>
> On Wed, Dec 09, 2015 at 02:59:21PM +0000, Liang, Kan wrote:
> > > diff --git a/kernel/events/core.c b/kernel/events/core.c index
> > > 36babfd..97aa610 100644
> > > --- a/kernel/events/core.c
> > > +++ b/kernel/events/core.c
> > > @@ -3508,11 +3515,6 @@ retry:
> > > if (!ctx)
> > > goto errout;
> > >
> > > - if (task_ctx_data) {
> > > - ctx->task_ctx_data = task_ctx_data;
> > > - task_ctx_data = NULL;
> > > - }
> > > -
> > > err = 0;
> > > mutex_lock(&task->perf_event_mutex);
> > > /*
> > > @@ -3526,6 +3528,10 @@ retry:
> > > else {
> > > get_ctx(ctx);
> > > ++ctx->pin_count;
> > > + if (task_ctx_data) {
> > > + ctx->task_ctx_data = task_ctx_data;
> > > + task_ctx_data = NULL;
> > > + }
> > > rcu_assign_pointer(task->perf_event_ctxp[ctxn],
> > > ctx);
> > > }
> > > mutex_unlock(&task->perf_event_mutex);
> > >
> > >
> > > Does that make sense? No point in setting task_ctx_data if we're
> > > going to free the ctx and try again.
> >
> > The task_ctx_data will be checked before use. So it wouldn't crash the
> > system if it's NULL.
>
> Yeah, I know, I checked :-)
>
> > The problem is that LBR stack info will not be save/store on context
> > switch anymore. The user probably get wrong call stack information.
>
> Yep
>
> > May I know why you want to do that?
>
> Because this seemed like a less fragile construct. When there's multiple
> event creations racing it seems possible (ableit entirely unlikely) to assign
> the allocated task_ctx_data to a ctx that we'll delete, and on the second go
> around re-allocate a ctx, but are left wihtout task_ctx_data to assign to it.
>
> So by only assigning the task_ctx_data when we _know_ we've succeeded,
> we'll avoid this scenario.
Yes, I think it make sense to that.
Thanks,
Kan
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
