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

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 1 Nov 2018 13:29:10 -0700

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);
> 
>