On Wed 2019-11-20 10:30:05, Sergey Senozhatsky wrote: > On (19/11/19 10:41), Petr Mladek wrote: > [..] > > > > I do not like this. As a result, normal printk() will always deadlock > > > > in the scheduler code, including WARN() calls. The chance of the > > > > deadlock is small now. It happens only when there is another > > > > process waiting for console_sem. > > > > > > Why would it *always* deadlock? If this is the case, why we don't *always* > > > deadlock doing the very same wake_up_process() from console_unlock()? > > > > I speak about _normal_ printk() and not about printk_deferred(). > > > > wake_up_process() is called in console_unlock() only when > > sem->wait_list is not empty, see up() in kernel/locking/semaphore.c. > > printk() itself uses console_trylock() and does not wait. > > > I believe that this is the rason why printk_sched() was added > > so late in 2012. > > Right. I also think scheduler people do pretty nice work avoiding printk > calls under ->rq lock. > > What I tried to say - it's really not that hard to have a non-empty > console_sem ->wait_list, any "wrong" printk() call from scheduler > will deadlock us, because we have something to wake_up(). I am sorry but I do not take this as an argument that it would be acceptable to replace irq_work_queue() with wake_up_interruptible(). It is the first time that I hear about problem caused by the irq_work(). But we deal with deadlocks caused by wake_up() for years. It would be like replacing a lightly dripping tap with a heavily dripping one. I see reports with WARN() from scheduler code from time to time. I would get reports about silent death instead. RT guys are going to make printk() fully lockless. It would be really great achievement. irq_work is lockless. While wake_up() is not. There must be a better way how to break the infinite loop caused by the irq_work. Best Regards, Petr
