linux-kernel - Re: [RFC] [PATCH] Pre-emption control for userspace

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Date:	Thu, 6 Mar 2014 15:04:47 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
cc:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Khalid Aziz <khalid.aziz@...cle.com>,
	Ingo Molnar <mingo@...nel.org>, akpm@...ux-foundation.org,
	andi.kleen@...el.com, rob@...dley.net, viro@...iv.linux.org.uk,
	oleg@...hat.com, venki@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] Pre-emption control for userspace

On Thu, 6 Mar 2014, Rasmus Villemoes wrote:

> Peter Zijlstra <peterz@...radead.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?

All these variants can sleep, as they might trip a page fault.....

If you want to access user space from a context which cant sleep you
need to use the __copy_to/from_user_inatomic variants. If the access
fails then they return -EFAULT and you need to deal with that
yourself.

Thanks,

	tglx



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