[PATCH v2 3/3] tty: Use RCU read lock to iterate tasks and threads in __do_SAK()

[Kirill Tkhai]

There were made several efforts to make __do_SAK()
working in process context long ago, but it does
not solves the problem completely. Since __do_SAK()
may take tasklist_lock for a long time, the concurent
processes, waiting for write lock with interrupts
disabled (e.g., forking), get into the same situation
like __do_SAK() would have been executed in interrupt
context. I've observed several hard lockups on 3.10
kernel running 200 containers, caused by long duration
of copy_process()->write_lock_irq() after SAK was sent
to a tty. Current mainline kernel has the same problem.

The solution is to use RCU to iterate processes and threads.
Task list integrity is the only reason we taken tasklist_lock
before, as tty subsys primitives mostly take it for reading
also (e.g., __proc_set_tty). RCU read lock is enough for that.
This patch solves the problem and makes __do_SAK() to be
not greedy of tasklist_lock. That should prevent hard lockups
I've pointed above.

Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
---
 drivers/tty/tty_io.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 89326cee2403..55115e65668d 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2724,7 +2724,9 @@ void __do_SAK(struct tty_struct *tty)
 			   task_pid_nr(p), p->comm);
 		send_sig(SIGKILL, p, 1);
 	} while_each_pid_task(session, PIDTYPE_SID, p);
+	read_unlock(&tasklist_lock);
 
+	rcu_read_lock();
 	/* Now kill any processes that happen to have the tty open */
 	for_each_process(p) {
 		if (p->signal->tty == tty) {
@@ -2754,7 +2756,7 @@ void __do_SAK(struct tty_struct *tty)
 kill:
 		send_sig(SIGKILL, p, 1);
 	}
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 #endif
 }