[Devel] [PATCH RHEL7 COMMIT] tty: Use RCU read lock to iterate tasks and threads in __do_SAK()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-693.11.6.vz7.42.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-693.11.6.vz7.42.1
------>
commit 6aecb63c35a56397228c2db879977db9937cb523
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Thu Jan 18 19:26:13 2018 +0300

    tty: Use RCU read lock to iterate tasks and threads in __do_SAK()
    
    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>
---
 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
 }