Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation

[Anup Patel]

On Wed, Mar 24, 2021 at 6:08 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Mar 24, 2021 at 05:58:58PM +0530, Anup Patel wrote:
> > On Wed, Mar 24, 2021 at 3:45 PM <guoren@xxxxxxxxxx> wrote:
> > >
> > > From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > >
> > > This patch introduces a ticket lock implementation for riscv, along the
> > > same lines as the implementation for arch/arm & arch/csky.
> > >
> > > Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> > > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> > > Cc: Will Deacon <will.deacon@xxxxxxx>
> > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Cc: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> > > Cc: Anup Patel <anup@xxxxxxxxxxxxxx>
> > > Cc: Arnd Bergmann <arnd@xxxxxxxx>
> > > ---
> > >  arch/riscv/Kconfig                      |   1 +
> > >  arch/riscv/include/asm/Kbuild           |   1 +
> > >  arch/riscv/include/asm/spinlock.h       | 158 ++++++++++++--------------------
> > >  arch/riscv/include/asm/spinlock_types.h |  19 ++--
> >
> > NACK from myside.
> >
> > Linux ARM64 has moved away from ticket spinlock to qspinlock.
> >
> > We should directly go for qspinlock.
>
> I think it is a sensible intermediate step, even if you want to go
> qspinlock. Ticket locks are more or less trivial and get you fairness
> and all that goodness without the mind bending complexity of qspinlock.
>
> Once you have the ticket lock implementation solid (and qrwlock) and
> everything, *then* start to carefully look at qspinlock.

I do understand qspinlock are relatively complex but the best thing
about qspinlock is it tries to ensure each CPU spins on it's own location.

Instead of adding ticket spinlock now and later replacing it with qspinlock,
it is better to straight away explore qspinlock hence my NACK.

>
> Now, arguably arm64 did the heavy lifting of making qspinlock good on
> weak architectures, but if you want to do it right, you still have to
> analyze the whole thing for your own architecture.

Most of the RISC-V implementations are weak memory ordering so it
makes more sense to explore qspinlock first.

Regards,
Anup