Messages in this thread | | | Date | Mon, 26 Oct 2015 12:27:05 -0700 | Subject | Re: [PATCH] perf/core: fix RCU issues with cgroup monitoring mode | From | Stephane Eranian <> |
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On Mon, Oct 19, 2015 at 2:12 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Mon, Oct 19, 2015 at 12:58:47AM -0700, Stephane Eranian wrote: > > Peter, > > > > On Sat, Oct 17, 2015 at 2:56 AM, Peter Zijlstra <peterz@infradead.org> 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. > > But perf_cgroup_attach()->perf_cgroup_switch() takes ctx->lock(). > > Therefore; if you hold ctx->lock, the cgroup is pinned. > Ok, that one was a bad example because yes, it grabs the ctx lock and the rcu_lock() already.
But the other path:
__perf_event_task_sched_out() -> perf_cgroup_sched_out() -> perf_cgroup_switch() is accessing in perf_cgroup_sched_out() task_subsys_state() without ctx->lock() or rcu_read lock. Same thing on the sched_in path.
The one place where we already hold the ctx->lock is __perf_install_in_context(). Same thing for __perf_event_read() -> update_cgrp_time_from_event(). So yes, we'd need a way to tell this is okay, the cgroup cannot disappear.
> > And the above sequence very much holds ctx->lock. > > Right? > > So it looks to me that we should teach perf_cgroup_from_task() about > ctx->lock or something.
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