linux-kernel - Re: [RFC] LKMM: Add volatile

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Message-ID: <YLpSEM7sxSmsuc5t@hirez.programming.kicks-ass.net>
Date:   Fri, 4 Jun 2021 18:17:20 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Will Deacon <will@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        paulmck@...nel.org, parri.andrea@...il.com, boqun.feng@...il.com,
        npiggin@...il.com, dhowells@...hat.com, j.alglave@....ac.uk,
        luc.maranget@...ia.fr, akiyks@...il.com,
        linux-kernel@...r.kernel.org, linux-toolchains@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [RFC] LKMM: Add volatile_if()

On Fri, Jun 04, 2021 at 11:51:54AM -0400, Alan Stern wrote:
> On Fri, Jun 04, 2021 at 05:42:28PM +0200, Peter Zijlstra wrote:

> > #define volatile_if(cond) if (({ bool __t = (cond); BUILD_BUG_ON(__builtin_constant_p(__t)); volatile_cond(__t); }))
> 
> That won't help with more complicated examples, such as:
> 
> 	volatile_if (READ_ONCE(*x) * 0 + READ_ONCE(*y))

That's effectively:

	volatile_if (READ_ONCE(*y))
		WRITE_ONCE(*y, 42);

which is a valid, but daft, LOAD->STORE order, no? A compiler might
maybe be able to WARN on that, but that's definitely beyond what we can
do with macros.