Re: [rfc 4.16-rt patch] crypto: cryptd - serialize RT request enqueue/dequeue with a local lock

[Mike Galbraith <efault@gmx.de>]

On Thu, 2018-07-26 at 18:58 +0200, Sebastian Andrzej Siewior wrote:
> On 2018-07-11 17:19:59 [+0200], Mike Galbraith wrote:
> > Note: patch is the result of code inspection...
> > 
> > cryptod disables preemption to provide request enqueue/dequeue exclusion,
> > use a LOCAL_IRQ_LOCK to do the same for RT, keeping preemption enabled.
> 
> What about this:

Looks fine to me, I'll ask my arm-thing box for its opinion.

> -------- >8
> 
> From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Date: Thu, 26 Jul 2018 18:52:00 +0200
> Subject: [PATCH] crypto: cryptd - add a lock instead
>  preempt_disable/local_bh_disable
> 
> cryptd has a per-CPU lock which protected with local_bh_disable() and
> preempt_disable().
> Add an explicit spin_lock to make the locking context more obvious and
> visible to lockdep. Since it is a per-CPU lock, there should be no lock
> contention on the actual spinlock.
> There is a small race-window where we could be migrated to another CPU
> after the cpu_queue has been obtain. This is not a problem because the
> actual ressource is protected by the spinlock.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  crypto/cryptd.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/crypto/cryptd.c b/crypto/cryptd.c
> index addca7bae33f..8ad657cddc0a 100644
> --- a/crypto/cryptd.c
> +++ b/crypto/cryptd.c
> @@ -39,6 +39,7 @@ MODULE_PARM_DESC(cryptd_max_cpu_qlen, "Set cryptd Max queue depth");
>  struct cryptd_cpu_queue {
>  	struct crypto_queue queue;
>  	struct work_struct work;
> +	spinlock_t qlock;
>  };
>  
>  struct cryptd_queue {
> @@ -117,6 +118,7 @@ static int cryptd_init_queue(struct cryptd_queue *queue,
>  		cpu_queue = per_cpu_ptr(queue->cpu_queue, cpu);
>  		crypto_init_queue(&cpu_queue->queue, max_cpu_qlen);
>  		INIT_WORK(&cpu_queue->work, cryptd_queue_worker);
> +		spin_lock_init(&cpu_queue->qlock);
>  	}
>  	pr_info("cryptd: max_cpu_qlen set to %d\n", max_cpu_qlen);
>  	return 0;
> @@ -141,8 +143,10 @@ static int cryptd_enqueue_request(struct cryptd_queue *queue,
>  	struct cryptd_cpu_queue *cpu_queue;
>  	atomic_t *refcnt;
>  
> -	cpu = get_cpu();
> -	cpu_queue = this_cpu_ptr(queue->cpu_queue);
> +	cpu_queue = raw_cpu_ptr(queue->cpu_queue);
> +	spin_lock_bh(&cpu_queue->qlock);
> +	cpu = smp_processor_id();
> +
>  	err = crypto_enqueue_request(&cpu_queue->queue, request);
>  
>  	refcnt = crypto_tfm_ctx(request->tfm);
> @@ -158,7 +162,7 @@ static int cryptd_enqueue_request(struct cryptd_queue *queue,
>  	atomic_inc(refcnt);
>  
>  out_put_cpu:
> -	put_cpu();
> +	spin_unlock_bh(&cpu_queue->qlock);
>  
>  	return err;
>  }
> @@ -174,16 +178,11 @@ static void cryptd_queue_worker(struct work_struct *work)
>  	cpu_queue = container_of(work, struct cryptd_cpu_queue, work);
>  	/*
>  	 * Only handle one request at a time to avoid hogging crypto workqueue.
> -	 * preempt_disable/enable is used to prevent being preempted by
> -	 * cryptd_enqueue_request(). local_bh_disable/enable is used to prevent
> -	 * cryptd_enqueue_request() being accessed from software interrupts.
>  	 */
> -	local_bh_disable();
> -	preempt_disable();
> +	spin_lock_bh(&cpu_queue->qlock);
>  	backlog = crypto_get_backlog(&cpu_queue->queue);
>  	req = crypto_dequeue_request(&cpu_queue->queue);
> -	preempt_enable();
> -	local_bh_enable();
> +	spin_unlock_bh(&cpu_queue->qlock);
>  
>  	if (!req)
>  		return;