Re: [PATCH] [PATCH] Gaurantee spinlocks implicit barrier for !PREEMPT_COUNT

[Peter Zijlstra <peterz@infradead.org>]

On Sun, 2013-04-07 at 21:48 -0700, Linus Torvalds wrote:
> That said, thinking about barriers and preemption made me realize that
> we do have a potential issue between: (a) non-preemption UP kernel
> (with no barriers in the preempt_enable/disable()) and (b)
> architectures that use inline asm without a memory clobber for
> get_user/put_user(). That includes x86.
> 
> The reason is that I could imagine code like
> 
>     if (get_user(val, addr))
>         return -EFAULT;
>     preempt_disable();
>     ... do something percpu ..
>     preempt_enable();
> 
> and it turns out that for non-preempt UP, we don't tell the compiler
> anywhere that it can't move the get_user past the preempt_disable. But
> the get_user() can cause a preemption point because of a page fault,
> obviously.
> 
> I suspect that the best way to fix this ends up relying on the gcc
> "asm volatile" rules, and make the rule be that:
>  - preempt_disable/enable have to generate an asm volatile() string
> (preferably just a ASM comment saying "preempt disable/enable")
>  - get_user/put_user doesn't need to add a memory clobber, but needs
> to be done with an asm volatile too.
> 
> Then the gcc "no reordering of volatile asms" should make the above be
> ok, without having to add an actual compiler memory barrier.
> 
> Ingo? Peter? I'm not sure anybody really uses UP+no-preempt on x86,
> but it does seem to be a bug.. Comments?

Right, stuff between preempt_disable() and preempt_enable() is supposed
to appear atomic wrt scheduling contexts, allowing any schedule to
happen in between would violate this.

I'm not seeing how this would be UP only though, I can see the same
thing happening on SMP+no-preempt.

Also, is {get,put}_user() the only construct that can do this? If so,
using the "asm volatile" rules as described might be the best way,
otherwise making the PREEMPT_COUNT=n operations be compiler barriers
seems like the safer option.

That said, I can't remember ever having seen a BUG like this, even
though !PREEMPT is (or at least was) the most popular distro setting.