Re: [PATCH v3] ipc/mqueue: Avoid relying on a stack reference past its expiry

[Manfred Spraul]

Hi Varad,

On 5/7/21 3:38 PM, Varad Gautam wrote:
> @@ -1005,11 +1022,9 @@ static inline void __pipelined_op(struct wake_q_head *wake_q,
>   				  struct ext_wait_queue *this)
>   {
>   	list_del(&this->list);
> -	get_task_struct(this->task);
> -
> +	wake_q_add(wake_q, this->task);
>   	/* see MQ_BARRIER for purpose/pairing */
>   	smp_store_release(&this->state, STATE_READY);
> -	wake_q_add_safe(wake_q, this->task);
>   }
>   
>   /* pipelined_send() - send a message directly to the task waiting in

First, I was too fast: I had assumed that wake_q_add() before 
smp_store_release() would be a potential lost wakeup.

As __pipelined_op() is called within spin_lock(&info->lock), and as 
wq_sleep() will reread this->state after acquiring 
spin_lock(&info->lock), I do not see a bug anymore.

But I don't like the change: Why should ipc/*.c differ from kernel/futex.c?

--

    Manfred