linux-kernel - Re: [PATCH] locking: Generic ticket lock

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Date:   Fri, 22 Oct 2021 14:31:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, Guo Ren <guoren@...nel.org>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Anup Patel <anup@...infault.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Christoph Müllner <christophm30@...il.com>,
        Stafford Horne <shorne@...il.com>
Subject: Re: [PATCH] locking: Generic ticket lock

On Fri, Oct 22, 2021 at 10:23:02AM +0100, Mark Rutland wrote:
> Hi Peter,
> 
> On Thu, Oct 21, 2021 at 03:05:15PM +0200, Peter Zijlstra wrote:
> > +static __always_inline void ticket_lock(arch_spinlock_t *lock)
> > +{
> > +	u32 val = atomic_fetch_add_acquire(ONE_TICKET, lock);
> 
> I wonder, should these atomics be arch_atomic_*(), in case an arch_ or raw_
> lock is used in noinstr code? The plain atomic_*() forms can have explicit
> inline instrumentation.
> 
> I haven't seen any issues with qspinlock so far, and that also uses the
> (instrumented) atomics, so maybe that's not actually a problem, but I'm not
> sure what we intend here w.r.t.  instrumentability.

So far it's not been a problem, and as you say, if we want to change
this, we need a larger audit/cleanup.

IIRC there's a potential problem in the arm idle code (noinstr'ing the
idle code is still on the TODO list somewhre, hampered by the need to
create more tooling).