linux-kernel - Re: [RFC][PATCH 7/7] kref: Implement using refcount

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Message-ID: <20161117124844.GD3117@twins.programming.kicks-ass.net>
Date:   Thu, 17 Nov 2016 13:48:44 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, keescook@...omium.org,
        will.deacon@....com, elena.reshetova@...el.com, arnd@...db.de,
        tglx@...utronix.de, mingo@...nel.org, hpa@...or.com,
        dave@...gbits.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 7/7] kref: Implement using refcount_t

On Thu, Nov 17, 2016 at 12:03:33PM +0100, Greg KH wrote:
> On Thu, Nov 17, 2016 at 11:39:27AM +0100, Peter Zijlstra wrote:

> > And let me note here that RCU users can use a fully relaxed put, because
> > call_rcu() guarantees a grace-period between the call_rcu and the
> > free(), which in turn provides a full memory barrier that orders things.
> > 
> > We could actually expose that to driver writers by doing something like:
> > 
> > 	kref_put_rcu(struct kref *kref, struct rcu_head *head, rcu_callback_t func)
> > 	{
> > 		if (refcount_dec_and_test_relaxed(&kref->refcount))
> > 			call_rcu(head, func);
> > 	}
> > 
> > Do we want to go there?
> 
> No, please no...

Hehe, fair enough. Although core kernel 'power' users might really want
that. The relaxed memory order stuff saves quite a few cycles on weak
archs.