linux-kernel - Re: [PATCH 1/4] rcu: Allow for page faults in NMI handlers

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Message-ID: <CA+55aFyHXW5smbkWdpz-v3n6=9u1+m68YH4rB8Kb99AWaz50Kw@mail.gmail.com>
Date:   Sun, 24 Sep 2017 12:42:32 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 1/4] rcu: Allow for page faults in NMI handlers

On Sat, Sep 23, 2017 at 1:56 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> +
> +       /* Page faults can happen in NMI handlers, so check... */
> +       if (READ_ONCE(rdtp->dynticks_nmi_nesting))
> +               return;
> +

What is the reason for the READ_ONCE() here (and in the other case)?

It doesn't seem to have any actual reason.  It's a "stable" per-cpu
value in that even if an NMI were to happen, it gets incremented and
then decremented, so there is nothing really volatile about it
anywhere that I can see.

So the READ_ONCE() seems to be just pure confusion.

What am I missing?

               Linus