Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state racy load

[Mathieu Desnoyers]

----- On Sep 4, 2019, at 7:49 AM, Peter Zijlstra peterz@xxxxxxxxxxxxx wrote:

> On Wed, Sep 04, 2019 at 01:28:19PM +0200, Peter Zijlstra wrote:
>> @@ -196,6 +198,17 @@ static int membarrier_register_global_expedited(void)
>>  		 */
>>  		smp_mb();
>>  	} else {
>> +		struct task_struct *g, *t;
>> +
>> +		read_lock(&tasklist_lock);
>> +		do_each_thread(g, t) {
>> +			if (t->mm == mm) {
>> +				atomic_or(MEMBARRIER_STATE_GLOBAL_EXPEDITED,
>> +					  &t->membarrier_state);
>> +			}
>> +		} while_each_thread(g, t);
>> +		read_unlock(&tasklist_lock);
>> +
>>  		/*
>>  		 * For multi-mm user threads, we need to ensure all
>>  		 * future scheduler executions will observe the new
> 
> Arguably, because this is exposed to unpriv users and a potential
> preemption latency issue, we could do it in 3 passes:
> 
>	- RCU, mark all found lacking, count
>	- RCU, mark all found lacking, count
>	- if count of last pass, tasklist_lock
> 
> That way, it becomes much harder to trigger the bad case.
> 
> Do we worry about that?

Allowing unprivileged processes to iterate over all processes/threads
with the tasklist lock held is something I try to avoid.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com