Re: [PATCH v3 3/3] sched, x86: Check that we're on the right stack in schedule and __might_sleep

[Andy Lutomirski]

On Wed, Nov 19, 2014 at 3:23 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Wed, Nov 19, 2014 at 3:04 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>
>> Does that include nmi?  I'm a bit afraid of touching that code.
>
> NMI is kind of special, since it's really not supposed to touch
> 'current' or anything like that, and that's how we do preempt-count
> (and that's where irq-count is) right now.
>
> I would prefer to have preempt_count be a percpu variable rather than
> a per-thread one, but there are historical reasons for that horror. Oh
> well.
>
>> It's certainly easy enough to bump irq_count in the paranoid entries.
>
> It definitely shouldn't be done from the assembly code. Usually it's
> "irq_enter/exit()" that does it, but for NMI you'd presumably just do
> it explicitly from do_nmi() or similar. But NMI really is nastier than
> other cases, see above.

My only real objection is that it's going to be ugly and error prone.
It'll have to be something like:

if (!user_mode_vm(regs))
    irq_enter();

...

if (!user_mode_vm(regs))
    irq_exit();

because the whole point of this series is to make the IST entries not
be atomic when they come from userspace.  We can wrap this in an
inline cond_ist_enter/cond_ist_exit or whatever, but still, blech.

Tony already sent the follow-up patch that will make do_machine_check
enable interrupts if it came from userspace.

--Andy

>
> (That said, I thought we did it anyway, but now that I look I can't
> find it. So I was probably just smoking something)
>
>              Linus



-- 
Andy Lutomirski
AMA Capital Management, LLC
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