Re: [PATCH] semaphore: Add might_sleep() to down_*() family

[Waiman Long <llong@redhat.com>]

On 8/13/21 1:27 PM, Thomas Gleixner wrote:
> On Sun, Aug 08 2021 at 23:01, Waiman Long wrote:
>> On 8/8/21 10:12 PM, Xiaoming Ni wrote:
>>> Semaphore is sleeping lock. Add might_sleep() to down*() family
>>> (with exception of down_trylock()) to detect atomic context sleep.
>>> @@ -157,6 +160,7 @@ int down_timeout(struct semaphore *sem, long timeout)
>>>    	unsigned long flags;
>>>    	int result = 0;
>>>    
>>> +	might_sleep();
>>>    	raw_spin_lock_irqsave(&sem->lock, flags);
>>>    	if (likely(sem->count > 0))
>>>    		sem->count--;
>> I think it is simpler to just put a "might_sleep()" in __down_common()
>> which is the function where sleep can actually happen.
> No. Putting it in __down_common() is wrong, because that covers only the
> contended case.
>
> But we want to cover the potential sleep, i.e. checking even in the
> non-contended case, which is what might_sleep() is about.

You are right. Thanks for the correction.

Cheers,
Longman