linux-kernel - Re: [PATCH v3 01/29] asm-generic: add generic futex for !CONFIG

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Date:	Tue, 23 Sep 2014 23:47:59 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"LF.Tan" <lftan.linux@...il.com>
cc:	Linux-Arch <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Arnd Bergmann <arnd@...db.de>, Ley Foon Tan <lftan@...era.com>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH v3 01/29] asm-generic: add generic futex for
 !CONFIG_SMP

On Tue, 23 Sep 2014, LF.Tan wrote:
> On Mon, Sep 8, 2014 at 5:22 PM, Ley Foon Tan <lftan@...era.com> wrote:
> > Follow m68k futex implementation for !CONFIG_SMP.

Great. Follow arch/random implementation blindly and copy all the bugs
in it.

> > +static inline int
> > +futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
> > +                             u32 oldval, u32 newval)
> > +{
> > +       u32 val;
> > +
> > +       if (unlikely(get_user(val, uaddr) != 0))
> > +               return -EFAULT;
> > +
> > +       if (val == oldval && unlikely(put_user(newval, uaddr) != 0))
> > +               return -EFAULT;
> > +
> > +       *uval = val;
> > +
> > +       return 0;
> > +}

Care to explain how this works reliably and resembles proper cmpxchg
semantics under all circumstances?

Thanks,

	tglx
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