signals: re-assign CLD_CONTINUED notification from the sender to reciever

Based on discussion with Jiri and Roland.

In short: currently handle_stop_signal(SIGCONT, p) sends the notification to
p->parent, with this patch p itself notifies its parent when it becomes
running.

handle_stop_signal(SIGCONT) has to drop ->siglock temporary in order to notify
the parent with do_notify_parent_cldstop().  This leads to multiple problems:

	- as Jiri Kosina pointed out, the stopped task can resume without
	  actually seeing SIGCONT which may have a handler.

	- we race with another sig_kernel_stop() signal which may come in
	  that window.

	- we race with sig_fatal() signals which may set SIGNAL_GROUP_EXIT
	  in that window.

	- we can't avoid taking tasklist_lock() while sending SIGCONT.

With this patch handle_stop_signal() just sets the new SIGNAL_CLD_CONTINUED
flag in p->signal->flags and returns.  The notification is sent by the first
task which returns from finish_stop() (there should be at least one) or any
other signalled thread from get_signal_to_deliver().

This is a user-visible change.  Say, currently kill(SIGCONT, stopped_child)
can't return without seeing SIGCHLD, with this patch SIGCHLD can be delayed
unpredictably.  Another difference is that if the child is ptraced by another
process, CLD_CONTINUED may be delivered to ->real_parent after ptrace_detach()
while currently it always goes to the tracer which doesn't actually need this
notification.  Hopefully not a problem.

The patch asks for the futher obvious cleanups, I'll send them separately.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 25 insertions, 10 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 1d02babdb2c7..ef5615270342 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -554,6 +554,12 @@ struct signal_struct {
 #define SIGNAL_STOP_DEQUEUED    0x00000002 /* stop signal dequeued */
 #define SIGNAL_STOP_CONTINUED   0x00000004 /* SIGCONT since WCONTINUED reap */
 #define SIGNAL_GROUP_EXIT       0x00000008 /* group exit in progress */
+/*
+ * Pending notifications to parent.
+ */
+#define SIGNAL_CLD_STOPPED      0x00000010
+#define SIGNAL_CLD_CONTINUED    0x00000020
+#define SIGNAL_CLD_MASK         (SIGNAL_CLD_STOPPED|SIGNAL_CLD_CONTINUED)
 
 /* If true, all threads except ->group_exit_task have pending SIGKILL */
 static inline int signal_group_exit(const struct signal_struct *sig)
diff --git a/kernel/signal.c b/kernel/signal.c
index 91d57f89f5a5..115c04f3f143 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -603,10 +603,8 @@ static void handle_stop_signal(int sig, struct task_struct *p)
                          * the SIGCHLD was pending on entry to this kill.
                          */
                         p->signal->group_stop_count = 0;
-                        p->signal->flags = SIGNAL_STOP_CONTINUED;
-                        spin_unlock(&p->sighand->siglock);
-                        do_notify_parent_cldstop(p, CLD_STOPPED);
-                        spin_lock(&p->sighand->siglock);
+                        p->signal->flags = SIGNAL_STOP_CONTINUED |
+                                                SIGNAL_CLD_STOPPED;
                 }
                 rm_from_queue(SIG_KERNEL_STOP_MASK, &p->signal->shared_pending);
                 t = p;
@@ -643,25 +641,23 @@ static void handle_stop_signal(int sig, struct task_struct *p)
                          * We were in fact stopped, and are now continued.
                          * Notify the parent with CLD_CONTINUED.
                          */
-                        p->signal->flags = SIGNAL_STOP_CONTINUED;
+                        p->signal->flags = SIGNAL_STOP_CONTINUED |
+                                                SIGNAL_CLD_CONTINUED;
                         p->signal->group_exit_code = 0;
-                        spin_unlock(&p->sighand->siglock);
-                        do_notify_parent_cldstop(p, CLD_CONTINUED);
-                        spin_lock(&p->sighand->siglock);
                 } else {
                         /*
                          * We are not stopped, but there could be a stop
                          * signal in the middle of being processed after
                          * being removed from the queue.  Clear that too.
                          */
-                        p->signal->flags = 0;
+                        p->signal->flags &= ~SIGNAL_STOP_DEQUEUED;
                 }
         } else if (sig == SIGKILL) {
                 /*
                  * Make sure that any pending stop signal already dequeued
                  * is undone by the wakeup for SIGKILL.
                  */
-                p->signal->flags = 0;
+                p->signal->flags &= ~SIGNAL_STOP_DEQUEUED;
         }
 }
 
@@ -1784,6 +1780,19 @@ relock:
         try_to_freeze();
 
         spin_lock_irq(&current->sighand->siglock);
+
+        if (unlikely(current->signal->flags & SIGNAL_CLD_MASK)) {
+                int why = (current->signal->flags & SIGNAL_STOP_CONTINUED)
+                                ? CLD_CONTINUED : CLD_STOPPED;
+                current->signal->flags &= ~SIGNAL_CLD_MASK;
+                spin_unlock_irq(&current->sighand->siglock);
+
+                read_lock(&tasklist_lock);
+                do_notify_parent_cldstop(current->group_leader, why);
+                read_unlock(&tasklist_lock);
+                goto relock;
+        }
+
         for (;;) {
                 struct k_sigaction *ka;