Commit e4420551 authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

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: default avatarOleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 3b5e9e53

include/linux/sched.h

+6 −0
Original line number Diff line number Diff line
@@ -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)

kernel/signal.c

+19 −10
Original line number Diff line number Diff line
@@ -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;

CRA Git | Maintained and supported by SUSTech CRA and CCSE