linux-kernel - Re: [PATCH] Documentation/atomic

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Date:   Mon, 11 Feb 2019 13:13:02 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Will Deacon <will.deacon@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation/atomic_t: Clarify signed vs unsigned

On Mon, Feb 11, 2019 at 05:28:00PM +0000, Will Deacon wrote:
> On Mon, Feb 11, 2019 at 06:09:43PM +0100, Peter Zijlstra wrote:
> > 
> > Clarify the whole signed vs unsigned issue for atomic_t.
> > 
> > There has been enough confusion on this topic to warrant a few explicit
> > words I feel.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > ---
> >  Documentation/atomic_t.txt | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt
> > index 913396ac5824..dca3fb0554db 100644
> > --- a/Documentation/atomic_t.txt
> > +++ b/Documentation/atomic_t.txt
> > @@ -56,6 +56,23 @@ The 'full' API consists of (atomic64_ and atomic_long_ prefixes omitted for
> >    smp_mb__{before,after}_atomic()
> >  
> >  
> > +TYPES (signed vs unsigned)
> > +-----
> > +
> > +While atomic_t, atomic_long_t and atomic64_t use int, long and s64
> > +respectively (for hysterical raisins), the kernel uses -fno-strict-overflow
> > +(which implies -fwrapv) and defines signed overflow to behave like
> > +2s-complement.
> > +
> > +Therefore, an explicitly unsigned variant of the atomic ops is strictly
> > +unnecessary and we can simply cast, there is no UB.
> > +
> > +There was a bug in UBSAN prior to GCC-8 that would generate UB warnings for
> > +signed types.
> > +
> > +With this we also conform to the C/C++ _Atomic behaviour and things like
> > +P1236R1.
> > +
> 
> Acked-by: Will Deacon <will.deacon@....com>

Queued with Will's ack, thank you both!

								Thanx, Paul