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SubjectRe: atomic copy_from_user?
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().


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