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 16:20:35 -0800
From:   Tyrel Datwyler <tyreld@...ux.ibm.com>
To:     Brian King <brking@...ux.vnet.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 12/4/20 1:35 PM, Brian King wrote:
> 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. 
> 

Yeah, this will also get its own lock and flags saved in the per-queue struct in
the next spin.

-Tyrel

>>  }
>>  
>>  /**
>>
> 
>