linux-kernel - Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

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Message-ID: <CAHk-=whjEq6uEt0o0Ur9Epa7EKVvEFUVJVFJ+heJCv9ehV7pyA@mail.gmail.com>
Date:   Sat, 17 Aug 2019 01:44:04 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alan Stern <stern@...land.harvard.edu>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

On Sat, Aug 17, 2019 at 1:28 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
>    unsigned int bits = some_global_value;
>    ...test different bits in in 'bits' ...
>
> can easily cause multiple reads (particularly on a CPU that has a
> "test bits in memory" instruction and a lack of registers.
>
> So then doing it as
>
>    unsigned int bits = READ_ONCE(some_global_value);
>    .. test different bits in 'bits'...

Side note: this is likely the best example of actual WRITE_ONCE() use
too: if you have that global value with multiple bits that actually
have some interdependencies, then doing

    some_global_value = some_complex_expression();

might be reasonably compiled to do several rmw instructions to update
'some_global_value'

So then

   WRITE_ONCE(some_global_value, some_complex_expression());

really can be a good thing - it clearly just writes things once, and
it also documents the whole "write one or the other" value, not some
mid-way one, when you then look at the READ_ONCE() thing.

But I'm seeing a lot of WRITE_ONCE(x, constantvalue) kind of things
and don't seem to find a lot of reason to think that they are any
inherently better than "x = constantvalue".

(In contrast, using "smp_store_release(flag, true)" has inherent
value, because it actually implies a memory barrier wrt previous
writes, in ways that WRITE_ONCE() or a direct assignment does not.)

Ok, enough blathering. I think I've made my point.

              Linus