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Date:   Fri, 22 Oct 2021 14:31:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Will Deacon <will@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        Waiman Long <longman@redhat.com>,
        Arnd Bergmann <arnd@arndb.de>, linux-arch@vger.kernel.org,
        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>,
        Christoph =?iso-8859-1?Q?M=FCllner?= <christophm30@gmail.com>,
        Stafford Horne <shorne@gmail.com>
Subject: Re: [PATCH] locking: Generic ticket lock
Message-ID: <20211022123100.GZ174703@worktop.programming.kicks-ass.net>
References: <YXFli3mzMishRpEq@hirez.programming.kicks-ass.net>
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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).