Messages in this thread Patch in this message | | | Subject | [PATCH v2 3/3] tty: Use RCU read lock to iterate tasks and threads in __do_SAK() | From | Kirill Tkhai <> | Date | Wed, 17 Jan 2018 15:39:54 +0300 |
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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 <ktkhai@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 89326cee2403..55115e65668d 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -2724,7 +2724,9 @@ 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); + read_unlock(&tasklist_lock); + rcu_read_lock(); /* Now kill any processes that happen to have the tty open */ for_each_process(p) { if (p->signal->tty == tty) { @@ -2754,7 +2756,7 @@ void __do_SAK(struct tty_struct *tty) kill: send_sig(SIGKILL, p, 1); } - read_unlock(&tasklist_lock); + rcu_read_unlock(); #endif }
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