Re: [PATCH] futex: Set USER_DS for the futex_detect_cmpxchg() test

Andy Lutomirski
2018-09-28T14:11:44Z

> On Sep 28, 2018, at 1:42 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
>> On Fri, 28 Sep 2018, Martin Schwidefsky wrote:
>> On Fri, 28 Sep 2018 09:12:10 +0200
>> Geert Uytterhoeven <geert@linux-m68k.org> wrote:
>>> I don't know if that has happened, and whether it would work on s390 now.
>>
>> commit 03b8c7b623c80af264c4c8d6111e5c6289933666
>> Author: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Date: Sun Mar 2 13:09:47 2014 +0100
>>
>> futex: Allow architectures to skip futex_atomic_cmpxchg_inatomic() test
>>
>> If an architecture has futex_atomic_cmpxchg_inatomic() implemented and there
>> is no runtime check necessary, allow to skip the test within futex_init().
>>
>> This allows to get rid of some code which would always give the same result,
>> and also allows the compiler to optimize a couple of if statements away.
>>
>> Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
>> Cc: Finn Thain <fthain@telegraphics.com.au>
>> Cc: Geert Uytterhoeven <geert@linux-m68k.org>
>> Link: http://lkml.kernel.org/r/20140302120947.GA3641@osiris
>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>>
>>
>> Heiko created the CONFIG_HAVE_FUTEX_CMPXCHG to get around this issue.
>> We just skip the runtime check as well as arc, m68k and sh. Not sure
>> about xtensa, the set it config option only for !MMU.
>
> Duh. grep would have told me. -ENOTENOUGHCOFFEE
>
>

There’s another way to skin this cat: keep KERNEL_DS but pass a valid pointer. I don’t suppose you remember why you didn’t do that?