From patchwork Thu Jan 18 09:48:58 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: [3/3] tty: Use RCU read lock to iterate tasks and threads in __do_SAK() From: Kirill Tkhai X-Patchwork-Id: 7214 Message-Id: <151626893719.1120.9984106521883691963.stgit@localhost.localdomain> To:,,, Date: Thu, 18 Jan 2018 12:48:58 +0300 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 --- 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 }