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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH] semaphore: Add might_sleep() to down_*() family
To:     Thomas Gleixner <tglx@linutronix.de>,
        Waiman Long <llong@redhat.com>,
        Xiaoming Ni <nixiaoming@huawei.com>,
        linux-kernel@vger.kernel.org, peterz@infradead.org,
        mingo@redhat.com, will@kernel.org, boqun.feng@gmail.com
Cc:     wangle6@huawei.com, xiaoqian9@huawei.com, shaolexi@huawei.com
References: <20210809021215.19991-1-nixiaoming@huawei.com>
 <48cddad4-0388-ae8b-f98b-1629b9ae590a@redhat.com> <877dgpp852.ffs@tglx>
Message-ID: <41bddbee-948a-987c-eb72-108a465a7082@redhat.com>
Date:   Fri, 13 Aug 2021 14:47:35 -0400
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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