Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call() users

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call() users
Date	2016-01-13 21:50 +0100
Message-ID	<qQAwG-50b-29@gated-at.bofh.it> (permalink)
References	<qPNFD-4Vc-3@gated-at.bofh.it> <qPNFE-4Vc-25@gated-at.bofh.it> <qQvdF-1tl-39@gated-at.bofh.it> <qQybv-3tX-1@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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On Wed, Jan 13, 2016 at 07:10:16PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 13, 2016 at 05:00:50PM +0200, Alexander Shishkin wrote:
> > I think I caught one, below.
> > 
> > Peter Zijlstra <peterz@infradead.org> writes:
> > 
> > > +static int event_function(void *info)
> > > +{
> > > +	struct event_function_struct *efs = info;
> > > +	struct perf_event *event = efs->event;
> > > +	struct perf_event_context *ctx = event->ctx;
> > > +	struct perf_cpu_context *cpuctx = __get_cpu_context(ctx);
> > > +	struct perf_event_context *task_ctx = cpuctx->task_ctx;
> > > +
> > > +	WARN_ON_ONCE(!irqs_disabled());
> > > +
> > > +	/*
> > > +	 * Since we do the IPI call without holding ctx->lock things can have
> > > +	 * changed, double check we hit the task we set out to hit.
> > > +	 *
> > > +	 * If ctx->task == current, we know things must remain valid because
> > > +	 * we have IRQs disabled so we cannot schedule.
> > > +	 */
> > > +	if (ctx->task) {
> > > +		if (ctx->task != current)
> > > +			return -EAGAIN;
> > > +
> > > +		WARN_ON_ONCE(task_ctx != ctx);
> > 
> > Looks like between dropping ctx::lock in event_function_call() and here,
> > cpuctx::task_ctx may still become NULL.
> 
> Hmm yes I think you're right. If the event is being migrated to another
> context concurrently the remove_from_context() might have gone through,
> we'll still be waiting for sync_rcu and then this (say enabled) happens
> and we're looking at a 'dead' context.

Hmm, no. This cannot be, all event_function_call() users should hold
ctx->mutex.

Thread

[RFC][PATCH 12/12] perf: Collapse and fix event_function_call() users Peter Zijlstra <peterz@infradead.org> - 2016-01-11 17:40 +0100
  Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call()  users Peter Zijlstra <peterz@infradead.org> - 2016-01-13 12:00 +0100
    Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call() users Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-13 14:50 +0100
      Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call()  users Peter Zijlstra <peterz@infradead.org> - 2016-01-13 18:40 +0100
  Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call() users Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-13 16:10 +0100
    Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call() users Alexander Shishkin <alexander.shishkin@linux.intel.com> - 2016-01-13 16:50 +0100
    Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call()  users Peter Zijlstra <peterz@infradead.org> - 2016-01-13 19:20 +0100
      Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call()  users Peter Zijlstra <peterz@infradead.org> - 2016-01-13 21:50 +0100
    Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call()  users Peter Zijlstra <peterz@infradead.org> - 2016-01-14 11:50 +0100
      Re: [RFC][PATCH 12/12] perf: Collapse and fix event_function_call()  users Peter Zijlstra <peterz@infradead.org> - 2016-01-14 17:40 +0100

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