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Date:   Tue, 17 Jan 2023 04:14:26 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Hyeonggon Yoo <42.hyeyoo@gmail.com>
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Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock
Message-ID: <Y8YgomKF189vmgLz@casper.infradead.org>
References: <20230109205336.3665937-1-surenb@google.com>
 <20230109205336.3665937-42-surenb@google.com>
 <Y8UxnqPCTLbbD+2F@localhost>
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On Mon, Jan 16, 2023 at 11:14:38AM +0000, Hyeonggon Yoo wrote:
> > @@ -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?

I don't understand how readers can starve a writer.  Readers do
atomic_inc_unless_negative() so a writer can always force readers
to fail.