Re: atomic copy_from_user?

[Andrew Morton <akpm@osdl.org>]

Linus Torvalds <torvalds@osdl.org> wrote:
>
> > From some naughty place in the code where might_sleep
>  > would trigger, I'd like to read from user memory.
>  > I'll pretty much assume that mlockall() has been
>  > called. Suppose that "current" is correct as well.
>  > I'd just use a pointer directly, except that:
>  > 
>  > a. it isn't OK for the 4g/4g feature, s390, or sparc64
>  > b. it causes the "sparse" type checker to complain
>  > c. it will oops or worse if the user screwed up
>  > 
>  > If the page is swapped out, I want a failed copy.
> 
>  the sequence
> 
>  	local_bh_disable();
>  	err = get_user(n, ptr);
>  	local_bh_enable();
>  	if (!err)
>  		.. 'n' .. was the value
> 
>  will do this in 2.6.x, except it will complain loudly about the unatomic 
>  access. Other than that, it will do what you ask for.

An explicit inc_preempt_count() would be clearer.  See how ia32's
kmap_atomic() does it.  And filemap_copy_from_user().