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

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Message-ID: <20210607225433.GR18427@gate.crashing.org>
Date:   Mon, 7 Jun 2021 17:54:33 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexander Monakov <amonakov@...ras.ru>,
        Jakub Jelinek <jakub@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nick Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Akira Yokosawa <akiyks@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-toolchains@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [RFC] LKMM: Add volatile_if()

On Mon, Jun 07, 2021 at 01:31:24PM -0700, Linus Torvalds wrote:
> > Is it useful in general for the kernel to have separate "read" and
> > "write" clobbers in asm expressions?  And for other applications?
> 
> See above. It's actually not all that uncommon that you have a "this
> doesn't modify memory, but you can't move writes around it". It's
> usually very much about cache handling or memory ordering operations,
> and that bit test example was probably a bad example exactly because
> it made it look like it's about some controlled range.
> 
> The "write memory barroer" is likely the best and simplest example,
> but it's in not the only one.

Thanks for the examples!  I opened <https://gcc.gnu.org/PR100953> so
that we can easily track it.


Segher