Re: [kvm-unit-tests PATCH 1/6] s390x: Use interrupts in SCLP and add locking

[Janosch Frank <frankja@linux.ibm.com>]

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On 8/30/19 2:21 PM, David Hildenbrand wrote:
> On 29.08.19 14:14, Janosch Frank wrote:
>> We need to properly implement interrupt handling for SCLP, because on
>> z/VM and LPAR SCLP calls are not synchronous!
>>
>> Also with smp CPUs have to compete for sclp. Let's add some locking,
>> so they execute sclp calls in an orderly fashion and don't compete for
>> the data buffer.

[...]

>> +
>> +void sclp_mark_busy(void)
>> +{
>> +	/*
>> +	 * With multiple CPUs we might need to wait for another CPU's
>> +	 * request before grabbing the busy indication.
>> +	 */
>> +retry_wait:
>> +	sclp_wait_busy();
>> +	spin_lock(&sclp_lock);
>> +	if (sclp_busy) {
>> +		spin_unlock(&sclp_lock);
>> +		goto retry_wait;
>> +	}
>> +	sclp_busy = true;
>> +	spin_unlock(&sclp_lock);
> 
> while (true) {
> 	sclp_wait_busy();
> 	spin_lock(&sclp_lock);
> 	if (!sclp_busy) {
> 		sclp_busy = true
> 		spin_unlock(&sclp_lock);
> 		break;
> 	}
> 	spin_unlock(&sclp_lock);
> }
> 
> Or can we simply switch to an atomic_t for sclp_busy and implement
> cmpxchg using __sync_bool_compare_and_swap/ __sync_val_compare_and_swap ?
> 
> I guess then we can drop the lock. But maybe I am missing something :)

If you want to add it and it works, I'm open for it :)
Until then I'm taking your suggestion.

> 
>> +}
[...]
>>  extern char _sccb[];
>> +void sclp_handle_ext(void);
>> +void sclp_wait_busy(void);
>> +void sclp_mark_busy(void);
> 
> I wonder if we can find better names ...
> 
> sclp_prepare()
> sclp_finalize()
> 
> or sth like that.

Hmm, IMHO my names are a bit better, since they clearly state, that we
wait until the other user has finished.



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