> On Fri, Mar 27, 2015 at 04:40:50PM +0100, David Hildenbrand wrote:
> > e.g. futex_atomic_op_inuser(): easy to fix, add preempt_enable/disable
> > respectively.
> >
> > e.g. futex_atomic_cmpxchg_inatomic(): not so easy / nice to fix.
> >
> > The "inatomic" variants rely on the caller to make sure that preemption is
> > disabled.
> >
> > pagefault_disable();
> > ret = futex_atomic_cmpxchg_inatomic(curval, uaddr, uval, newval);
> > pagefault_enable();
>
> Typically the _inatomic() variants of functions have the exception
> tables required for fixups and can return -EFAULT. In that regard the
> futex_atomic_cmpxchg_inatomic() is consistently named.
>
> In specific the above is taken from cmpxchg_futex_value_locked(), which
> is private to futex.c, so we don't really need to worry about it.
>
> Furthermore, the futex.c helpers that wrap them in pagefault_disable()
> do so because they want to handle the fault themselves. I don't think we
> need to worry about that.
I totally agree with pagefault_disable() and that -EFAULT logic to handle that
themselves. I'm basically only concerned about implicitly used disabled
preemption.
>
> > 1. We could simply add preempt_disable/enable to the calling code. However that
> > results in _all_ futex_atomic_cmpxchg_inatomic() running with disabled
> > preemption, although the implementation doesn't really need it. So there is not
> > really a "decoupling", but to counters to set.
>
> Not really needed, the few callsites where they are not already under a
> lock is where we want to explicitly handle the -EFAULT case ourselves.
>
> > 2. We could add the preempt_disable/enable to the implementations that only
> > need it, leaving calling code as is. However, then the name
> > "futex_atomic_cmpxchg_inatomic" is misleading, because it has nothing to do
> > with "inatomic" anymore.
>
> The _inatomic() naming is because it _can_ be called from atomic
> context, like __copy_to_user_inatomic(). It doesn't mean it has to.
Well, they have to be called from an pagefault_disabled environment (for now
atomic). Atomic context is optional, with a few exceptions (see next section).
> These functions work just fine outside of atomic regions.
To make clear what I'm worried about, have a look at the following code taken
from include/asm-generic/futex.h):
static inline int
futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
u32 oldval, u32 newval)
{
u32 val;
if (unlikely(get_user(val, uaddr) != 0))
return -EFAULT;
if (val == oldval && unlikely(put_user(newval, uaddr) != 0))
return -EFAULT;
*uval = val;
return 0;
}
This _has to_ be called from an atomic context. Otherwise the logic is broken
(mutual exclusion). Not adding a preempt_disable() somewhere in the calling code
(or the function itself) will not allow this function to work properly. At
least that's my understanding :)
And we have exactly that case when we drop preempt_disable() from pagefault_disable()
in the futex code.
My quick hack for this special case would be to add preempt_disable/enable to
that function body. But maybe I am totally wrong about that given code and
preemption.
>
> And they still can be used in atomic regions, but now
> pagefault_disable() will also trigger the exception fixup.
>
> I don't think we should worry too much about this.
>
> > The same applies to other "inatomic" functions. I think most of these functions
> > rely on pagefaults to be disabled in order to work correctly, not disabled
> > preemption.
I agree. The kmap_atomic stuff is another candidate I identified that needs
additional preempt_disable().
> >
> > Any idea how to fix this or what would be the way to go?
>
> I have the feeling you're over thinking this. _inatomic() has exception
> fixups and will return -EFAULT when it cannot do the pagefault in place,
> for whatever reason -- traditionally because of atomic context, now also
> pagefault_disable().
Haha, well I don't want to break things. And places like the futex code look
suspicious. That's why I better double check with an expert.
>
> And esp. things like futexes have been extensively used under -rt and
> are known good.
Yes, on most configuration, but maybe not all (archs that use asm-generic code
+ !CONFIG_SMP + CONFIG_PREEMPT)
Thanks for your reply.
David
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