Re: [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff

[Waiman Long]

On 10/19/22 03:05, Hillf Danton wrote:
> On 18 Oct 2022 22:49:02 -0400 Waiman Long <longman@xxxxxxxxxx>
> > On 10/18/22 22:29, Hillf Danton wrote:
> > > If handoff grants rwsem to a read waiter then the read fast path may revive.
> > I don't quite understand what you mean by "read fast path may revive".
> Subsequent readers may take rwsem without going the slow path after a
> read waiter is granted.
That may not be true. As long as there are still waiters in the wait 
queue, a reader has to go into the slow path and queued up in the wait 
queue. This is is to prevent a continuous stream of readers from 
starving writers in the wait queue.
> OTOH even after the check for single BIAS, another reader may come in
> and add its BIAS to count, which builds the same count as multiple
> readers came in before the check.

That is true, but hopefully we will eventually run out reader trying to 
get a read lock on a given rwsem.

Cheers,
Longman