[RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed

[Peter Zijlstra]

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);