[PATCH 6/9] signal: Always call do_notify_parent_cldstop with siglock held

["Eric W. Biederman" <ebiederm@xmission.com>]

Now that siglock keeps tsk->parent and tsk->real_parent constant
require that do_notify_parent_cldstop is called with tsk->siglock held
instead of the tasklist_lock.

As all of the callers of do_notify_parent_cldstop had to drop the
siglock and take tasklist_lock this simplifies all of it's callers.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/signal.c | 156 +++++++++++++++++-------------------------------
 1 file changed, 55 insertions(+), 101 deletions(-)

diff --git a/kernel/signal.c b/kernel/signal.c
index 72d96614effc..584d67deb3cb 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -2121,11 +2121,13 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
 				     bool for_ptracer, int why)
 {
 	struct kernel_siginfo info;
-	unsigned long flags;
 	struct task_struct *parent;
 	struct sighand_struct *sighand;
+	bool lock;
 	u64 utime, stime;
 
+	assert_spin_locked(&tsk->sighand->siglock);
+
 	if (for_ptracer) {
 		parent = tsk->parent;
 	} else {
@@ -2164,7 +2166,9 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
  	}
 
 	sighand = parent->sighand;
-	spin_lock_irqsave(&sighand->siglock, flags);
+	lock = tsk->sighand != sighand;
+	if (lock)
+		spin_lock_nested(&sighand->siglock, SINGLE_DEPTH_NESTING);
 	if (sighand->action[SIGCHLD-1].sa.sa_handler != SIG_IGN &&
 	    !(sighand->action[SIGCHLD-1].sa.sa_flags & SA_NOCLDSTOP))
 		send_signal_locked(SIGCHLD, &info, parent, PIDTYPE_TGID);
@@ -2172,7 +2176,8 @@ static void do_notify_parent_cldstop(struct task_struct *tsk,
 	 * Even if SIGCHLD is not generated, we must wake up wait4 calls.
 	 */
 	__wake_up_parent(tsk, parent);
-	spin_unlock_irqrestore(&sighand->siglock, flags);
+	if (lock)
+		spin_unlock(&sighand->siglock);
 }
 
 /*
@@ -2193,7 +2198,6 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
 	__acquires(&current->sighand->siglock)
 {
 	bool gstop_done = false;
-	bool read_code = true;
 
 	if (arch_ptrace_stop_needed()) {
 		/*
@@ -2209,6 +2213,34 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
 		spin_lock_irq(&current->sighand->siglock);
 	}
 
+	/* Don't stop if current is not ptraced */
+	if (unlikely(!current->ptrace))
+		return (clear_code) ? 0 : exit_code;
+
+	/*
+	 * If @why is CLD_STOPPED, we're trapping to participate in a group
+	 * stop.  Do the bookkeeping.  Note that if SIGCONT was delievered
+	 * across siglock relocks since INTERRUPT was scheduled, PENDING
+	 * could be clear now.  We act as if SIGCONT is received after
+	 * TASK_TRACED is entered - ignore it.
+	 */
+	if (why == CLD_STOPPED && (current->jobctl & JOBCTL_STOP_PENDING))
+		gstop_done = task_participate_group_stop(current);
+
+	/*
+	 * Notify parents of the stop.
+	 *
+	 * While ptraced, there are two parents - the ptracer and
+	 * the real_parent of the group_leader.  The ptracer should
+	 * know about every stop while the real parent is only
+	 * interested in the completion of group stop.  The states
+	 * for the two don't interact with each other.  Notify
+	 * separately unless they're gonna be duplicates.
+	 */
+	do_notify_parent_cldstop(current, true, why);
+	if (gstop_done && ptrace_reparented(current))
+		do_notify_parent_cldstop(current, false, why);
+
 	/*
 	 * schedule() will not sleep if there is a pending signal that
 	 * can awaken the task.
@@ -2239,15 +2271,6 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
 	current->last_siginfo = info;
 	current->exit_code = exit_code;
 
-	/*
-	 * If @why is CLD_STOPPED, we're trapping to participate in a group
-	 * stop.  Do the bookkeeping.  Note that if SIGCONT was delievered
-	 * across siglock relocks since INTERRUPT was scheduled, PENDING
-	 * could be clear now.  We act as if SIGCONT is received after
-	 * TASK_TRACED is entered - ignore it.
-	 */
-	if (why == CLD_STOPPED && (current->jobctl & JOBCTL_STOP_PENDING))
-		gstop_done = task_participate_group_stop(current);
 
 	/* any trap clears pending STOP trap, STOP trap clears NOTIFY */
 	task_clear_jobctl_pending(current, JOBCTL_TRAP_STOP);
@@ -2257,56 +2280,19 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
 	/* entering a trap, clear TRAPPING */
 	task_clear_jobctl_trapping(current);
 
+	/*
+	 * Don't want to allow preemption here, because
+	 * sys_ptrace() needs this task to be inactive.
+	 *
+	 * XXX: implement spin_unlock_no_resched().
+	 */
+	preempt_disable();
 	spin_unlock_irq(&current->sighand->siglock);
-	read_lock(&tasklist_lock);
-	if (likely(current->ptrace)) {
-		/*
-		 * Notify parents of the stop.
-		 *
-		 * While ptraced, there are two parents - the ptracer and
-		 * the real_parent of the group_leader.  The ptracer should
-		 * know about every stop while the real parent is only
-		 * interested in the completion of group stop.  The states
-		 * for the two don't interact with each other.  Notify
-		 * separately unless they're gonna be duplicates.
-		 */
-		do_notify_parent_cldstop(current, true, why);
-		if (gstop_done && ptrace_reparented(current))
-			do_notify_parent_cldstop(current, false, why);
 
