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Date:   Thu, 1 Nov 2018 15:59:26 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        "linux@roeck-us.net" <linux@roeck-us.net>,
        "paul.burton@mips.com" <paul.burton@mips.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "ralf@linux-mips.org" <ralf@linux-mips.org>,
        "jlayton@kernel.org" <jlayton@kernel.org>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        "bfields@fieldses.org" <bfields@fieldses.org>,
        "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "anna.schumaker@netapp.com" <anna.schumaker@netapp.com>,
        "jhogan@kernel.org" <jhogan@kernel.org>,
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        "paulus@samba.org" <paulus@samba.org>,
        "mpe@ellerman.id.au" <mpe@ellerman.id.au>,
        "benh@kernel.crashing.org" <benh@kernel.crashing.org>,
        Will Deacon <will.deacon@arm.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed
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On Thu, Nov 01, 2018 at 01:18:46PM +0000, Mark Rutland wrote:
> > My one question (and the reason why I went with cmpxchg() in the first
> > place) would be about the overflow behaviour for atomic_fetch_inc() and
> > friends. I believe those functions should be OK on x86, so that when we
> > overflow the counter, it behaves like an unsigned value and wraps back
> > around.  Is that the case for all architectures?
> > 
> > i.e. are atomic_t/atomic64_t always guaranteed to behave like u32/u64
> > on increment?
> > 
> > I could not find any documentation that explicitly stated that they
> > should.
> 
> Peter, Will, I understand that the atomic_t/atomic64_t ops are required
> to wrap per 2's-complement. IIUC the refcount code relies on this.
> 
> Can you confirm?

There is quite a bit of core code that hard assumes 2s-complement. Not
only for atomics but for any signed integer type. Also see the kernel
using -fno-strict-overflow which implies -fwrapv, which defines signed
overflow to behave like 2s-complement (and rids us of that particular
UB).