Re: [PATCH V7 1/5] asm-generic: ticket-lock: Remove unnecessary atomic_read

Guo Ren
2022-06-29T02:12:34Z

On Wed, Jun 29, 2022 at 2:06 AM Waiman Long <longman@redhat.com> wrote:
>
> On 6/28/22 04:17, guoren@kernel.org wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> >
> > Remove unnecessary atomic_read in arch_spin_value_unlocked(lock),
> > because the value has been in lock. This patch could prevent
> > arch_spin_value_unlocked contend spin_lock data again.
> >
> > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@kernel.org>
> > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Arnd Bergmann <arnd@arndb.de>
> > Cc: Palmer Dabbelt <palmer@rivosinc.com>
> > ---
> > include/asm-generic/spinlock.h | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/asm-generic/spinlock.h b/include/asm-generic/spinlock.h
> > index fdfebcb050f4..f1e4fa100f5a 100644
> > --- a/include/asm-generic/spinlock.h
> > +++ b/include/asm-generic/spinlock.h
> > @@ -84,7 +84,9 @@ static __always_inline int arch_spin_is_contended(arch_spinlock_t *lock)
> >
> > static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> > {
> > - return !arch_spin_is_locked(&lock);
> > + u32 val = lock.counter;
> > +
> > + return ((val >> 16) == (val & 0xffff));
> > }
> >
> > #include <asm/qrwlock.h>
>
> lockref.c is the only current user of arch_spin_value_unlocked(). This
> change is probably OK with this particular use case. Do you have any
> performance data about the improvement due to this change?
I don't have performance data and I just check the asm code, previous
version has an additional unnecessary atomic_read.

About this point, we've talked before, but I & palmer missed that
point when we pick peter's patch again.
https://lore.kernel.org/linux-riscv/YHbmXXvuG442ZDfN@hirez.programming.kicks-ass.net/
----
> > +static __always_inline int ticket_value_unlocked(arch_spinlock_t lock)
> > +{
> > + return !ticket_is_locked(&lock);
> Are you sure to let ticket_is_locked->atomic_read(lock) again, the
> lock has contained all information?
>
> return lock.tickets.owner == lock.tickets.next;

Yeah, I wrote then the wrong way around. Couldn't be bothered to go back
when I figured it out.
---
It's just a small typo.

>
> You may have to document that we have to revisit that if another use
> case shows up.
>
> Cheers,
> Longman
>

--
Best Regards
Guo Ren

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