Re: [RFC PATCH 5/5] locking/rwsem: Remove reader optimistic spinning

[Davidlohr Bueso]

On Fri, 20 Nov 2020, David Laight wrote:
> I got massive performance improvements from changing a driver
> we have to use mutex instead of the old semaphores (the driver
> was written a long time ago).
>
> While these weren't 'rw' the same issue will apply.
>
> The problem was that the semaphore/mutex was typically only held over
> a few instructions (eg to add an item to a list).
> But with semaphore if you got contention the process always slept.
> OTOH mutex spin 'for a while' before sleeping so the code rarely slept.

The caveat here is if you are using trylock/unlock from irq, which
is the only reason why regular semaphores are still around today. If
not, indeed a mutex is better.

Thanks,
Davidlohr