Re: [PATCH v5 09/10] mmap locking API: add mmap_assert_locked

[Matthew Wilcox <willy@infradead.org>]

On Tue, Apr 21, 2020 at 07:10:39PM -0700, Michel Lespinasse wrote:
> On Tue, Apr 21, 2020 at 5:14 PM Michel Lespinasse <walken@google.com> wrote:
> > +static inline void mmap_assert_locked(struct mm_struct *mm)
> > +{
> > +       if (IS_ENABLED(CONFIG_LOCKDEP) && debug_locks)
> > +               VM_BUG_ON_MM(!lockdep_is_held(&mm->mmap_sem), mm);
> > +       else
> > +               VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
> > +}
> 
> Turns out this function definition does not work for !CONFIG_LOCKDEP
> as lockdep_is_held is not defined in that case.

Oops, sorry.  It only defines

#define lockdep_is_held_type(l, r)              (1)
#define lockdep_assert_held(l)                  do { (void)(l); } while (0)
#define lockdep_assert_held_write(l)    do { (void)(l); } while (0)
#define lockdep_assert_held_read(l)             do { (void)(l); } while (0)
#define lockdep_assert_held_once(l)             do { (void)(l); } while (0)

which seems like an oversight, but not one that you should be fixing.

> The following should work instead:
> 
> static inline void mmap_assert_locked(struct mm_struct *mm)
> {
> #ifdef CONFIG_LOCKDEP
>         if (debug_locks) {
>                 VM_BUG_ON_MM(!lockdep_is_held(&mm->mmap_lock), mm);
>                 return;
>         }
> #endif
>         VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_lock), mm);
> }

Yes, I agree.