On 12/28, Oleg Nesterov wrote:
int __wake_up(struct wait_queue_head *wq_head, unsigned int mode, int nr_exclusive, void *key) {
- if (list_empty(&wq_head->head)) {
struct list_head *pn;/** pairs with spin_unlock_irqrestore(&wq_head->lock);* We actually do not need to acquire wq_head->lock, we just* need to be sure that there is no prepare_to_wait() that* completed on any CPU before __wake_up was called.* Thus instead of load_acquiring the spinlock and dropping* it again, we load_acquire the next list entry and check* that the list is not empty.*/pn = smp_load_acquire(&wq_head->head.next);if(pn == &wq_head->head)return 0;- }
Too subtle for me ;)
I have some concerns, but I need to think a bit more to (try to) actually understand this change.
If nothing else, consider
int CONDITION; wait_queue_head_t WQ;
void wake(void) { CONDITION = 1; wake_up(WQ); }
void wait(void) { DEFINE_WAIT_FUNC(entry, woken_wake_function);
add_wait_queue(WQ, entry); if (!CONDITION) wait_woken(entry, ...); remove_wait_queue(WQ, entry); }
this code is correct even if LOAD(CONDITION) can leak into the critical section in add_wait_queue(), so CPU running wait() can actually do
// add_wait_queue spin_lock(WQ->lock); LOAD(CONDITION); // false! list_add(entry, head); spin_unlock(WQ->lock);
if (!false) // result of the LOAD above wait_woken(entry, ...);
Now suppose that another CPU executes wake() between LOAD(CONDITION) and list_add(entry, head). With your patch wait() will miss the event. The same for __pollwait(), I think...
No?
Oleg.