RE: [RFC][PATCH 2/7] kref: Add kref_read()

From: "Reshetova, Elena" <elena.reshetova@intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: David Windsor <dwindsor@gmail.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Kees Cook <keescook@chromium.org>,
	Greg KH <gregkh@linuxfoundation.org>,
	Will Deacon <will.deacon@arm.com>,
	"Arnd Bergmann" <arnd@arndb.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>
Subject: RE: [RFC][PATCH 2/7] kref: Add kref_read()
Date: Mon, 21 Nov 2016 19:27:29 +0000	[thread overview]
Message-ID: <2236FBA76BA1254E88B949DDB74E612B41C156AE@IRSMSX102.ger.corp.intel.com> (raw)
In-Reply-To: <20161121160059.GB3174@twins.programming.kicks-ass.net>

> 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?