linux-kernel - Re: Control Dependencies vs C Compilers

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Date:   Wed, 7 Oct 2020 23:07:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Florian Weimer <fweimer@...hat.com>,
        linux-toolchains@...r.kernel.org, Will Deacon <will@...nel.org>,
        linux-kernel@...r.kernel.org, stern@...land.harvard.edu,
        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, dlustig@...dia.com, joel@...lfernandes.org,
        torvalds@...ux-foundation.org
Subject: Re: Control Dependencies vs C Compilers

On Wed, Oct 07, 2020 at 10:11:07AM -0700, Paul E. McKenney wrote:

> Challenges include:
> 
> o	Unmarked accesses.  Compilers are quite aggressive about
> 	moving normal code.

Which is why this thread exists :-) We wants to dis-allow lifting the
stores over our volatile-if.

> o	Separately compiled code.  For example, does the compiler have
> 	unfortunatel optimization opportunities when "volatile if" 
> 	appears in one translation unit and the dependent stores in
> 	some other translation unit?

It can hardly lift anything outside a TU (barring the next point). So I
don't see how it can go wrong here. This is in fact the case with the
perf ringbuffer. The ctrl-dep lives in a different TU from the
stores.

> o	LTO, as has already been mentioned in this thread.

So I would probably advocate the volatile-if to be a full sync point,
and LTO would have to preserve that.