Re: [PATCH] rseq: Protect event mask against membarrier IPI

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-08-23 08:31, Thomas Gleixner wrote:
> On Wed, Aug 20 2025 at 09:23, Mathieu Desnoyers wrote:
>> On 2025-08-13 11:02, Thomas Gleixner wrote:
>> We should also update this comment in include/linux/sched.h:
>>
>>           /*
>>            * RmW on rseq_event_mask must be performed atomically
>>            * with respect to preemption.
>>            */
>>           unsigned long rseq_event_mask;
>>
>> to e.g.:
>>
>> /*
>>    * RmW on rseq_event_mask must be performed atomically
>>    * with respect to preemption and membarrier IPIs.
>>    */
> 
> True.
> 
>>> +
>>>    /*
>>>     * Map the event mask on the user-space ABI enum rseq_cs_flags
>>>     * for direct mask checks.
>>> @@ -41,9 +47,8 @@ static inline void rseq_handle_notify_re
>>>    static inline void rseq_signal_deliver(struct ksignal *ksig,
>>>    				       struct pt_regs *regs)
>>>    {
>>> -	preempt_disable();
>>> -	__set_bit(RSEQ_EVENT_SIGNAL_BIT, &current->rseq_event_mask);
>>> -	preempt_enable();
>>> +	scoped_guard(RSEQ_EVENT_GUARD)
>>> +		__set_bit(RSEQ_EVENT_SIGNAL_BIT, &current->rseq_event_mask);
>>
>> Then we have more to worry about interaction of the following
>> rseq events with membarrier IPI:
>>
>> - rseq_preempt, rseq_migrate, rseq_signal_deliver.
>>
>> Both rseq_preempt and rseq_migrate are documented as only being required
>> to be called with preempt off, not irq off.
> 
> They are always invoked from the middle of the scheduler with interrupts
> disabled, so just the documentation is wrong.

OK

> 
>> Can we just move the event sources requiring preempt-off to their own
>> word, and use a separate word for membarrier IPI instead ? This would
>> allow us to partition the problem into two distinct states each
>> protected by their respective mechanism.
> 
> signal delivery can just use set_bit() which is atomic vs. the IPI no?

Good point!

> 
> But as I pointed out in the other series, we don't need that whole zoo
> of event bits at all. There is absolutely no point.
> 
> signal delivery does not need to set an event in the first place. It can
> just unconditionally invoke this stuff, no?

Yes, as long as it can take a page fault from there.

Thanks,

Mathieu

> 
> Thanks,
> 
>          tglx
> 
> 


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