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

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Message-ID: <1065930957.23914.1566054178444.JavaMail.zimbra@efficios.com>
Date:   Sat, 17 Aug 2019 11:02:58 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     paulmck <paulmck@...ux.ibm.com>,
        Valentin Schneider <valentin.schneider@....com>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alan Stern <stern@...land.harvard.edu>,
        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 Aug 17, 2019, at 4:44 AM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> 
> 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".

If the only states that "x" can take is 1 or 0, then indeed there seems
to be no point in using a WRITE_ONCE() when paired with a READ_ONCE()
other than for documentation purposes.

However, if the state of "x" can be any pointer value, or a reference
count value, then not using "WRITE_ONCE()" to store a constant leaves
the compiler free to perform that store in more than one memory access.
Based on [1], section "Store tearing", there are situations where this
happens on x86 in the wild today when storing 64-bit constants: the
compiler is then free to decide to use two 32-bit immediate store
instructions.

Thanks,

Mathieu

[1] https://lwn.net/Articles/793253/

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com