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.