linux-kernel - Re: [PATCH v3 18/18] ibmvfc: drop host lock when completing commands in CRQ

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Date:   Fri, 4 Dec 2020 15:35:25 -0600
From:   Brian King <brking@...ux.vnet.ibm.com>
To:     Tyrel Datwyler <tyreld@...ux.ibm.com>,
        james.bottomley@...senpartnership.com
Cc:     martin.petersen@...cle.com, linux-scsi@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        brking@...ux.ibm.com
Subject: Re: [PATCH v3 18/18] ibmvfc: drop host lock when completing commands
 in CRQ

On 12/2/20 8:08 PM, Tyrel Datwyler wrote:
> The legacy CRQ holds the host lock the even while completing commands.
> This presents a problem when in legacy single queue mode and
> nr_hw_queues is greater than one since calling scsi_done() introduces
> the potential for deadlock.
> 
> If nr_hw_queues is greater than one drop the hostlock in the legacy CRQ
> path when completing a command.
> 
> Signed-off-by: Tyrel Datwyler <tyreld@...ux.ibm.com>
> ---
>  drivers/scsi/ibmvscsi/ibmvfc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/ibmvscsi/ibmvfc.c b/drivers/scsi/ibmvscsi/ibmvfc.c
> index e499599662ec..e2200bdff2be 100644
> --- a/drivers/scsi/ibmvscsi/ibmvfc.c
> +++ b/drivers/scsi/ibmvscsi/ibmvfc.c
> @@ -2969,6 +2969,7 @@ static void ibmvfc_handle_crq(struct ibmvfc_crq *crq, struct ibmvfc_host *vhost)
>  {
>  	long rc;
>  	struct ibmvfc_event *evt = (struct ibmvfc_event *)be64_to_cpu(crq->ioba);
> +	unsigned long flags;
>  
>  	switch (crq->valid) {
>  	case IBMVFC_CRQ_INIT_RSP:
> @@ -3039,7 +3040,12 @@ static void ibmvfc_handle_crq(struct ibmvfc_crq *crq, struct ibmvfc_host *vhost)
>  	del_timer(&evt->timer);
>  	list_del(&evt->queue);
>  	ibmvfc_trc_end(evt);
> -	evt->done(evt);
> +	if (nr_scsi_hw_queues > 1) {
> +		spin_unlock_irqrestore(vhost->host->host_lock, flags);
> +		evt->done(evt);
> +		spin_lock_irqsave(vhost->host->host_lock, flags);
> +	} else
> +		evt->done(evt);

Similar comment here as previously. The flags parameter is an output for
spin_lock_irqsave but an input for spin_unlock_irqrestore. You'll need
to rethink the locking here. You could just do a spin_unlock_irq / spin_lock_irq
here and that would probably be OK, but probably isn't the best. 

>  }
>  
>  /**
> 


-- 
Brian King
Power Linux I/O
IBM Linux Technology Center