-		/*
-		 * Don't want to allow preemption here, because
-		 * sys_ptrace() needs this task to be inactive.
-		 *
-		 * XXX: implement read_unlock_no_resched().
-		 */
-		preempt_disable();
-		read_unlock(&tasklist_lock);
-		cgroup_enter_frozen();
-		preempt_enable_no_resched();
-		freezable_schedule();
-		cgroup_leave_frozen(true);
-	} else {
-		/*
-		 * By the time we got the lock, our tracer went away.
-		 * Don't drop the lock yet, another tracer may come.
-		 *
-		 * If @gstop_done, the ptracer went away between group stop
-		 * completion and here.  During detach, it would have set
-		 * JOBCTL_STOP_PENDING on us and we'll re-enter
-		 * TASK_STOPPED in do_signal_stop() on return, so notifying
-		 * the real parent of the group stop completion is enough.
-		 */
-		if (gstop_done)
-			do_notify_parent_cldstop(current, false, why);
-
-		/* tasklist protects us from ptrace_freeze_traced() */
-		__set_current_state(TASK_RUNNING);
-		read_code = false;
-		if (clear_code)
-			exit_code = 0;
-		read_unlock(&tasklist_lock);
-	}
+	cgroup_enter_frozen();
+	preempt_enable_no_resched();
+	freezable_schedule();
+	cgroup_leave_frozen(true);
 
 	/*
 	 * We are back.  Now reacquire the siglock before touching
@@ -2314,8 +2300,7 @@ static int ptrace_stop(int exit_code, int why, int clear_code,
 	 * any signal-sending on another CPU that wants to examine it.
 	 */
 	spin_lock_irq(&current->sighand->siglock);
-	if (read_code)
-		exit_code = current->exit_code;
+	exit_code = current->exit_code;
 	current->last_siginfo = NULL;
 	current->ptrace_message = 0;
 	current->exit_code = 0;
@@ -2444,34 +2429,17 @@ static bool do_signal_stop(int signr)
 	}
 
 	if (likely(!current->ptrace)) {
-		int notify = 0;
-
 		/*
 		 * 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.
+		 * report to the real_parent.
 		 */
 		if (task_participate_group_stop(current))
-			notify = CLD_STOPPED;
+			do_notify_parent_cldstop(current, false, CLD_STOPPED);
 
 		set_special_state(TASK_STOPPED);
 		spin_unlock_irq(&current->sighand->siglock);
 
-		/*
-		 * Notify the parent of the group stop completion.  Because
-		 * we're not holding either the siglock or tasklist_lock
-		 * here, ptracer may attach inbetween; however, this is for
-		 * group stop and should always be delivered to the real
-		 * parent of the group leader.  The new ptracer will get
-		 * its notification when this task transitions into
-		 * TASK_TRACED.
-		 */
-		if (notify) {
-			read_lock(&tasklist_lock);
-			do_notify_parent_cldstop(current, false, notify);
-			read_unlock(&tasklist_lock);
-		}
-
 		/* Now we don't run again until woken by SIGCONT or SIGKILL */
 		cgroup_enter_frozen();
 		freezable_schedule();
@@ -2665,8 +2633,6 @@ bool get_signal(struct ksignal *ksig)
 
 		signal->flags &= ~SIGNAL_CLD_MASK;
 
-		spin_unlock_irq(&sighand->siglock);
-
 		/*
 		 * Notify the parent that we're continuing.  This event is
 		 * always per-process and doesn't make whole lot of sense
@@ -2675,15 +2641,10 @@ bool get_signal(struct ksignal *ksig)
 		 * the ptracer of the group leader too unless it's gonna be
 		 * a duplicate.
 		 */
-		read_lock(&tasklist_lock);
 		do_notify_parent_cldstop(current, false, why);
-
 		if (ptrace_reparented(current->group_leader))
 			do_notify_parent_cldstop(current->group_leader,
 						true, why);
-		read_unlock(&tasklist_lock);
-
-		goto relock;
 	}
 
 	for (;;) {
@@ -2940,7 +2901,6 @@ static void retarget_shared_pending(struct task_struct *tsk, sigset_t *which)
 
 void exit_signals(struct task_struct *tsk)
 {
-	int group_stop = 0;
 	sigset_t unblocked;
 
 	/*
@@ -2971,21 +2931,15 @@ void exit_signals(struct task_struct *tsk)
 	signotset(&unblocked);
 	retarget_shared_pending(tsk, &unblocked);
 
-	if (unlikely(tsk->jobctl & JOBCTL_STOP_PENDING) &&
-	    task_participate_group_stop(tsk))
-		group_stop = CLD_STOPPED;
-out:
-	spin_unlock_irq(&tsk->sighand->siglock);
-
 	/*
 	 * If group stop has completed, deliver the notification.  This
 	 * should always go to the real parent of the group leader.
 	 */
-	if (unlikely(group_stop)) {
-		read_lock(&tasklist_lock);
-		do_notify_parent_cldstop(tsk, false, group_stop);
-		read_unlock(&tasklist_lock);
-	}
+	if (unlikely(tsk->jobctl & JOBCTL_STOP_PENDING) &&
+	    task_participate_group_stop(tsk))
+		do_notify_parent_cldstop(tsk, false, CLD_STOPPED);
+out:
+	spin_unlock_irq(&tsk->sighand->siglock);
 }
 
 /*
-- 
2.35.3