linux-kernel - Re: Fw: [isocpp-parallel] OOTA fix (via fake branch-after-load) discussion

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Date:   Thu, 9 Nov 2023 10:24:23 -0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Jonas Oberhauser <jonas.oberhauser@...weicloud.com>
Cc:     paulmck@...nel.org,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Peter Zijlstra <peterz@...radead.org>, j.alglave@....ac.uk,
        will@...nel.org, catalin.marinas@....com, linux@...linux.org.uk,
        mpe@...erman.id.au, npiggin@...il.com, palmer@...belt.com,
        parri.andrea@...il.com, linux-kernel@...r.kernel.org,
        linux-toolchains@...r.kernel.org, davidtgoldblatt@...il.com
Subject: Re: Fw: [isocpp-parallel] OOTA fix (via fake branch-after-load)
 discussion

On Thu, Nov 09, 2023 at 05:25:05PM +0100, Jonas Oberhauser wrote:
[...]
> > 4.	Semantics of volatile.	Perhaps the current state is the best
> > 	that can be hoped for, but given that the current state is a
> > 	few vague words in the standard in combination with the fact
> > 	that C-language device drivers must be able to use volatile
> > 	to reliably and concurrently access memory shared with device
> > 	firmware, one would hope for better.
> 
> 
> Is it really so bad? I think the definition in the manual is quite precise,
> if confusing. (volatiles are visible side effects and must therefore have
> the same program order in the abstract machine and in the implementation,
> and that's pretty much it).
> 

But I don't think there is any mention on whether current volatile
accesses can be excluded from data race, or whether a volatile access
on a machine-word size natually aligned object can be teared or not.

Regards,
Boqun

> There should just be a large explanatory note about what it implies and what
> it doesn't imply.
> 
>