Re: [PATCH v5.5 09/10] mmap locking API: add mmap_assert_locked() and mmap_assert_write_locked()

[Vlastimil Babka <vbabka@suse.cz>]

On 4/24/20 3:38 AM, Michel Lespinasse wrote:
> Add new APIs to assert that mmap_sem is held.
> 
> Using this instead of rwsem_is_locked and lockdep_assert_held[_write]
> makes the assertions more tolerant of future changes to the lock type.
> 
> Signed-off-by: Michel Lespinasse <walken@google.com>
> ---
>  arch/x86/events/core.c    |  2 +-
>  fs/userfaultfd.c          |  6 +++---
>  include/linux/mmap_lock.h | 14 ++++++++++++++
>  mm/gup.c                  |  2 +-
>  mm/hmm.c                  |  2 +-
>  mm/memory.c               |  2 +-
>  mm/mmu_notifier.c         |  6 +++---
>  mm/pagewalk.c             |  6 +++---
>  mm/util.c                 |  2 +-
>  9 files changed, 28 insertions(+), 14 deletions(-)
> 

...

> @@ -73,4 +75,16 @@ static inline void mmap_read_unlock_non_owner(struct mm_struct *mm)
>  	up_read_non_owner(&mm->mmap_sem);
>  }
>  
> +static inline void mmap_assert_locked(struct mm_struct *mm)
> +{
> +	VM_BUG_ON_MM(!lockdep_is_held_type(&mm->mmap_sem, -1), mm);
> +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
> +}
> +
> +static inline void mmap_assert_write_locked(struct mm_struct *mm)
> +{
> +	VM_BUG_ON_MM(!lockdep_is_held_type(&mm->mmap_sem, 0), mm);
> +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
> +}

I would remove VM_BUG_ON_MM() from the lockdep part. If kernel has lockdep
enabled, it's already in heavy debugging mode enough so let's just use it and
not depend on DEBUG_VM. Many sites you convert don't require DEBUG_VM for the
lockdep checks.

With that you can also use the standard lockdep_assert_held() and
lockdep_assert_held_write() wrappers.

If user has both lockdep and DEBUG_VM enabled, should we run both variants?
Perhaps lockdep is enough as it's more comprehensive? Your initial v5 version
was doing that.