From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed Date: Thu, 1 Nov 2018 18:27:39 +0100 Message-ID: <20181101172739.GA3196@hirez.programming.kicks-ass.net> References: <20181031213240.zhh7dfcm47ucuyfl@pburton-laptop> <20181031220253.GA15505@roeck-us.net> <20181031233235.qbedw3pinxcuk7me@pburton-laptop> <4e2438a23d2edf03368950a72ec058d1d299c32e.camel@hammerspace.com> <20181101131846.biyilr2msonljmij@lakrids.cambridge.arm.com> <20181101145926.GE3178@hirez.programming.kicks-ass.net> <20181101163212.GF3159@hirez.programming.kicks-ass.net> <20181101171432.GH3178@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Trond Myklebust , "mark.rutland@arm.com" , "linux-kernel@vger.kernel.org" , "ralf@linux-mips.org" , "jlayton@kernel.org" , "linuxppc-dev@lists.ozlabs.org" , "bfields@fieldses.org" , "linux-mips@linux-mips.org" , "linux@roeck-us.net" , "linux-nfs@vger.kernel.org" , "akpm@linux-foundation.org" , "will.deacon@arm.com" , "boqun.feng@gmail.com" , "paul.burton@mips.com" , "anna.schumaker@netapp.com" , "jhogan@kerne To: Eric Dumazet Return-path: Content-Disposition: inline In-Reply-To: <20181101171432.GH3178@hirez.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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);