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Date:   Tue, 17 Jan 2023 18:23:34 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Hyeonggon Yoo <42.hyeyoo@gmail.com>, akpm@linux-foundation.org,
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Subject: Re: [PATCH 41/41] mm: replace rw_semaphore with atomic_t in vma_lock
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On Mon, Jan 16, 2023 at 09:58:35PM -0800, Suren Baghdasaryan wrote:
> On Mon, Jan 16, 2023 at 9:46 PM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Mon, Jan 16, 2023 at 08:34:36PM -0800, Suren Baghdasaryan wrote:
> > > On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox <willy@infradead.org> wrote:
> > > >
> > > > 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.
> > >
> > > I think the point here was that if page faults keep occuring and they
> > > prevent vm_lock->count from reaching 0 then a writer will be blocked
> > > and there is no reader throttling mechanism (no max time that writer
> > > will be waiting).
> >
> > Perhaps I misunderstood your description; I thought that a _waiting_
> > writer would make the count negative, not a successfully acquiring
> > writer.
> 
> A waiting writer does not modify the counter, instead it's placed on
> the wait queue and the last reader which sets the count to 0 while
> releasing its read lock will wake it up. Once the writer is woken it
> will try to set the count to negative and if successful will own the
> lock, otherwise it goes back to sleep.

Then yes, that's a starvable lock.  Preventing starvation on the mmap
sem was the original motivation for making rwsems non-starvable, so
changing that behaviour now seems like a bad idea.  For efficiency, I'd
suggest that a waiting writer set the top bit of the counter.  That way,
all new readers will back off without needing to check a second variable
and old readers will know that they *may* need to do the wakeup when
atomic_sub_return_release() is negative.

(rwsem.c has a more complex bitfield, but I don't think we need to go
that far; the important point is that the waiting writer indicates its
presence in the count field so that readers can modify their behaviour)