Re: [patch v2 1/2] mm, mmu_notifier: annotate mmu notifiers with blockable invalidate callbacks

[Tetsuo Handa]

On 2017/12/16 8:04, Andrew Morton wrote:
>> The implementation is steered toward an expensive slowpath, such as after
>> the oom reaper has grabbed mm->mmap_sem of a still alive oom victim.
> 
> some tweakage, please review.
> 
> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Subject: mm-mmu_notifier-annotate-mmu-notifiers-with-blockable-invalidate-callbacks-fix
> 
> make mm_has_blockable_invalidate_notifiers() return bool, use rwsem_is_locked()
> 

> @@ -240,13 +240,13 @@ EXPORT_SYMBOL_GPL(__mmu_notifier_invalid
>   * Must be called while holding mm->mmap_sem for either read or write.
>   * The result is guaranteed to be valid until mm->mmap_sem is dropped.
>   */
> -int mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
> +bool mm_has_blockable_invalidate_notifiers(struct mm_struct *mm)
>  {
>  	struct mmu_notifier *mn;
>  	int id;
> -	int ret = 0;
> +	bool ret = false;
>  
> -	WARN_ON_ONCE(down_write_trylock(&mm->mmap_sem));
> +	WARN_ON_ONCE(!rwsem_is_locked(&mm->mmap_sem));
>  
>  	if (!mm_has_notifiers(mm))
>  		return ret;

rwsem_is_locked() test isn't equivalent with __mutex_owner() == current test, is it?
If rwsem_is_locked() returns true because somebody else has locked it, there is
no guarantee that current thread has locked it before calling this function.

down_write_trylock() test isn't equivalent with __mutex_owner() == current test, is it?
What if somebody else held it for read or write (the worst case is registration path),
down_write_trylock() will return false even if current thread has not locked it for
read or write.

I think this WARN_ON_ONCE() can not detect incorrect call to this function.