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Date:   Tue, 20 Aug 2019 15:56:12 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Joel Fernandes <joel@joelfernandes.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        rostedt <rostedt@goodmis.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        paulmck <paulmck@linux.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        David Howells <dhowells@redhat.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
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On Sat, Aug 17, 2019 at 01:08:02AM -0700, Linus Torvalds wrote:

> The data tearing issue is almost a non-issue. We're not going to add
> WRITE_ONCE() to these kinds of places for no good reason.

Paulmck actually has an example of that somewhere; ISTR that particular
case actually got fixed by GCC, but I'd really _love_ for some compiler
people (both GCC and LLVM) to state that their respective compilers will
not do load/store tearing for machine word sized load/stores.

Without this written guarantee (which supposedly was in older GCC
manuals but has since gone missing), I'm loathe to rely on it.

Yes, it is very rare, but it is a massive royal pain to debug if/when it
does do happen.