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

[Stephane Eranian]

Peter,

On Sat, Oct 17, 2015 at 2:56 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> 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.
>
I am assuming that is the checker detects something suspicious there is likely
a problem.

Take for instance:
 perf_cgroup_sched_out()->perf_cgroup_from_task() ->task_subsys_state()

That one fires the checker. I think because we are accessing the css
state without
protection.

The other places are similar.

I am also trying to avoid double taking the rcu-read-lock(), so I have moved
the rcu_read_lock() out of some functions to place them in the caller. The
perf_cgroup_switch() is a performance sensitive path.

If this is not the way to fix this, let me know and I'll rewrite the patch.
Thanks.




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