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Date: Thu, 6 Mar 2014 15:02:43 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Khalid Aziz <khalid.aziz@oracle.com>,
        tglx@linutronix.de, Ingo Molnar <mingo@kernel.org>,
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        viro@zeniv.linux.org.uk, oleg@redhat.com, venki@google.com,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace
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On Thu, Mar 06, 2014 at 02:45:00PM +0100, Rasmus Villemoes wrote:
> Peter Zijlstra <peterz@infradead.org> writes:
> 
> > On Thu, Mar 06, 2014 at 02:24:43PM +0100, Rasmus Villemoes wrote:
> >> Is it possible to implement non-sleeping versions of {get,put}_user()?
> >
> > __{get,put}_user()
> 
> Huh?
> 
> arch/x86/include/asm/uaccess.h:
> 
> /**
>  * __get_user: - Get a simple variable from user space, with less checking.
>  * @x:   Variable to store result.
>  * @ptr: Source address, in user space.
>  *
>  * Context: User context only.  This function may sleep.
> 
> What am I missing?

__get_user() -> __get_user_nocheck() -> __get_user_size() -> __get_user_asm()

And __get_user_asm() seems to generate the required .fixup section for
this to work in pagefault_disable() context.