Re: [PATCH v8 07/19] locking/rwsem: Implement lock handoff to prevent lock starvation

[Waiman Long <longman@redhat.com>]

On 6/4/19 5:12 AM, Boqun Feng wrote:
> On Tue, Jun 04, 2019 at 11:26:30AM +0800, Yuyang Du wrote:
>> On Tue, 4 Jun 2019 at 11:03, Yuyang Du <duyuyang@gmail.com> wrote:
>>> Hi Waiman,
>>>
>>> On Tue, 21 May 2019 at 05:01, Waiman Long <longman@redhat.com> wrote:
>>>> Because of writer lock stealing, it is possible that a constant
>>>> stream of incoming writers will cause a waiting writer or reader to
>>>> wait indefinitely leading to lock starvation.
>>>>
>>>> This patch implements a lock handoff mechanism to disable lock stealing
>>>> and force lock handoff to the first waiter or waiters (for readers)
>>>> in the queue after at least a 4ms waiting period unless it is a RT
>>>> writer task which doesn't need to wait. The waiting period is used to
>>>> avoid discouraging lock stealing too much to affect performance.
>>> I was working on a patchset to solve read-write lock deadlock
>>> detection problem (https://lkml.org/lkml/2019/5/16/93).
>>>
>>> One of the mistakes in that work is that I considered the following
>>> case as deadlock:
>> Sorry everyone, but let me rephrase:
>>
>> One of the mistakes in that work is that I considered the following
>> case as no deadlock:
>>
>>>   T1            T2
>>>   --            --
>>>
>>>   down_read1    down_write2
>>>
>>>   down_write2   down_read1
>>>
> Not sure I understand the whole context here, but isn't adding a third
> independent task makes this a deadlock?
>
> 	 T1            T2		T3
> 	 --            --		--
>
> 	 down_read1    down_write2
> 	 				down_write1
> 	 down_write2   down_read1
>
> from the perspective of lockdep, we cannot be sure whether there will
> a T3 or not.

Yes, that will be a deadlock even with the my rwsem patch applied, as it
will still try to preserve the reader-writer ordering. So it will
certainly be safer to have the same lock ordering for both tasks.

>
> In case that I mis-understood you, maybe your point is about in the
> above case whether "down_read1" on T2 can *gauranteedly* steal (in the
> sense of breaking the fairness) the read lock after Waiman modification?
> If so, I will wait for Waiman's response ;-)

With my patchset applied, the reader-writer ordering is still supposed
to be preserved. Of course, there can be exceptions depending on the
exact timing, but we can't rely on that to prevent deadlock.

Cheers,
Longman