Re: [PATCH v3 1/1] vfio-ccw: Prevent quiesce function going into an infinite loop

[Farhan Ali <alifm@linux.ibm.com>]

On 04/17/2019 05:03 AM, Cornelia Huck wrote:
> On Tue, 16 Apr 2019 17:23:14 -0400
> Farhan Ali <alifm@linux.ibm.com> wrote:
> 
>> The quiesce function calls cio_cancel_halt_clear() and if we
>> get an -EBUSY we go into a loop where we:
>> 	- wait for any interrupts
>> 	- flush all I/O in the workqueue
>> 	- retry cio_cancel_halt_clear
>>
>> During the period where we are waiting for interrupts or
>> flushing all I/O, the channel subsystem could have completed
>> a halt/clear action and turned off the corresponding activity
>> control bits in the subchannel status word. This means the next
>> time we call cio_cancel_halt_clear(), we will again start by
>> calling cancel subchannel and so we can be stuck between calling
>> cancel and halt forever.
>>
>> Rather than calling cio_cancel_halt_clear() immediately after
>> waiting, let's try to disable the subchannel. If we succeed in
>> disabling the subchannel then we know nothing else can happen
>> with the device.
>>
>> Suggested-by: Eric Farman <farman@linux.ibm.com>
>> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
>> ---
>> ChangeLog:
>> v2 -> v3
>>     - Log an error message when cio_cancel_halt_clear
>>       returns EIO and break out of the loop.
>>     
>>     - Did not include past change log as the other patches
>>       of the original series have been queued by Conny.
>>       Old series (v2) can be found here:
>>       https://marc.info/?l=kvm&m=155475754101769&w=2
>>
>>   drivers/s390/cio/vfio_ccw_drv.c | 32 ++++++++++++++++++--------------
>>   1 file changed, 18 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/s390/cio/vfio_ccw_drv.c b/drivers/s390/cio/vfio_ccw_drv.c
>> index 78517aa..66a66ac 100644
>> --- a/drivers/s390/cio/vfio_ccw_drv.c
>> +++ b/drivers/s390/cio/vfio_ccw_drv.c
>> @@ -43,26 +43,30 @@ int vfio_ccw_sch_quiesce(struct subchannel *sch)
>>   	if (ret != -EBUSY)
>>   		goto out_unlock;
>>   
>> +	iretry = 255;
>>   	do {
>> -		iretry = 255;
>>   
>>   		ret = cio_cancel_halt_clear(sch, &iretry);
>> -		while (ret == -EBUSY) {
>> -			/*
>> -			 * Flush all I/O and wait for
>> -			 * cancel/halt/clear completion.
>> -			 */
>> -			private->completion = &completion;
>> -			spin_unlock_irq(sch->lock);
>>   
>> -			wait_for_completion_timeout(&completion, 3*HZ);
>> +		if (ret == -EIO) {
>> +			pr_err("vfio_ccw: could not quiesce subchannel 0.%x.%04x!\n",
>> +			       sch->schid.ssid, sch->schid.sch_no);
> 
> What about using
> 	dev_err(&sch->dev, "could not quiesce");
> instead?
> 
> (Can make that change while applying, no need to resend for that.)

Sure, the change is fine with me, dev_err would be more appropriate.

> 
>> +			break;
>> +		}
>> +
>> +		/*
>> +		 * Flush all I/O and wait for
>> +		 * cancel/halt/clear completion.
>> +		 */
>> +		private->completion = &completion;
>> +		spin_unlock_irq(sch->lock);
>>   
>> -			private->completion = NULL;
>> -			flush_workqueue(vfio_ccw_work_q);
>> -			spin_lock_irq(sch->lock);
>> -			ret = cio_cancel_halt_clear(sch, &iretry);
>> -		};
>> +		if (ret == -EBUSY)
>> +			wait_for_completion_timeout(&completion, 3*HZ);
>>   
>> +		private->completion = NULL;
>> +		flush_workqueue(vfio_ccw_work_q);
>> +		spin_lock_irq(sch->lock);
>>   		ret = cio_disable_subchannel(sch);
>>   	} while (ret == -EBUSY);
>>   out_unlock:
> 
> Otherwise, looks good to me. Will queue when I get some ack/r-b.
> 
> Thanks again for reviewing :).

Thanks
Farhan