[PATCHv3 2/6] tty/ldsem: Update waiter->task before waking up reader

[Dmitry Safonov <dima@arista.com>]

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.

Cc: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 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