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Date:   Mon, 16 Jan 2023 11:14:38 +0000
From:   Hyeonggon Yoo <42.hyeyoo@gmail.com>
To:     Suren Baghdasaryan <surenb@google.com>
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Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock
Message-ID: <Y8UxnqPCTLbbD+2F@localhost>
References: <20230109205336.3665937-1-surenb@google.com>
 <20230109205336.3665937-42-surenb@google.com>
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On Mon, Jan 09, 2023 at 12:53:36PM -0800, Suren Baghdasaryan wrote:
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d40bf8a5e19e..294dd44b2198 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -627,12 +627,16 @@ static inline void vma_write_lock(struct vm_area_struct *vma)
>  	 * mm->mm_lock_seq can't be concurrently modified.
>  	 */
>  	mm_lock_seq = READ_ONCE(vma->vm_mm->mm_lock_seq);
> -	if (vma->vm_lock_seq == mm_lock_seq)
> +	if (vma->vm_lock->lock_seq == mm_lock_seq)
>  		return;
>  
> -	down_write(&vma->vm_lock->lock);
> -	vma->vm_lock_seq = mm_lock_seq;
> -	up_write(&vma->vm_lock->lock);
> +	if (atomic_cmpxchg(&vma->vm_lock->count, 0, -1))
> +		wait_event(vma->vm_mm->vma_writer_wait,
> +			   atomic_cmpxchg(&vma->vm_lock->count, 0, -1) == 0);
> +	vma->vm_lock->lock_seq = mm_lock_seq;
> +	/* Write barrier to ensure lock_seq change is visible before count */
> +	smp_wmb();
> +	atomic_set(&vma->vm_lock->count, 0);
>  }
>  
>  /*
> @@ -643,20 +647,28 @@ static inline void vma_write_lock(struct vm_area_struct *vma)
>  static inline bool vma_read_trylock(struct vm_area_struct *vma)
>  {
>  	/* Check before locking. A race might cause false locked result. */
> -	if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
> +	if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))
>  		return false;
>  
> -	if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0))
> +	if (unlikely(!atomic_inc_unless_negative(&vma->vm_lock->count)))
>  		return false;
>  
> +	/* If atomic_t overflows, restore and fail to lock. */
> +	if (unlikely(atomic_read(&vma->vm_lock->count) < 0)) {
> +		if (atomic_dec_and_test(&vma->vm_lock->count))
> +			wake_up(&vma->vm_mm->vma_writer_wait);
> +		return false;
> +	}
> +
>  	/*
>  	 * Overflow might produce false locked result.
>  	 * False unlocked result is impossible because we modify and check
>  	 * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq
>  	 * modification invalidates all existing locks.
>  	 */
> -	if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
> -		up_read(&vma->vm_lock->lock);
> +	if (unlikely(vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) {
> +		if (atomic_dec_and_test(&vma->vm_lock->count))
> +			wake_up(&vma->vm_mm->vma_writer_wait);
>  		return false;
>  	}

With this change readers can cause writers to starve.
What about checking waitqueue_active() before or after increasing
vma->vm_lock->count?

--
Thanks,
Hyeonggon