linux-kernel - Re: [RFC PATCH] lib: Introduce generic __cmpxchg

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Message-Id: <20181102133837.GS4170@linux.ibm.com>
Date:   Fri, 2 Nov 2018 06:38:37 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     David Laight <David.Laight@...LAB.COM>,
        Trond Myklebust <trondmy@...merspace.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ralf@...ux-mips.org" <ralf@...ux-mips.org>,
        "jlayton@...nel.org" <jlayton@...nel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "bfields@...ldses.org" <bfields@...ldses.org>,
        "linux-mips@...ux-mips.org" <linux-mips@...ux-mips.org>,
        "linux@...ck-us.net" <linux@...ck-us.net>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "will.deacon@....com" <will.deacon@....com>,
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        "paul.burton@...s.com" <paul.burton@...s.com>,
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        "davem@...emloft.net" <davem@...emloft.net>,
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        "dvyukov@...gle.com" <dvyukov@...gle.com>
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed

On Fri, Nov 02, 2018 at 01:23:28PM +0100, Peter Zijlstra wrote:
> On Fri, Nov 02, 2018 at 10:56:31AM +0000, David Laight wrote:
> > From: Paul E. McKenney
> > > Sent: 01 November 2018 17:02
> > ...
> > > And there is a push to define C++ signed arithmetic as 2s complement,
> > > but there are still 1s complement systems with C compilers.  Just not
> > > C++ compilers.  Legacy...
> > 
> > Hmmm... I've used C compilers for DSPs where signed integer arithmetic
> > used the 'data registers' and would saturate, unsigned used the 'address
> > registers' and wrapped.
> > That was deliberate because it is much better to clip analogue values.
> 
> Seems a dodgy heuristic if you ask me.
> 
> > Then there was the annoying cobol run time that didn't update the
> > result variable if the result wouldn't fit.
> > Took a while to notice that the sum of a list of values was even wrong!
> > That would be perfectly valid for C - if unexpected.
> 
> That's just insane ;-)
> 
> > > > But for us using -fno-strict-overflow which actually defines signed
> > > > overflow
> > 
> > I wonder how much real code 'strict-overflow' gets rid of?
> > IIRC gcc silently turns loops like:
> > 	int i; for (i = 1; i != 0; i *= 2) ...
> > into infinite ones.
> > Which is never what is required.
> 
> Nobody said C was a 'safe' language. But less UB makes a better language
> IMO. Ideally we'd get all UBs filled in -- but I realise C has a few
> very 'interesting' ones that might be hard to get rid of.

There has been an effort to reduce UB, but not sure how far they got.

							Thanx, Paul