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

[Thomas Gleixner]

On Thu, 6 Mar 2014, Rasmus Villemoes wrote:

> Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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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