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SubjectRe: [RFC PATCH] asynchronous page fault.
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On Fri, 2009-12-25 at 10:51 +0900, KAMEZAWA Hiroyuki wrote:
> /*
> + * Returns vma which contains given address. This scans rb-tree in speculative
> + * way and increment a reference count if found. Even if vma exists in rb-tree,
> + * this function may return NULL in racy case. So, this function cannot be used
> + * for checking whether given address is valid or not.
> + */
> +struct vm_area_struct *
> +find_vma_speculative(struct mm_struct *mm, unsigned long addr)
> +{
> + struct vm_area_struct *vma = NULL;
> + struct vm_area_struct *vma_tmp;
> + struct rb_node *rb_node;
> +
> + if (unlikely(!mm))
> + return NULL;;
> +
> + rcu_read_lock();
> + rb_node = rcu_dereference(mm->mm_rb.rb_node);
> + vma = NULL;
> + while (rb_node) {
> + vma_tmp = rb_entry(rb_node, struct vm_area_struct, vm_rb);
> +
> + if (vma_tmp->vm_end > addr) {
> + vma = vma_tmp;
> + if (vma_tmp->vm_start <= addr)
> + break;
> + rb_node = rcu_dereference(rb_node->rb_left);
> + } else
> + rb_node = rcu_dereference(rb_node->rb_right);
> + }
> + if (vma) {
> + if ((vma->vm_start <= addr) && (addr < vma->vm_end)) {
> + if (!atomic_inc_not_zero(&vma->refcnt))

And here you destroy pretty much all advantage of having done the
lockless lookup ;-)

The idea is to let the RCU lock span whatever length you need the vma
for, the easy way is to simply use PREEMPT_RCU=y for now, the hard way
is to also incorporate the drop-mmap_sem on blocking patches from a
while ago.

> + vma = NULL;
> + } else
> + vma = NULL;
> + }
> + rcu_read_unlock();
> + return vma;
> +}



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