On Mon, Oct 19, 2015 at 2:12 AM, Peter Zijlstra <[email protected]> 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 <[email protected]> > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
