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

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Date:   Thu, 1 Nov 2018 13:29:10 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Eric Dumazet <eric.dumazet@...il.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>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "paul.burton@...s.com" <paul.burton@...s.com>,
        "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>,
        aryabinin@...tuozzo.com, dvyukov@...gle.com
Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it
 where needed

On Thu, Nov 01, 2018 at 06:27:39PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 01, 2018 at 06:14:32PM +0100, Peter Zijlstra wrote:
> > > This reminds me of this sooooo silly patch :/
> > > 
> > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=adb03115f4590baa280ddc440a8eff08a6be0cb7
> 
> You'd probably want to write it like so; +- some ordering stuff, that
> code didn't look like it really needs the memory barriers implied by
> these, but I didn't look too hard.

The atomic_fetch_add() API would need to be propagated out to the other
architectures, correct?

							Thanx, Paul

> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index c0a9d26c06ce..11deb1d7e96b 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -485,16 +485,10 @@ u32 ip_idents_reserve(u32 hash, int segs)
>  	u32 now = (u32)jiffies;
>  	u32 new, delta = 0;
> 
> -	if (old != now && cmpxchg(p_tstamp, old, now) == old)
> +	if (old != now && try_cmpxchg(p_tstamp, &old, now))
>  		delta = prandom_u32_max(now - old);
> 
> -	/* Do not use atomic_add_return() as it makes UBSAN unhappy */
> -	do {
> -		old = (u32)atomic_read(p_id);
> -		new = old + delta + segs;
> -	} while (atomic_cmpxchg(p_id, old, new) != old);
> -
> -	return new - segs;
> +	return atomic_fetch_add(segs + delta, p_id) + delta;
>  }
>  EXPORT_SYMBOL(ip_idents_reserve);
> 
>