[Devel] [PATCH RHEL8 COMMIT] tty: Use RCU read lock to iterate tasks and threads in __do_SAK()

[Konstantin Khorenko]

The commit is pushed to "branch-rh8-4.18.0-240.1.1.vz8.5.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh8-4.18.0-240.1.1.vz8.5.27
------>
commit 2d6adea14caacb91cc588fa1ab44fe391d5cd98c
Author: Kirill Tkhai <ktkhai at virtuozzo.com>
Date:   Tue May 11 19:19:06 2021 +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>
    
    (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>
---
 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 c20eb63d700b..1bd5a9d4e416 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2790,8 +2790,10 @@ 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",
@@ -2820,7 +2822,7 @@ 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();
 #endif
 }