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SubjectRe: [patch] x86: some lock annotations for user copy paths
On Wed, Sep 10, 2008 at 05:01:04PM +0200, Peter Zijlstra wrote:
> On Wed, 2008-09-10 at 16:48 +0200, Nick Piggin wrote:
>
> > @@ -3016,3 +3016,18 @@ void print_vma_addr(char *prefix, unsign
> > }
> > up_read(&current->mm->mmap_sem);
> > }
> > +
> > +void might_fault(void)
> > +{
> > + /*
> > + * it would be nicer only to annotatea paths which are not under
> > + * pagefault_disable, however that requires a larger audit and
> > + * providing helpers like get_user_atomic.
> > + */
> > + if (!in_atomic()) {
> > + might_sleep();
> > + if (current->mm)
> > + might_lock_read(&current->mm->mmap_sem);
> > + }
> > +}
> > +EXPORT_SYMBOL(might_fault);
>
> >From the nitpick squad :-), I prefer the form:
>
> void might_fault(void)
> {
> if (in_atomic())
> return;
>
> might_sleep();
>
> if (!current->mm)
> might_lock_read(&current->mm->mmap_sem);
> }
>
>
> Due to it being one nesting level less.

Well... yeah. I find it doesn't matter unless the function is complex or
nesting level high in absolute terms ;)


> > Index: linux-2.6/include/linux/kernel.h
> > ===================================================================
> > --- linux-2.6.orig/include/linux/kernel.h
> > +++ linux-2.6/include/linux/kernel.h
> > @@ -140,6 +140,15 @@ extern int _cond_resched(void);
> > (__x < 0) ? -__x : __x; \
> > })
> >
> > +#ifdef CONFIG_LOCKDEP
> > +void might_fault(void);
> > +#else
> > +static inline void might_fault(void)
> > +{
> > + might_sleep();
> > +}
> > +#endif
> > +
> > extern struct atomic_notifier_head panic_notifier_list;
> > extern long (*panic_blink)(long time);
> > NORET_TYPE void panic(const char * fmt, ...)
>
> This forgets that in_atomic() again - possibly triggering might_sleep()
> where not appropriate.

OK, drat... maybe we'll just noop that path and lose a couple of
might_sleep() points -- testing and developing should be done mostly
under lockdep anyway.


> I'm not sure its worth it to out-of-line the thing though (its only big
> on debug builds), and CONFIG_LOCKDEP is the wrong CONFIG_* variable, I
> think CONFIG_PROVE_LOCKING would be the appropriate one.

It is so that we don't need to pull sched.h and lockdep.h into those low
level user access functions.


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