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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 21 Oct 2021 17:31:51 +0200
Message-ID: <CAK8P3a14NUvo40GFY5DfQcF28OO22=BiHJO1TzBTEMK0RAwtHg@mail.gmail.com>
Subject: Re: [PATCH] locking: Generic ticket lock
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Arnd Bergmann <arnd@arndb.de>, Will Deacon <will@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Waiman Long <longman@redhat.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Guo Ren <guoren@kernel.org>,
        Palmer Dabbelt <palmerdabbelt@google.com>,
        Anup Patel <anup@brainfault.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        =?UTF-8?Q?Christoph_M=C3=BCllner?= <christophm30@gmail.com>,
        Stafford Horne <shorne@gmail.com>
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On Thu, Oct 21, 2021 at 5:14 PM Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Oct 21, 2021 at 03:49:51PM +0200, Arnd Bergmann wrote:
> I think for a load-store arch this thing should generate pretty close to
> optimal code. x86 can do ticket_unlock() slightly better using a single
> INCW (or ADDW 1) on the owner subword, where this implementation will to
> separate load-add-store instructions.
>
> If that is actually measurable is something else entirely.

Ok, so I guess such an architecture could take the generic implementation
and override just arch_spin_unlock() or just arch_spin_lock(), if that
makes a difference for them.

Should we perhaps turn your modified openrisc asm/spinlock.h
and asm/spin_lock_types.h the fallback in asm-generic, and
remove the ones for the architectures that have no overrides
at all?

Or possibly a version that can do both based on
CONFIG_ARCH_USE_QUEUED_SPINLOCKS? That would
let us remove even more architecture specific headers, but
it increases the risk of some architecture using qspinlock
when they really should not.

> > or a trivial test-and-set?
>
> If your SMP arch is halfway sane (no fwd progress issues etc..) then
> ticket should behave well and avoid the starvation/variablilty of TaS
> lock.

Ok, and I guess we still need to keep the parisc and sparc32 versions
anyway.

> The big exception there is virtualized architectures, ticket is
> absolutely horrendous for 'guests' (any fair lock is for that matter).

This might be useful information to put into the header, at least
I had no idea about this distinction.

       Arnd