with the active task_ctx?

With __ARCH_WANT_INTERRUPTS_ON_CTXSW the IPI might land before we did
perf_event_task_sched_in(), in which case we need to set the task_ctx
our-selves.
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