Re: [PATCH] perf/core: fix RCU issues with cgroup monitoring mode

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Oct 17, 2015 at 03:28:11AM +0200, Stephane Eranian wrote:
> 
> This patch eliminates all known RCU violations detected
> by the RCU  checker (PROVE_RCU). The impact code paths
> were all related to cgroup mode monitoring and involved
> access a task's cgrp.

But were they right? This patch provides no clues.

> Signed-off-by: Stephane Eranian <eranian@google.com>
> ---

> @@ -2094,6 +2104,7 @@ static int  __perf_install_in_context(void *info)
>  		cpuctx->task_ctx = task_ctx;
>  		task = task_ctx->task;
>  	}
> +	rcu_read_lock();
>  
>  	cpu_ctx_sched_out(cpuctx, EVENT_ALL);
>  
> @@ -2112,6 +2123,8 @@ static int  __perf_install_in_context(void *info)
>  	 */
>  	perf_event_sched_in(cpuctx, task_ctx, task);
>  
> +	rcu_read_unlock();
> +
>  	perf_pmu_enable(cpuctx->ctx.pmu);
>  	perf_ctx_unlock(cpuctx, task_ctx);
>  
> @@ -2398,7 +2411,9 @@ static void ctx_sched_out(struct perf_event_context *ctx,
>  		return;
>  
>  	update_context_time(ctx);
> +	rcu_read_lock();
>  	update_cgrp_time_from_cpuctx(cpuctx);
> +	rcu_read_unlock();
>  	if (!ctx->nr_active)
>  		return;
>  

And these impact !cgroup code, and the last looks like it should be
inside update_cgrp_time_from_cpuctx() if at all.