Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

[Peter Zijlstra]

On Mon, Dec 16, 2024 at 11:24:13AM -0800, Suren Baghdasaryan wrote:
> +static inline void vma_refcount_put(struct vm_area_struct *vma)
> +{
> +	int refcnt;
> +
> +	if (!__refcount_dec_and_test(&vma->vm_refcnt, &refcnt)) {
> +		rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> +
> +		if (refcnt & VMA_STATE_LOCKED)
> +			rcuwait_wake_up(&vma->vm_mm->vma_writer_wait);
> +	}
> +}
> +
>  /*
>   * Try to read-lock a vma. The function is allowed to occasionally yield false
>   * locked result to avoid performance overhead, in which case we fall back to
> @@ -710,6 +728,8 @@ static inline void vma_lock_init(struct vm_area_struct *vma)
>   */
>  static inline bool vma_start_read(struct vm_area_struct *vma)
>  {
> +	int oldcnt;
> +
>  	/*
>  	 * Check before locking. A race might cause false locked result.
>  	 * We can use READ_ONCE() for the mm_lock_seq here, and don't need
> @@ -720,13 +740,20 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
>  	if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence))
>  		return false;
>  
> +
> +	rwsem_acquire_read(&vma->vmlock_dep_map, 0, 0, _RET_IP_);
> +	/* Limit at VMA_STATE_LOCKED - 2 to leave one count for a writer */
> +	if (unlikely(!__refcount_inc_not_zero_limited(&vma->vm_refcnt, &oldcnt,
> +						      VMA_STATE_LOCKED - 2))) {
> +		rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
>  		return false;
> +	}
> +	lock_acquired(&vma->vmlock_dep_map, _RET_IP_);
>  
>  	/*
> +	 * Overflow of vm_lock_seq/mm_lock_seq might produce false locked result.
>  	 * False unlocked result is impossible because we modify and check
> +	 * vma->vm_lock_seq under vma->vm_refcnt protection and mm->mm_lock_seq
>  	 * modification invalidates all existing locks.
>  	 *
>  	 * We must use ACQUIRE semantics for the mm_lock_seq so that if we are
> @@ -734,10 +761,12 @@ static inline bool vma_start_read(struct vm_area_struct *vma)
>  	 * after it has been unlocked.
>  	 * This pairs with RELEASE semantics in vma_end_write_all().
>  	 */
> +	if (oldcnt & VMA_STATE_LOCKED ||
> +	    unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) {
> +		vma_refcount_put(vma);

Suppose we have detach race with a concurrent RCU lookup like:

					vma = mas_lookup();

	vma_start_write();
	mas_detach();
					vma_start_read()
					rwsem_acquire_read()
					inc // success
	vma_mark_detach();
	dec_and_test // assumes 1->0
		     // is actually 2->1

					if (vm_lock_seq == vma->vm_mm_mm_lock_seq) // true
					  vma_refcount_put
					    dec_and_test() // 1->0
					      *NO* rwsem_release()



>  		return false;
>  	}
> +
>  	return true;
>  }