Re: [RFC][PATCH] locking: Generic ticket-lock

From: Peter Zijlstra <peterz@infradead.org>
To: Guo Ren <guoren@kernel.org>
Cc: "Christoph Müllner" <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>,
	"Jonas Bonn" <jonas@southpole.se>,
	"Stefan Kristiansson" <stefan.kristiansson@saunalahti.fi>,
	"Stafford Horne" <shorne@gmail.com>
Subject: Re: [RFC][PATCH] locking: Generic ticket-lock
Date: Wed, 14 Apr 2021 15:08:34 +0200	[thread overview]
Message-ID: <YHbpUtIWaaP1Qod1@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <YHbmXXvuG442ZDfN@hirez.programming.kicks-ass.net>

On Wed, Apr 14, 2021 at 02:55:57PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 14, 2021 at 08:39:33PM +0800, Guo Ren wrote:

> > > + * It further assumes atomic_*_release() + atomic_*_acquire() is RCpc and hence
> > > + * uses atomic_fetch_add() which is SC to create an RCsc lock.
> 
> This ^^^ then vvv
> 
> > > +static __always_inline void ticket_lock(arch_spinlock_t *lock)
> > > +{
> > > +       u32 val = atomic_fetch_add(1<<16, lock); /* SC, gives us RCsc */
> > atomic_fetch_add_acquire ?
> 
> Then we must rely on the arch to implement RCsc atomics. And I for one
> can never tell wth Risc-V actually does.

Anyway, if we can mandate that atomic acquire/release is RCsc, then yes
we can do that but then we also need:

#define smp_mb__after_spinlock()	smp_mb__after_atomic()

But currently atomic acquire/release is not RCsc (at the very least
Power does RCtso -- and power gets to deal with any and all pain caused
by that).

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