Re: [PATCH V7 4/5] asm-generic: spinlock: Add combo spinlock (ticket & queued)

Guo Ren
2022-07-01T12:18:45Z

On Wed, Jun 29, 2022 at 4:30 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Wed, Jun 29, 2022 at 10:24 AM Guo Ren <guoren@kernel.org> wrote:
> > On Wed, Jun 29, 2022 at 3:09 PM Arnd Bergmann <arnd@arndb.de> wrote:
> > > On Wed, Jun 29, 2022 at 3:34 AM Waiman Long <longman@redhat.com> wrote:
> > >
> > > From looking at the header file dependencies on arm64, I know that
> > > putting jump labels into core infrastructure like the arch_spin_lock()
> > > makes a big mess of indirect includes and measurably slows down
> > > the kernel build.
> > arm64 needn't combo spinlock, it could use pure qspinlock with keeping
> > current header files included.
>
> arm64 has a different problem: there are two separate sets of atomic
> instructions, and the decision between those is similarly done using
> jump labels. I definitely like the ability to choose between qspinlock
> and ticket spinlock on arm64 as well. This can be done as a
> compile-time choice, but both of them still depend on jump labels.
1. xchg use ALTERNATIVE, but cmpxchg to jump labels.
2. arm64 is still using qspinlock when ll/sc, and I think they give
strong enough fwd guarantee with "prfm pstl1strm".
But another question is if ll/sc could give enough strong fwd
guarantee, why arm64 introduce LSE, for code size reduction? Why
instructions fusion technology is not enough?

>
> Arnd

--
Best Regards
Guo Ren

ML: https://lore.kernel.org/linux-csky/