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Date: Mon, 21 Nov 2016 15:12:33 -0500 From: David Windsor <dwindsor@...il.com> To: "Reshetova, Elena" <elena.reshetova@...el.com> Cc: Peter Zijlstra <peterz@...radead.org>, Alexei Starovoitov <alexei.starovoitov@...il.com>, Kees Cook <keescook@...omium.org>, Greg KH <gregkh@...uxfoundation.org>, Will Deacon <will.deacon@....com>, Arnd Bergmann <arnd@...db.de>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, "H. Peter Anvin" <hpa@...or.com>, LKML <linux-kernel@...r.kernel.org>, Daniel Borkmann <daniel@...earbox.net> Subject: Re: [RFC][PATCH 2/7] kref: Add kref_read() On Mon, Nov 21, 2016 at 2:27 PM, Reshetova, Elena <elena.reshetova@...el.com> wrote: >> On Mon, Nov 21, 2016 at 04:49:15PM +0100, Peter Zijlstra wrote: >> > > Speaking of non-fitting patterns. This one is quite common in >> > > networking code for refcounters: >> > > >> > > if (atomic_cmpxchg(&cur->refcnt, 1, 0) == 1) {} This is from >> > > net/netfilter/nfnetlink_acct.c, but there are similar ones in other >> > > places. >> > >> > Cute, but weird it doesn't actually decrement if not 1. >> >> Hurgh.. creative refcounting that. The question is how much of that do >> we want to support? It really must not decrement there. > > And one more creative usage: > > http://lxr.free-electrons.com/source/net/ipv4/udp.c#L1940 > > if (!sk || !atomic_inc_not_zero_hint(&sk->sk_refcnt, 2)) > return; > > I didn't even guess anyone is using atomic_inc_not_zero_hint... > But network code keeps surprising me today :) > So, yes, I guess the question is what to do with these cases really? Many of the calls to non-supported functions can be decomposed into calls to supported functions. The ones that may prove interesting are ones like atomic_cmpxchg(), in which some sort of external locking is going to be required to achieve the same atomicity guarantees provided by cmpxchg, like so: mutex_lock(lock); cnt = refcount_read(ref); if (cnt == val1) { refcount_set(ref, val2); } mutex_unlock(lock); return cnt;
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