linux-kernel - Re: [PATCH] semaphore: Add might_sleep() to down

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Message-ID: <877dgpp852.ffs@tglx>
Date:   Fri, 13 Aug 2021 19:27:37 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Waiman Long <llong@...hat.com>,
        Xiaoming Ni <nixiaoming@...wei.com>,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, will@...nel.org, boqun.feng@...il.com
Cc:     wangle6@...wei.com, xiaoqian9@...wei.com, shaolexi@...wei.com
Subject: Re: [PATCH] semaphore: Add might_sleep() to down_*() family

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.

Thanks,

        tglx