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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH] semaphore: Add might_sleep() to down_*() family
To:     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>
Message-ID: <48cddad4-0388-ae8b-f98b-1629b9ae590a@redhat.com>
Date:   Sun, 8 Aug 2021 23:01:07 -0400
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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.
>
> Previously discussed with Peter Zijlstra, see link:
>   https://lore.kernel.org/lkml/20210806082320.GD22037@worktop.programming.kicks-ass.net
>
> Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
> ---
>   kernel/locking/semaphore.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> index 9aa855a96c4a..9ee381e4d2a4 100644
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -54,6 +54,7 @@ void down(struct semaphore *sem)
>   {
>   	unsigned long flags;
>   
> +	might_sleep();
>   	raw_spin_lock_irqsave(&sem->lock, flags);
>   	if (likely(sem->count > 0))
>   		sem->count--;
> @@ -77,6 +78,7 @@ int down_interruptible(struct semaphore *sem)
>   	unsigned long flags;
>   	int result = 0;
>   
> +	might_sleep();
>   	raw_spin_lock_irqsave(&sem->lock, flags);
>   	if (likely(sem->count > 0))
>   		sem->count--;
> @@ -103,6 +105,7 @@ int down_killable(struct semaphore *sem)
>   	unsigned long flags;
>   	int result = 0;
>   
> +	might_sleep();
>   	raw_spin_lock_irqsave(&sem->lock, flags);
>   	if (likely(sem->count > 0))
>   		sem->count--;
> @@ -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.

Cheers,
Longman