[Devel] [PATCH 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.

https://jira.sw.ru/browse/PSBM-80340

Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 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 50b3170e091c..14d05eb6f354 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3027,9 +3027,11 @@ 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);
+	qread_unlock(&tasklist_lock);
 	/* Now kill any processes that happen to have the
 	 * tty open.
 	 */
+	rcu_read_lock();
 	for_each_process(p) {
 		if (p->signal->tty == tty) {
 			printk(KERN_NOTICE "SAK: killed process %d"
@@ -3060,7 +3062,7 @@ void __do_SAK(struct tty_struct *tty)
 kill:
 		send_sig(SIGKILL, p, 1);
 	}
-	qread_unlock(&tasklist_lock);
+	rcu_read_unlock();
 #endif
 }