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

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Message-ID: <20190820202932.GW28441@linux.ibm.com>
Date:   Tue, 20 Aug 2019 13:29:32 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joel Fernandes <joel@...lfernandes.org>,
        Alan Stern <stern@...land.harvard.edu>,
        rostedt <rostedt@...dmis.org>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.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 Tue, Aug 20, 2019 at 03:56:12PM +0200, Peter Zijlstra wrote:
> 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.

I do very much recall such an example, but I am now unable to either
find it or reproduce it.  :-/

If I cannot turn it up in a few days, I will ask the LWN editors to
make appropriate changes to the "Who is afraid" article.

> 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.

But from what I can see, Linus is OK with use of WRITE_ONCE() for data
races on any variable for which there is at least one READ_ONCE().
So we can still use WRITE_ONCE() as we would like in our own code.
Yes, you or I might be hit by someone else's omission of WRITE_ONCE(),
it is better than the proverbial kick in the teeth.

Of course, if anyone knows of a compiler/architecture combination that
really does tear stores of 32-bit constants, please do not keep it
a secret!  After all, it would be good to get that addressed easily
starting now rather than after a difficult and painful series of
debugging sessions.

							Thanx, Paul