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Date:   Thu, 1 Nov 2018 18:27:39 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Eric Dumazet <eric.dumazet@gmail.com>
Cc:     Trond Myklebust <trondmy@hammerspace.com>,
        "mark.rutland@arm.com" <mark.rutland@arm.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>,
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        "linux-mips@linux-mips.org" <linux-mips@linux-mips.org>,
        "linux@roeck-us.net" <linux@roeck-us.net>,
        "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
        "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "will.deacon@arm.com" <will.deacon@arm.com>,
        "boqun.feng@gmail.com" <boqun.feng@gmail.com>,
        "paul.burton@mips.com" <paul.burton@mips.com>,
        "anna.schumaker@netapp.com" <anna.schumaker@netapp.com>,
        "jhogan@kernel.org" <jhogan@kernel.org>,
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        "davem@davemloft.net" <davem@davemloft.net>,
        "arnd@arndb.de" <arnd@arndb.de>,
        "paulus@samba.org" <paulus@samba.org>,
        "mpe@ellerman.id.au" <mpe@ellerman.id.au>,
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        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        aryabinin@virtuozzo.com, dvyukov@google.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 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.

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);