There is a couple of reports about lockup in ldsem_down_read() without anyone holding write end of ldisc semaphore: lkml.kernel.org/r/<[email protected]> 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. Cc: [email protected] Cc: Greg Kroah-Hartman <[email protected]> Cc: Jiri Slaby <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: "Paul E. McKenney" <[email protected]> Reported-by: kernel test robot <[email protected]> Signed-off-by: Dmitry Safonov <[email protected]> --- drivers/tty/tty_ldsem.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c index 0c98d88f795a..832accbbcb6d 100644 --- 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(); wake_up_process(tsk); put_task_struct(tsk); } @@ -217,7 +219,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout) for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); - if (!waiter.task) + if (!READ_ONCE(waiter.task)) break; if (!timeout) break; -- 2.13.6
