linux-kernel - Re: [RFC][PATCH 6/7] freelist: Lock less freelist

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Message-ID: <20200828013410.GA49492@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>
Date:   Fri, 28 Aug 2020 09:34:10 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Cameron <cameron@...dycamel.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        Eddy_Wu@...ndmicro.com, x86@...nel.org, davem@...emloft.net,
        rostedt@...dmis.org, naveen.n.rao@...ux.ibm.com,
        anil.s.keshavamurthy@...el.com, linux-arch@...r.kernel.org,
        oleg@...hat.com, will@...nel.org, paulmck@...nel.org
Subject: Re: [RFC][PATCH 6/7] freelist: Lock less freelist

On Thu, Aug 27, 2020 at 03:57:22PM -0400, Cameron wrote:
> On Thu, Aug 27, 2020 at 3:08 PM Boqun Feng <boqun.feng@...il.com> wrote:
> > So if try_cmpxchg_acquire() fails, we don't have ACQUIRE semantics on
> > read of the new list->head, right? Then probably a
> > smp_mb__after_atomic() is needed in that case?
> 
> Yes, there needs to be an acquire on the head after a failed cmpxchg;
> does the atomic_fetch_add following that not have acquire semantics?
> 

Yes, you're right, the atomic_fecth_add() following is a fully-ordered
atomic, so could provide the necessary ACQUIRE semantics. I was missing
that. Maybe a few words explaining this helps.

Regards,
Boqun

> Cameron