[Devel] [PATCH RH9 2/2] 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>
Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

(cherry picked from vz7 commit 6aecb63c35a5 ("tty: Use RCU read lock to iterate
tasks and threads in __do_SAK()"))
Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
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 535f40164c2b..e7268372edb6 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3055,8 +3055,10 @@ void __do_SAK(struct tty_struct *tty)
 			   task_pid_nr(p), p->comm);
 		group_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_SID);
 	} while_each_pid_task(session, PIDTYPE_SID, p);
+	read_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) {
 			tty_notice(tty, "SAK: killed process %d (%s): by controlling tty\n",
@@ -3085,7 +3087,7 @@ void __do_SAK(struct tty_struct *tty)
 kill:
 		group_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_SID);
 	}
-	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 	put_pid(session);
 #endif
 }