RE: [rfc] x86, bts: fix crash

Metzger, Markus T
2009-03-30T07:25:07Z

>-----Original Message-----
>From: Oleg Nesterov [mailto:oleg@redhat.com]
>Sent: Friday, March 27, 2009 10:30 PM
>To: Markus Metzger

>> Regarding the race on task->thread.ds_ctx between ds_release_bts() and
>> ds_switch_to(), how would I prevent a task from being rescheduled for
>> a small amount of time?
>
>I don't see how we can do this. We can split wait_task_inactive() into
>2 functions, the first one returns with task_rq_lock() held and interrupts
>disabled. But this is nasty, and in any case wait_task_inactive(p) can't
>force "p" to be deactivated.
>
>Can't we do something different?
>
>For simplicity, let's suppose that we have only task_struct->bts and it
>is just a blob of memory which can be used by CPU somehow.
>
>First, we add "struct rcu_head" into task_struct->bts, and then
>
> void free_bts((struct rcu_head *rcu)
> {
> struct bts_tracer *bts = container_of();
> ...
> kfree(bts);
> }
>
> void ds_release_bts(struct bts_tracer *tracer)
> {
> struct task_struct *child = tracer->ds.context->task;
> struct bts_tracer *bts = child->bts;
>
> child->bts = NULL;
>
> // make sure child will NOT use ->bts
> // after the next context switch,
> // clear TIF_DS_AREA_MSR or something
> ...

But that's exactly the problem if the bts_tracer is released by another
task (!= current).
We first need to disable tracing by clearing bits in debugctlmsr.
Then, we can set ->bts to NULL and clear TIF_DS_AREA_MSR.

If the traced task is currently in a context switch, clearing debugctlmsr bits
might come too late. We need to wait for the next context switch until we can
clear TIF_DS_AREA_MSR.

The benefit would be that I don't need to hook into do_exit() anymore.
This would rid us of the nasty ->ptraced loop.
I will give it a try.

I use something like this to wait for the context switch:
nvcsw = task->nvcsw + 1;
nivcsw = task->nivcsw + 1;
for (;;) {
if (nvcsw < task->nvcsw)
break;
if (nivcsw < task->nivcsw)
break;
if (task->state != TASK_RUNNING)
break;
}

I would need to check for overflows, but for my examples, I don't expect any.

regards,
markus.

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