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

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Message-ID: <YHbj987Ks0JOKw/X@hirez.programming.kicks-ass.net>
Date:   Wed, 14 Apr 2021 14:45:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Guo Ren <guoren@...nel.org>
Cc:     Christoph Müllner <christophm30@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Anup Patel <anup@...infault.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guo Ren <guoren@...ux.alibaba.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, jonas@...thpole.se,
        stefan.kristiansson@...nalahti.fi, shorne@...il.com
Subject: Re: [RFC][PATCH] locking: Generic ticket-lock

On Wed, Apr 14, 2021 at 12:16:38PM +0200, Peter Zijlstra wrote:
> On Wed, Apr 14, 2021 at 11:05:24AM +0200, Peter Zijlstra wrote:
> 
> > That made me look at the qspinlock code, and queued_spin_*lock() uses
> > atomic_try_cmpxchg_acquire(), which means any arch that uses qspinlock
> > and has RCpc atomics will give us massive pain.
> > 
> > Current archs using qspinlock are: x86, arm64, power, sparc64, mips and
> > openrisc (WTF?!).
> > 
> > Of those, x86 and sparc are TSO archs with SC atomics, arm64 has RCsc
> > atomics, power has RCtso atomics (and is the arch we all hate for having
> > RCtso locks).
> > 
> > Now MIPS has all sorts of ill specified barriers, but last time looked
> > at it it didn't actually use any of that and stuck to using smp_mb(), so
> > it will have RCsc atomics.
> > 
> > /me goes look at wth openrisc is..  doesn't even appear to have
> > asm/barrier.h :-/ Looking at wikipedia it also doesn't appear to
> > actually have hardware ...
> 
> FWIW this is broken, anything SMP *MUST* define mb(), at the very least.

As near as I can tell this should do. The arch spec only lists this one
instruction and the text makes it sound like a completion barrier.

---
 arch/openrisc/include/asm/barrier.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/openrisc/include/asm/barrier.h b/arch/openrisc/include/asm/barrier.h
new file mode 100644
index 000000000000..7538294721be
--- /dev/null
+++ b/arch/openrisc/include/asm/barrier.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __ASM_BARRIER_H
+#define __ASM_BARRIER_H
+
+#define mb() asm volatile ("l.msync" ::: "memory")
+
+#include <asm-generic/barrier.h>
+
+#endif /* __ASM_BARRIER_H */