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

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Date:	Wed, 24 Sep 2014 18:29:27 +0800
From:	Ley Foon Tan <lftan@...era.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH v3 01/29] asm-generic: add generic futex for !CONFIG_SMP

On Wed, Sep 24, 2014 at 5:47 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 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?

kernel/futex.c:cmpxchg_futex_value_locked() is already called to
pagefault_disable() before call to futex_atomic_cmpxchg_inatomic().
For UP kernel and pagefault_disable() also means there is no preemption.

Regards
Ley Foon
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