linux-kernel - Re: [RFC][PATCH 2/7] kref: Add kref

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Message-ID: <20161121154915.GB3124@twins.programming.kicks-ass.net>
Date:   Mon, 21 Nov 2016 16:49:15 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     David Windsor <dwindsor@...il.com>,
        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 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.