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Date: Mon, 6 Feb 2017 09:12:37 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: elena.reshetova@...el.com, gregkh@...uxfoundation.org,
keescook@...omium.org, arnd@...db.de, tglx@...utronix.de,
mingo@...nel.org, h.peter.anvin@...el.com, will.deacon@....com,
dwindsor@...il.com, dhowells@...hat.com,
linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 4/5] atomic: Introduce atomic_try_cmpxchg()
On Mon, Feb 06, 2017 at 12:24:28PM +0800, Boqun Feng wrote:
> On Fri, Feb 03, 2017 at 02:26:02PM +0100, Peter Zijlstra wrote:
> >
> > for (;;) {
> > new = val $op $imm;
> > if (try_cmpxchg(ptr, &val, new))
> > break;
> > }
> >
> > while also generating better code (GCC6 and onwards).
> >
>
> But switching to try_cmpxchg() will make @val a memory location, which
> could not be put in a register. And this will generate unnecessary
> memory accesses on archs having enough registers(PPC, e.g.).
GCC was perfectly capable of making @val a register in the code I was
looking at.
> > +#ifndef atomic_try_cmpxchg
> > +
> > +#define __atomic_try_cmpxchg(type, _p, _po, _n) \
> > +({ \
> > + typeof(_po) __po = (_po); \
> > + typeof(*(_po)) __o = *__po; \
> > + bool success = (atomic_cmpxchg##type((_p), __o, (_n)) == __o); \
> > + *__po = __o; \
>
> Besides, is this part correct? atomic_cmpxchg_*() wouldn't change the
> value of __o, so *__po wouldn't be changed.. IOW, in case of failure,
> *ptr wouldn't be updated to a new value.
>
> Maybe this should be:
>
> bool success;
> *__po = atomic_cmpxchg##type((_p), __o, (_n));
> sucess = (*__po == _o);
>
> , right?
Yes, botched that. Don't think I even compiled it to be honest :/
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