netdev - Re: [PATCH net-next v2 5/7] ibmvnic: serialize access to work queue

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Date:   Tue, 12 Jan 2021 11:56:28 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     Sukadev Bhattiprolu <sukadev@...ux.ibm.com>, netdev@...r.kernel.org
Cc:     Dany Madden <drt@...ux.ibm.com>, Lijun Pan <ljp@...ux.ibm.com>,
        Rick Lindsley <ricklind@...ux.ibm.com>
Subject: Re: [PATCH net-next v2 5/7] ibmvnic: serialize access to work queue

On Tue, 2021-01-12 at 10:14 -0800, Sukadev Bhattiprolu wrote:
> The work queue is used to queue reset requests like CHANGE-PARAM or
> FAILOVER resets for the worker thread. When the adapter is being
> removed
> the adapter state is set to VNIC_REMOVING and the work queue is
> flushed
> so no new work is added. However the check for adapter being removed
> is
> racy in that the adapter can go into REMOVING state just after we
> check
> and we might end up adding work just as it is being flushed (or
> after).
> 
> Use a new spin lock, ->remove_lock to ensure that after (while) the
> work
> queue is (being) flushed, no new work is queued.
> 
> A follow-on patch will convert the existing ->state_lock from a spin
> lock
> to to a mutex to allow us to hold it for longer periods of time.
> Since
> work can be scheduled from a tasklet, we cannot block on the mutex,
> so
> use a new spin lock.
> 
> Fixes: 6954a9e4192b ("ibmvnic: Flush existing work items before
> device removal")
> Signed-off-by: Sukadev Bhattiprolu <sukadev@...ux.ibm.com>
> ---
>  drivers/net/ethernet/ibm/ibmvnic.c | 15 ++++++++++++++-
>  drivers/net/ethernet/ibm/ibmvnic.h |  2 ++
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/ibm/ibmvnic.c
> b/drivers/net/ethernet/ibm/ibmvnic.c
> index ad551418ac63..7645df37e886 100644
> --- a/drivers/net/ethernet/ibm/ibmvnic.c
> +++ b/drivers/net/ethernet/ibm/ibmvnic.c
> @@ -2435,6 +2435,7 @@ static int ibmvnic_reset(struct ibmvnic_adapter
> *adapter,
>  			 enum ibmvnic_reset_reason reason)
>  {
>  	struct net_device *netdev = adapter->netdev;
> +	unsigned long flags;
>  	int ret;
>  
>  	if (adapter->state == VNIC_PROBING) {
> @@ -2443,6 +2444,8 @@ static int ibmvnic_reset(struct ibmvnic_adapter
> *adapter,
>  		goto err;
>  	}
>  
> +	spin_lock_irqsave(&adapter->remove_lock, flags);
> +
>  	/*
>  	 * If failover is pending don't schedule any other reset.
>  	 * Instead let the failover complete. If there is already a
> @@ -2465,8 +2468,9 @@ static int ibmvnic_reset(struct ibmvnic_adapter
> *adapter,
>  	netdev_dbg(adapter->netdev, "Scheduling reset (reason %d)\n",
> reason);
>  	schedule_work(&adapter->ibmvnic_reset);
>  
> -	return 0;
> +	ret = 0;
>  err:
> +	spin_unlock_irqrestore(&adapter->remove_lock, flags);
>  	return -ret;
>  }
>  
> @@ -5387,6 +5391,7 @@ static int ibmvnic_probe(struct vio_dev *dev,
> const struct vio_device_id *id)
>  	memset(&adapter->pending_resets, 0, sizeof(adapter-
> >pending_resets));
>  	spin_lock_init(&adapter->rwi_lock);
>  	spin_lock_init(&adapter->state_lock);
> +	spin_lock_init(&adapter->remove_lock);
>  	mutex_init(&adapter->fw_lock);
>  	init_completion(&adapter->init_done);
>  	init_completion(&adapter->fw_done);
> @@ -5467,7 +5472,15 @@ static int ibmvnic_remove(struct vio_dev *dev)
>  		return -EBUSY;
>  	}
>  
> +	/* If ibmvnic_reset() is scheduling a reset, wait for it to
> +	 * finish. Then prevent it from scheduling any more resets
> +	 * and have the reset functions ignore any resets that have
> +	 * already been scheduled.
> +	 */
> +	spin_lock_irqsave(&adapter->remove_lock, flags);
>  	adapter->state = VNIC_REMOVING;
> +	spin_unlock_irqrestore(&adapter->remove_lock, flags);
> +

Why irqsave/restore variants ? are you expecting this spinlock to be
held in interruptcontext ?

>  	spin_unlock_irqrestore(&adapter->state_lock, flags);
>  
>  	flush_work(&adapter->ibmvnic_reset);
> diff --git a/drivers/net/ethernet/ibm/ibmvnic.h
> b/drivers/net/ethernet/ibm/ibmvnic.h
> index 1179a95a3f92..2779696ade09 100644
> --- a/drivers/net/ethernet/ibm/ibmvnic.h
> +++ b/drivers/net/ethernet/ibm/ibmvnic.h
> @@ -1081,6 +1081,8 @@ struct ibmvnic_adapter {
>  	spinlock_t rwi_lock;
>  	enum ibmvnic_reset_reason pending_resets[VNIC_RESET_MAX-1];
>  	short next_reset;
> +	/* serialize ibmvnic_reset() and ibmvnic_remove() */
> +	spinlock_t remove_lock;
>  	struct work_struct ibmvnic_reset;
>  	struct delayed_work ibmvnic_delayed_reset;
>  	unsigned long resetting;