Date: Mon, 21 Nov 2016 16:49:15 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: "Reshetova, Elena" <elena.reshetova@intel.com>
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()
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On Mon, Nov 21, 2016 at 03:39:19PM +0000, Reshetova, Elena wrote:
> > By the way, there are several sites where the use of
> > atomic_t/atomic_wrap_t as a counter ventures beyond the standard (inc,
> > dec, add, sub, read, set) operations we're planning on implementing
> > for both refcount_t and stats_t. 
> 
> 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.

> Also, simple atomic_dec() is used pretty much everywhere for counters,
> which we don’t have a straight match in refcount_t API.

WARN_ON(refcount_dec_and_test(refs));

And seeing how I've implememented refcount_inc() in similar terms:

WARN_ON(!refcount_inc_not_zero(refs));

It might make sense to actually provide that.