On Thu, May 28, 2015 at 01:48:25AM +0300, Cyrill Gorcunov wrote:
> On Thu, May 28, 2015 at 01:29:42AM +0300, Alexey Dobriyan wrote:
> > > > +
> > > > + page = (char *)__get_free_page(GFP_TEMPORARY);
> > > > + if (!page) {
> > > > + rv = -ENOMEM;
> > > > + goto out_mmput;
> > > > + }
> > > > +
> > > > + down_read(&mm->mmap_sem);
> > > > + arg_start = mm->arg_start;
> > > > + arg_end = mm->arg_end;
> > > > + env_start = mm->env_start;
> > > > + env_end = mm->env_end;
> > > > + up_read(&mm->mmap_sem);
> > >
> > > Could you please explain why this down/up is needed?
> >
> > Code is written this way to get constistent snapshot of data.
>
> it does not. you fetch data into local variables which is the
> same as simply read them locklessly in general (because later
> you refer to local vars).
It is snapshot w.r.t getting both pairs not snapshot w.r.t atomicity or
something (unsigned long access is atomic after all). Once down_write()
is used in the other place, it even becomes obviously correct code!
> > If you look at PR_SET_MM_* code, you'll notice down_read(&mm->mmap_sem)
> > as well which is a separate bug because you're _writing_ those fields
> > eventually in prctl_set_mm(), yuck!
>
> yes, there members are modified under read-lock and initially
> i didn't see any problem with that except one can have inconsistent
> statistics output because another process modified these fields
> (we validate that new members are having sane values at least
> in new interface and after your first patch). But now I think
> that down_write may be more suitable here.
>
> Cyrill
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