On Mon, Jul 18, 2016 at 04:39:25PM -0400, Waiman Long wrote: > Ding Tianhong reported a live-lock situation where a constant stream > of incoming optimistic spinners blocked a task in the wait list from > getting the mutex. > > This patch attempts to fix this live-lock condition by enabling the > woken task in the wait queue to enter into an optimistic spinning > loop itself in parallel with the regular spinners in the OSQ. This > should prevent the live-lock condition from happening.
No, two spinners are not in fact starvation proof. It makes the reported life-lock scenario much less likely, but it does not guarantee anything. > + /* > + * Optimistically spinning on the mutex without the wait lock There should either be a '.' at the end of that line, or the next line should not start with a capital. Also, I don't see how the two sentences are related, should they be in the same paragraph? > + * The state has to be set to running to avoid another waker > + * spinning on the on_cpu flag while the woken waiter is > + * spinning on the mutex. > + */ > + acquired = mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, > + true); > spin_lock_mutex(&lock->wait_lock, flags); > } > __set_task_state(task, TASK_RUNNING);
