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.
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