Re: [PATCH v4 07/16] locking/rwsem: Implement lock handoff to prevent lock starvation

[Waiman Long]

On 04/16/2019 12:15 PM, Peter Zijlstra wrote:
> On Tue, Apr 16, 2019 at 05:49:37PM +0200, Peter Zijlstra wrote:
>> See, if you first write that function in the form:
>>
>> 	long new;
>>
>> 	do {
>> 		new = count | RWSEM_WRITER_LOCKED;
>>
>> 		if (count & RWSEM_LOCK_MASK)
>> 			return false;
>>
>> 		if (list_is_singular(&sem->wait_list))
>> 			new &= ~RWSEM_FLAG_WAITERS;
>>
>> 	} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
>>
>> 	rwsem_set_owner(sem);
>> 	return true;
>>
>> And then add the HANDOFF bits like:
>>
>> 	long new;
>>
>> 	do {
>> +		bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
>>
>> +		new = (count | RWSEM_WRITER_LOCKED) & ~RWSEM_FLAG_HANDOFF;
>>
>> 		if (count & RWSEM_LOCK_MASK) {
>> +			if (has_handoff && wstate != WRITER_HANDOFF)
>> +				return false;
>> 			new |= RWSEM_FLAG_HANDOFF;
>> 		}
>>
>> +		if (has_handoff && wstate == WRITER_NOT_FIRST)
>> +			return false;
>>
>> 		if (list_is_singular(&sem->wait_list))
>> 			new &= ~RWSEM_FLAG_WAITERS;
>>
>> 	} while (atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
> obviously that should be !
>

Right.

-Longman