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Date:   Wed, 7 Oct 2020 23:07:17 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     Florian Weimer <fweimer@redhat.com>,
        linux-toolchains@vger.kernel.org, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org, stern@rowland.harvard.edu,
        parri.andrea@gmail.com, boqun.feng@gmail.com, npiggin@gmail.com,
        dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr,
        akiyks@gmail.com, dlustig@nvidia.com, joel@joelfernandes.org,
        torvalds@linux-foundation.org
Subject: Re: Control Dependencies vs C Compilers
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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.