On Tue, Sep 11, 2018 at 02:48:17AM +0100, Dmitry Safonov wrote:
There is a couple of reports about lockup in ldsem_down_read() without anyone holding write end of ldisc semaphore: lkml.kernel.org/r/20171121132855.ajdv4k6swzhvktl6@wfg-t540p.sh.intel.com lkml.kernel.org/r/20180907045041.GF1110@shao2-debian
They all looked like a missed wake up. I wasn't lucky enough to reproduce it, but it seems like reader on another CPU can miss waiter->task update and schedule again, resulting in indefinite (MAX_SCHEDULE_TIMEOUT) sleep.
Make sure waked up reader will see waiter->task == NULL.
--- a/drivers/tty/tty_ldsem.c +++ b/drivers/tty/tty_ldsem.c @@ -118,6 +118,8 @@ static void __ldsem_wake_readers(struct ld_semaphore *sem) tsk = waiter->task; smp_mb(); waiter->task = NULL;
/* Make sure down_read_failed() will see !waiter->task update */smp_wmb();
This is 'wrong', wake_up_process() should imply sufficient for this to already be true.
put_task_struct(tsk);}