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
signature.asc
Description: PGP signature