linux-kernel - Re: [BUG] uml panics with "Segfault with no mm" in v3.19-rc

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Date:	Wed, 10 Dec 2014 13:13:25 +0100
From:	Richard Weinberger <richard@....at>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	Miklos Szeredi <miklos@...redi.hu>,
	Ley Foon Tan <lftan@...era.com>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Daniel Walter <sahne@...0.at>,
	user-mode-linux-devel <user-mode-linux-devel@...ts.sourceforge.net>
Subject: Re: [BUG] uml panics with "Segfault with no mm" in v3.19-rc

Am 10.12.2014 um 12:59 schrieb Geert Uytterhoeven:
> On Wed, Dec 10, 2014 at 11:49 AM, Richard Weinberger <richard@....at> wrote:
>> Am 10.12.2014 um 11:46 schrieb Miklos Szeredi:
>>> The guilty commit is:
>>>
>>>   00f634bc522d "asm-generic: add generic futex for !CONFIG_SMP"
>>
>> Thanks a lot Miklos!
>> Your bisecting faster than I do.
>>
>> Let's dig into the issue!
> 
> Do you need "select HAVE_FUTEX_CMPXCHG if FUTEX"?

> Cfr. commit e571c58f313d35c5 ("m68k: Skip futex_atomic_cmpxchg_inatomic()
> test") and commit 03b8c7b623c80af2 ("futex: Allow architectures to skip
> futex_atomic_cmpxchg_inatomic() test").

Bingo!

If UML selects HAVE_FUTEX_CMPXCHG the code path I've described in my previous mail
is no longer taken and makes it work again.
I did only a small test but nothing exploded so far.

> BTW, I still think the real problem is the wrong address space, cfr.
> "[PATCH/RFC] futex: Switch to USER_DS for futex test"
> (http://www.spinics.net/lists/linux-m68k/msg06597.html), so you may also
> want to try that.
> However, that caused problems on s390, as it ran too early:
> http://permalink.gmane.org/gmane.linux.kernel.next/30165

Yeah, this would also make sense for UML.

Thanks,
//richard
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