RE: [PATCH net] net: hns: Fix the stray netpoll locks causing deadlock in NAPI path

[Salil Mehta <salil.mehta@huawei.com>]

Hi Marc,

> From: Marc Zyngier [mailto:maz@kernel.org]
> Sent: Thursday, November 7, 2019 2:13 PM
> To: Salil Mehta <salil.mehta@huawei.com>
> 
> Hi Salil,
> 
> On 2019-11-06 20:03, Salil Mehta wrote:
> > This patch fixes the problem of the spin locks, originally
> > meant for the netpoll path of hns driver, causing deadlock in
> > the normal NAPI poll path. The issue happened due presence of
> > the stray leftover spin lock code related to the netpoll,
> > whose support was earlier removed from the HNS[1], got activated
> > due to enabling of NET_POLL_CONTROLLER switch.
> >
> > Earlier background:
> > The netpoll handling code originally had this bug(as identified
> > by Marc Zyngier[2]) of wrong spin lock API being used which did
> > not disable the interrupts and hence could cause locking issues.
> > i.e. if the lock were first acquired in context to thread like
> > 'ip' util and this lock if ever got later acquired again in
> > context to the interrupt context like TX/RX (Interrupts could
> > always pre-empt the lock holding task and acquire the lock again)
> > and hence could cause deadlock.
> >
> > Proposed Solution:
> > 1. If the netpoll was enabled in the HNS driver, which is not
> >    right now, we could have simply used spin_[un]lock_irqsave()
> > 2. But as netpoll is disabled, therefore, it is best to get rid
> >    of the existing locks and stray code for now. This should
> >    solve the problem reported by Marc.
> >
> > @Marc,
> > Could you please test this patch and confirm if the problem is
> > fixed at your end?
> 
> Yes, this fixes it, although you may want to fully get rid of
> the now useless lock:


Oops..missed them. I will fix these as well and float V2 version
of this patch.


> diff --git a/drivers/net/ethernet/hisilicon/hns/hnae.c
> b/drivers/net/ethernet/hisilicon/hns/hnae.c
> index 6d0457eb4faa..08339278c722 100644
> --- a/drivers/net/ethernet/hisilicon/hns/hnae.c
> +++ b/drivers/net/ethernet/hisilicon/hns/hnae.c
> @@ -199,7 +199,6 @@ hnae_init_ring(struct hnae_queue *q, struct
> hnae_ring *ring, int flags)
> 
>   	ring->q = q;
>   	ring->flags = flags;
> -	spin_lock_init(&ring->lock);
>   	ring->coal_param = q->handle->coal_param;
>   	assert(!ring->desc && !ring->desc_cb && !ring->desc_dma_addr);
> 
> diff --git a/drivers/net/ethernet/hisilicon/hns/hnae.h
> b/drivers/net/ethernet/hisilicon/hns/hnae.h
> index e9c67c06bfd2..6ab9458302e1 100644
> --- a/drivers/net/ethernet/hisilicon/hns/hnae.h
> +++ b/drivers/net/ethernet/hisilicon/hns/hnae.h
> @@ -274,9 +274,6 @@ struct hnae_ring {
>   	/* statistic */
>   	struct ring_stats stats;
> 
> -	/* ring lock for poll one */
> -	spinlock_t lock;
> -
>   	dma_addr_t desc_dma_addr;
>   	u32 buf_size;       /* size for hnae_desc->addr, preset by AE */
>   	u16 desc_num;       /* total number of desc */
> 
> With that:
> 
> Acked-by: Marc Zyngier <maz@kernel.org>
> Tested-by: Marc Zyngier <maz@kernel.org>


Many thanks for your review and testing.


Best Regards
Salil.