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
