Re: [PATCH RFC] ptr_ring: fix race conditions when resizing

[Jason Wang]

On 2017å02æ11æ 06:31, Michael S. Tsirkin wrote:
> Resizing currently drops consumer lock.  This can cause entries to be
> reordered, which isn't good in itself.  More importantly, consumer can
> detect a false ring empty condition and block forever.
>
> Further, nesting of consumer within producer lock is problematic for
> tun, since it produces entries in an interrupt, which causes a

In an bh actually?

Except for this.

Acked-by: Jason Wang <jasowang@xxxxxxxxxx>

> lock order reversal:
>
>         CPU0                    CPU1
>         ----                    ----
>    consume:
>    lock(&(&r->consumer_lock)->rlock);
>                                 resize:
>                                 local_irq_disable();
>                                 lock(&(&r->producer_lock)->rlock);
>                                 lock(&(&r->consumer_lock)->rlock);
>    <Interrupt>
>    produce:
>    lock(&(&r->producer_lock)->rlock);
>
> To fix, nest producer lock within consumer lock during resize,
> and keep consumer lock during the whole swap operation.
>
> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>   include/linux/ptr_ring.h | 36 +++++++++++++++++++++++++++++++-----
>   1 file changed, 31 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
> index 2052011..6c70444 100644
> --- a/include/linux/ptr_ring.h
> +++ b/include/linux/ptr_ring.h
> @@ -111,6 +111,11 @@ static inline int __ptr_ring_produce(struct ptr_ring *r, void *ptr)
>   	return 0;
>   }
>   
> +/*
> + * Note: resize (below) nests producer lock within consumer lock, so if you
> + * consume in interrupt or BH context, you must disable interrupts/BH when
> + * calling this.
> + */
>   static inline int ptr_ring_produce(struct ptr_ring *r, void *ptr)
>   {
>   	int ret;
> @@ -242,6 +247,11 @@ static inline void *__ptr_ring_consume(struct ptr_ring *r)
>   	return ptr;
>   }
>   
> +/*
> + * Note: resize (below) nests producer lock within consumer lock, so if you
> + * call this in interrupt or BH context, you must disable interrupts/BH when
> + * producing.
> + */
>   static inline void *ptr_ring_consume(struct ptr_ring *r)
>   {
>   	void *ptr;
> @@ -357,7 +367,7 @@ static inline void **__ptr_ring_swap_queue(struct ptr_ring *r, void **queue,
>   	void **old;
>   	void *ptr;
>   
> -	while ((ptr = ptr_ring_consume(r)))
> +	while ((ptr = __ptr_ring_consume(r)))
>   		if (producer < size)
>   			queue[producer++] = ptr;
>   		else if (destroy)
> @@ -372,6 +382,12 @@ static inline void **__ptr_ring_swap_queue(struct ptr_ring *r, void **queue,
>   	return old;
>   }
>   
> +/*
> + * Note: producer lock is nested within consumer lock, so if you
> + * resize you must make sure all uses nest correctly.
> + * In particular if you consume ring in interrupt or BH context, you must
> + * disable interrupts/BH when doing so.
> + */
>   static inline int ptr_ring_resize(struct ptr_ring *r, int size, gfp_t gfp,
>   				  void (*destroy)(void *))
>   {
> @@ -382,17 +398,25 @@ static inline int ptr_ring_resize(struct ptr_ring *r, int size, gfp_t gfp,
>   	if (!queue)
>   		return -ENOMEM;
>   
> -	spin_lock_irqsave(&(r)->producer_lock, flags);
> +	spin_lock_irqsave(&(r)->consumer_lock, flags);
> +	spin_lock(&(r)->producer_lock);
>   
>   	old = __ptr_ring_swap_queue(r, queue, size, gfp, destroy);
>   
> -	spin_unlock_irqrestore(&(r)->producer_lock, flags);
> +	spin_unlock(&(r)->producer_lock);
> +	spin_unlock_irqrestore(&(r)->consumer_lock, flags);
>   
>   	kfree(old);
>   
>   	return 0;
>   }
>   
> +/*
> + * Note: producer lock is nested within consumer lock, so if you
> + * resize you must make sure all uses nest correctly.
> + * In particular if you consume ring in interrupt or BH context, you must
> + * disable interrupts/BH when doing so.
> + */
>   static inline int ptr_ring_resize_multiple(struct ptr_ring **rings, int nrings,
>   					   int size,
>   					   gfp_t gfp, void (*destroy)(void *))
> @@ -412,10 +436,12 @@ static inline int ptr_ring_resize_multiple(struct ptr_ring **rings, int nrings,
>   	}
>   
>   	for (i = 0; i < nrings; ++i) {
> -		spin_lock_irqsave(&(rings[i])->producer_lock, flags);
> +		spin_lock_irqsave(&(rings[i])->consumer_lock, flags);
> +		spin_lock(&(rings[i])->producer_lock);
>   		queues[i] = __ptr_ring_swap_queue(rings[i], queues[i],
>   						  size, gfp, destroy);
> -		spin_unlock_irqrestore(&(rings[i])->producer_lock, flags);
> +		spin_unlock(&(rings[i])->producer_lock);
> +		spin_unlock_irqrestore(&(rings[i])->consumer_lock, flags);
>   	}
>   
>   	for (i = 0; i < nrings; ++i)