linux-kernel - Re: [PATCH v2 21/29] nios2: Futex operations

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Message-ID: <CAFiDJ5-uDPCZha-s2-DrLbHLjezBkLTEjJ0nbnsMocof7kO=1Q@mail.gmail.com>
Date:	Fri, 18 Jul 2014 14:07:42 +0800
From:	Ley Foon Tan <lftan@...era.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	Chung-Lin Tang <cltang@...esourcery.com>
Subject: Re: [PATCH v2 21/29] nios2: Futex operations

On Thu, Jul 17, 2014 at 7:07 PM, Arnd Bergmann <arnd@...db.de> wrote:
> The get_user/put_user functions really need to be annotated might_fault(),
> because that's what they do.
>
> The whole point of get_user() is to access an unchecked user space
> pointer, which  can do a number of things based on what the pointer
> points to:
>
> - access a user space variable that resides in memory
> - access a kernel pointer and fail because of the access_ok()
>   check
> - access a user space pointer that is not mapped and return
>   through the __ex_table fixup.
> - access a user space pointer that has a valid VMA but not PTE,
>   causing a page fault to be resolved.
>
> It's the last case that breaks here.
So, do you mean that we can't use get_user/put_user in futex support?
BTW, some architectures like sh,parisc, m68k use get_user in futex
function as well.
Any recommendation way to support futex if we can't use get_user.
Note, nios2 doesn't have atomic instruction.
Thanks.

Regards
Ley Foon
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