linux-kernel - Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and spin

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Message-ID: <20180306130059.GE25201@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Mar 2018 14:00:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Will Deacon <will.deacon@....com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Daniel Lustig <dlustig@...dia.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrea Parri <parri.andrea@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Palmer Dabbelt <palmer@...ive.com>,
        Albert Ou <albert@...ive.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Akira Yokosawa <akiyks@...il.com>,
        Ingo Molnar <mingo@...nel.org>, linux-riscv@...ts.infradead.org
Subject: Re: [RFC PATCH] riscv/locking: Strengthen spin_lock() and
 spin_unlock()

On Mon, Feb 26, 2018 at 09:00:43AM -0800, Linus Torvalds wrote:
> On Mon, Feb 26, 2018 at 8:24 AM, Will Deacon <will.deacon@....com> wrote:
> >
> > Strictly speaking, that's not what we've got implemented on arm64: only
> > the read part of the RmW has Acquire semantics, but there is a total
> > order on the lock/unlock operations for the lock.
> 
> Hmm.
> 
> I thought we had exactly that bug on some architecture with the queued
> spinlocks, and people decided it was wrong.

So ARM64 and Power have the acquire-on-load only thing, but qspinlock
has it per construction on anything that allowes reordering stores.

Given that unlock/lock are ordered, which covers about 99% of the users
out there, and fixing the issue would make things significantly slower
on the weak architectures we let it be.

But yes, its a pesky detail.