2009-07-29 01:51:38

by Oleg Nesterov

Subject: [PATCH 1/1] do_signal_stop: do not call tracehook_notify_jctl() in TASK_STOPPED state

do_signal_stop() can call tracehook_notify_jctl() before decrementing
->group_stop_count and setting TASK_STOPPED/SIGNAL_STOP_STOPPED.

This way the tracing hooks can drop and reacquire the siglock freely
and do any blocking hooks without potential SIGCONT races.

With this patch TASK_STOPPED/SIGNAL_STOP_STOPPED is set only when we
know for sure we are going to schedule() after unlock(siglock).

Signed-off-by: Oleg Nesterov <[email protected]>
---

kernel/signal.c | 40 ++++++++++++++++++----------------------
1 file changed, 18 insertions(+), 22 deletions(-)

--- __UTRACE/kernel/signal.c~1_DO_SIGNAL_STOP 2009-07-29 03:10:15.000000000 +0200
+++ __UTRACE/kernel/signal.c 2009-07-29 03:17:49.000000000 +0200
@@ -1682,16 +1682,9 @@ void ptrace_notify(int exit_code)
static int do_signal_stop(int signr)
{
struct signal_struct *sig = current->signal;
- int stop_count;
int notify;

- if (sig->group_stop_count > 0) {
- /*
- * There is a group stop in progress. We don't need to
- * start another one.
- */
- stop_count = --sig->group_stop_count;
- } else {
+ if (!sig->group_stop_count) {
struct task_struct *t;

if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||
@@ -1703,7 +1696,7 @@ static int do_signal_stop(int signr)
*/
sig->group_exit_code = signr;

- stop_count = 0;
+ sig->group_stop_count = 1;
for (t = next_thread(current); t != current; t = next_thread(t))
/*
* Setting state to TASK_STOPPED for a group
@@ -1712,25 +1705,28 @@ static int do_signal_stop(int signr)
*/
if (!(t->flags & PF_EXITING) &&
!task_is_stopped_or_traced(t)) {
- stop_count++;
+ sig->group_stop_count++;
signal_wake_up(t, 0);
}
- sig->group_stop_count = stop_count;
}
-
- if (stop_count == 0)
- sig->flags = SIGNAL_STOP_STOPPED;
- current->exit_code = sig->group_exit_code;
- __set_current_state(TASK_STOPPED);
-
/*
* If there are no other threads in the group, or if there is
- * a group stop in progress and we are the last to stop,
- * report to the parent. When ptraced, every thread reports itself.
+ * a group stop in progress and we are the last to stop, report
+ * to the parent. When ptraced, every thread reports itself.
*/
- notify = tracehook_notify_jctl(stop_count == 0 ? CLD_STOPPED : 0,
- CLD_STOPPED);
-
+ notify = sig->group_stop_count == 1 ? CLD_STOPPED : 0;
+ notify = tracehook_notify_jctl(notify, CLD_STOPPED);
+ /*
+ * tracehook_notify_jctl() can drop and reacquire siglock, so
+ * we keep ->group_stop_count != 0 before the call. If SIGCONT
+ * or SIGKILL comes in between ->group_stop_count == 0.
+ */
+ if (sig->group_stop_count) {
+ if (!--sig->group_stop_count)
+ sig->flags = SIGNAL_STOP_STOPPED;
+ current->exit_code = sig->group_exit_code;
+ __set_current_state(TASK_STOPPED);
+ }
spin_unlock_irq(&current->sighand->siglock);

if (notify) {

2009-07-29 22:46:00

by Roland McGrath

Subject: Re: [PATCH 1/1] do_signal_stop: do not call tracehook_notify_jctl() in TASK_STOPPED state

Acked-by: Roland McGrath <[email protected]>