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

[Peter Zijlstra <peterz@infradead.org>]

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.

Thanks!