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From:   Guo Ren <guoren@kernel.org>
Date:   Wed, 14 Apr 2021 10:26:57 +0800
Message-ID: <CAJF2gTQaF8wBCp-L6vgJPcu6EnFRWmh_qZMX2PiEfj0Z70-Ykg@mail.gmail.com>
Subject: Re: [PATCH] riscv: locks: introduce ticket-based spinlock implementation
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     =?UTF-8?Q?Christoph_M=C3=BCllner?= <christophm30@gmail.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Anup Patel <anup@brainfault.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Guo Ren <guoren@linux.alibaba.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Arnd Bergmann <arnd@arndb.de>
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Thx Peter,

On Tue, Apr 13, 2021 at 4:17 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Apr 13, 2021 at 10:03:01AM +0200, Peter Zijlstra wrote:
>
> > For ticket locks you really only needs atomic_fetch_add() and
> > smp_store_release() and an architectural guarantees that the
> > atomic_fetch_add() has fwd progress under contention and that a sub-word
> > store (through smp_store_release()) will fail the SC.
> >
> > Then you can do something like:
> >
> > void lock(atomic_t *lock)
> > {
> >       u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> >       u16 ticket = val >> 16;
> >
> >       for (;;) {
> >               if (ticket == (u16)val)
> >                       break;
> >               cpu_relax();
> >               val = atomic_read_acquire(lock);
> >       }
Should it be?
       for (;;) {
               if (ticket == (u16)val) {
                       __atomic_acquire_fence();
                       break;
               }

>
> A possibly better might be:
>
>         if (ticket == (u16)val)
>                 return;
Should it be?
         if (ticket == (u16)val) {
                 __atomic_acquire_fence();
                 return;
         }

>
>         atomic_cond_read_acquire(lock, ticket == (u16)VAL);
>
> Since that allows architectures to use WFE like constructs.
>
> > }
> >
> > void unlock(atomic_t *lock)
> > {
> >       u16 *ptr = (u16 *)lock + (!!__BIG_ENDIAN__);
> >       u32 val = atomic_read(lock);
> >
> >       smp_store_release(ptr, (u16)val + 1);
> > }
> >
> > That's _almost_ as simple as a test-and-set :-) It isn't quite optimal
> > on x86 for not being allowed to use a memop on unlock, since its being
> > forced into a load-store because of all the volatile, but whatever.


-- 
Best Regards
 Guo Ren

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