linux-kernel - Re: [PATCH v6 13/13] riscv: Add qspinlock support

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Message-ID: <20241112120510.GA21181@willie-the-truck>
Date: Tue, 12 Nov 2024 12:05:10 +0000
From: Will Deacon <will@...nel.org>
To: Guo Ren <guoren@...nel.org>
Cc: Alexandre Ghiti <alexghiti@...osinc.com>,
	Jonathan Corbet <corbet@....net>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Conor Dooley <conor@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Andrea Parri <parri.andrea@...il.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>, Arnd Bergmann <arnd@...db.de>,
	Leonardo Bras <leobras@...hat.com>, linux-doc@...r.kernel.org,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v6 13/13] riscv: Add qspinlock support

On Tue, Nov 12, 2024 at 09:49:15AM +0800, Guo Ren wrote:
> On Tue, Nov 12, 2024 at 12:43 AM Will Deacon <will@...nel.org> wrote:
> >
> > On Sun, Nov 03, 2024 at 03:51:53PM +0100, Alexandre Ghiti wrote:
> > > In order to produce a generic kernel, a user can select
> > > CONFIG_COMBO_SPINLOCKS which will fallback at runtime to the ticket
> > > spinlock implementation if Zabha or Ziccrse are not present.
> > >
> > > Note that we can't use alternatives here because the discovery of
> > > extensions is done too late and we need to start with the qspinlock
> > > implementation because the ticket spinlock implementation would pollute
> > > the spinlock value, so let's use static keys.
> >
> > I think the static key toggling takes a mutex (jump_label_lock()) which
> > can take a spinlock (lock->wait_lock) internally, so I don't grok how
> > this works:
> >
> > > +static void __init riscv_spinlock_init(void)
> > > +{
> > > +     char *using_ext = NULL;
> > > +
> > > +     if (IS_ENABLED(CONFIG_RISCV_TICKET_SPINLOCKS)) {
> > > +             pr_info("Ticket spinlock: enabled\n");
> > > +             return;
> > > +     }
> > > +
> > > +     if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) &&
> > > +         IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) &&
> > > +         riscv_isa_extension_available(NULL, ZABHA) &&
> > > +         riscv_isa_extension_available(NULL, ZACAS)) {
> > > +             using_ext = "using Zabha";
> > > +     } else if (riscv_isa_extension_available(NULL, ZICCRSE)) {
> > > +             using_ext = "using Ziccrse";
> > > +     }
> > > +#if defined(CONFIG_RISCV_COMBO_SPINLOCKS)
> > > +     else {
> > > +             static_branch_disable(&qspinlock_key);
> > > +             pr_info("Ticket spinlock: enabled\n");
> > > +             return;
> > > +     }
> > > +#endif
> >
> > i.e. we've potentially already used the qspinlock at this point.
> Yes, I've used qspinlock here. But riscv_spinlock_init is called with
> irq_disabled and smp_off. That means this qspinlock only performs a
> test-set lock behavior by qspinlock fast-path.

That's... horrendous.

Will