netdev - Re: [RFC PATCH] lib: Introduce generic __cmpxchg

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Message-ID: <20181101145926.GE3178@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Nov 2018 15:59:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Trond Myklebust <trondmy@...merspace.com>,
        "linux@...ck-us.net" <linux@...ck-us.net>,
        "paul.burton@...s.com" <paul.burton@...s.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-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>,
        "jhogan@...nel.org" <jhogan@...nel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "arnd@...db.de" <arnd@...db.de>,
        "paulus@...ba.org" <paulus@...ba.org>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed

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).