Re: [PATCH 5/5] aio-posix: make AioHandler dispatch O(1) with epoll

[Stefan Hajnoczi]

On Wed, Feb 19, 2020 at 12:13:40PM +0100, Paolo Bonzini wrote:
> On 14/02/20 18:17, Stefan Hajnoczi wrote:
> > +    while ((node = QLIST_FIRST(ready_list))) {
> > +        QLIST_SAFE_REMOVE(node, node_ready);
> 
> Why does this need safe remove?

Yes, it's necessary.  QLIST_SAFE_REMOVE() has two properties that make
it "safe":
1. It doesn't crash if the node is currently not on a list.
2. It clears the node's linked list pointers so that future linked
   list operations (like QLIST_SAFE_REMOVE()) aren't accidentally
   performed on stale pointers.

The node has a long lifespan and will be inserted into ready_lists
multiple times.  We need to safely remove it from ready_list to protect
against a corruption the next time the node is inserted into a
ready_list again:

  /* Add a handler to a ready list */
  static void add_ready_handler(AioHandlerList *ready_list,
                                AioHandler *node,
                                int revents)
  {
      QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list 
*/
      ^---- would cause corruption if node->node_ready was stale!

Would you like me to add a comment?

Stefan

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