linux-kernel - Re: [PATCH 0/3] task: Making tasks on the runqueue rcu protected

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Date:   Tue, 3 Sep 2019 21:46:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 0/3] task: Making tasks on the runqueue rcu protected

On Tue, Sep 03, 2019 at 08:44:40AM -0700, Linus Torvalds wrote:

> That said, it won't affect any of the core architectures much, because
> smp_store_release() isn't that expensive (it's just a compiler barrier
> on x86, it's a cheap instruction on arm64, and it should be very cheap
> on any other architecture too unless they do insane things - even on
> powerpc, which is about the worst case for any barriers, it's just an
> lwsync).

Right, x86/s390/Sparc it's a compiler barrier, ARM64 has a store-release
op which is relatively cheap, Power has an LWSYNC, but the rest does
store-release with smp_mb() -- and this includes ARM.