On 11/19, Ivo Sieben wrote: > > Hi > > 2012/11/19 Oleg Nesterov <oleg@xxxxxxxxxx>: > > > > I am wondering if it makes sense unconditionally. A lot of callers do > > > > if (waitqueue_active(q)) > > wake_up(...); > > > > this patch makes the optimization above pointless and adds mb(). > > > > > > But I won't argue. > > > > Oleg. > > > > This patch solved an issue for me that I had with the TTY line > discipline idle handling: > Testing on a PREEMPT_RT system with TTY serial communication. Each > time the TTY line discipline is dereferenced the Idle handling wait > queue is woken up (see function put_ldisc in /drivers/tty/tty_ldisc.c) > However line discipline idle handling is not used very often so the > wait queue is empty most of the time. But still the wake_up() function > enters the critical section guarded by spin locks. This causes > additional scheduling overhead when a lower priority thread has > control of that same lock. > > The /drivers/tty/tty_ldisc.c did not use the waitqueue_active() call > to check if the waitqueue was filled.... maybe I should solve this > problem the other way around: and make tty_ldisc.c first do the > waitqueue_active() call? IMHO yes... Because on a second thought I suspect this change is wrong. Just for example, please look at kauditd_thread(). It does set_current_state(TASK_INTERRUPTIBLE); add_wait_queue(&kauditd_wait, &wait); if (!CONDITION) // <-- LOAD schedule(); And the last LOAD can leak into the critical section protected by wait_queue_head_t->lock, and it can be reordered with list_add() inside this critical section. In this case we can race with wake_up() unless it takes the same lock. Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
