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

[Yuyang Du]

On Tue, 4 Jun 2019 at 11:03, Yuyang Du <duyuyang@xxxxxxxxx> wrote:
>
> Hi Waiman,
>
> On Tue, 21 May 2019 at 05:01, Waiman Long <longman@xxxxxxxxxx> 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
>
> So I was trying to understand what really went wrong and find the
> problem is that if I understand correctly the current rwsem design
> isn't showing real fairness but priority in favor of write locks, and
> thus one of the bad effects is that read locks can be starved if write
> locks keep coming.
>
> Luckily, I noticed you are revamping rwsem and seem to have thought
> about it already. I am not crystal sure what is your work's
> ramification on the above case, so hope that you can shed some light
> and perhaps share your thoughts on this.
>
> Thanks,
> Yuyang