6 files changed, 442 insertions, 180 deletions

diff --git a/fs/exec.c b/fs/exec.c
index 5e62d26a4fec..8328beb9016f 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1659,6 +1659,7 @@ static int zap_process(struct task_struct *start, int exit_code)
 
         t = start;
         do {
+                task_clear_group_stop_pending(t);
                 if (t != current && t->mm) {
                         sigaddset(&t->pending.signal, SIGKILL);
                         signal_wake_up(t, 1);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 83bd2e2982fc..3f53c25aa008 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -653,9 +653,8 @@ struct signal_struct {
  * Bits in flags field of signal_struct.
  */
 #define SIGNAL_STOP_STOPPED     0x00000001 /* job control stop in effect */
-#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 */
+#define SIGNAL_STOP_CONTINUED   0x00000002 /* SIGCONT since WCONTINUED reap */
+#define SIGNAL_GROUP_EXIT       0x00000004 /* group exit in progress */
 /*
  * Pending notifications to parent.
  */
@@ -1261,6 +1260,7 @@ struct task_struct {
         int exit_state;
         int exit_code, exit_signal;
         int pdeath_signal;  /*  The signal sent when the parent dies  */
+        unsigned int group_stop;        /* GROUP_STOP_*, siglock protected */
         /* ??? */
         unsigned int personality;
         unsigned did_exec:1;
@@ -1777,6 +1777,17 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
 #define tsk_used_math(p) ((p)->flags & PF_USED_MATH)
 #define used_math() tsk_used_math(current)
 
+/*
+ * task->group_stop flags
+ */
+#define GROUP_STOP_SIGMASK      0xffff    /* signr of the last group stop */
+#define GROUP_STOP_PENDING      (1 << 16) /* task should stop for group stop */
+#define GROUP_STOP_CONSUME      (1 << 17) /* consume group stop count */
+#define GROUP_STOP_TRAPPING     (1 << 18) /* switching from STOPPED to TRACED */
+#define GROUP_STOP_DEQUEUED     (1 << 19) /* stop signal dequeued */
+
+extern void task_clear_group_stop_pending(struct task_struct *task);
+
 #ifdef CONFIG_PREEMPT_RCU
 
 #define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 3a2e66d88a32..b073f3c8adc3 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -469,33 +469,6 @@ static inline int tracehook_get_signal(struct task_struct *task,
 }
 
 /**
- * tracehook_notify_jctl - report about job control stop/continue
- * @notify:             zero, %CLD_STOPPED or %CLD_CONTINUED
- * @why:                %CLD_STOPPED or %CLD_CONTINUED
- *
- * This is called when we might call do_notify_parent_cldstop().
- *
- * @notify is zero if we would not ordinarily send a %SIGCHLD,
- * or is the %CLD_STOPPED or %CLD_CONTINUED .si_code for %SIGCHLD.
- *
- * @why is %CLD_STOPPED when about to stop for job control;
- * we are already in %TASK_STOPPED state, about to call schedule().
- * It might also be that we have just exited (check %PF_EXITING),
- * but need to report that a group-wide stop is complete.
- *
- * @why is %CLD_CONTINUED when waking up after job control stop and
- * ready to make a delayed @notify report.
- *
- * Return the %CLD_* value for %SIGCHLD, or zero to generate no signal.
- *
- * Called with the siglock held.
- */
-static inline int tracehook_notify_jctl(int notify, int why)
-{
-        return notify ?: (current->ptrace & PT_PTRACED) ? why : 0;
-}
-
-/**
  * tracehook_finish_jctl - report about return from job control stop
  *
  * This is called by do_signal_stop() after wakeup.
diff --git a/kernel/exit.c b/kernel/exit.c
index 6a488ad2dce5..5cbc83e83a5d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1538,33 +1538,83 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace,
                 return 0;
         }
 
-        if (likely(!ptrace) && unlikely(task_ptrace(p))) {
+        /* dead body doesn't have much to contribute */
+        if (p->exit_state == EXIT_DEAD)
+                return 0;
+
+        /* slay zombie? */
+        if (p->exit_state == EXIT_ZOMBIE) {
                 /*
-                 * This child is hidden by ptrace.
-                 * We aren't allowed to see it now, but eventually we will.
+                 * A zombie ptracee is only visible to its ptracer.
+                 * Notification and reaping will be cascaded to the real
+                 * parent when the ptracer detaches.
                  */
-                wo->notask_error = 0;
-                return 0;
-        }
+                if (likely(!ptrace) && unlikely(task_ptrace(p))) {
+                        /* it will become visible, clear notask_error */
+                        wo->notask_error = 0;
+                        return 0;
+                }
 
-        if (p->exit_state == EXIT_DEAD)
-                return 0;
+                /* we don't reap group leaders with subthreads */
+                if (!delay_group_leader(p))
+                        return wait_task_zombie(wo, p);
 
-        /*
-         * We don't reap group leaders with subthreads.
-         */
-        if (p->exit_state == EXIT_ZOMBIE && !delay_group_leader(p))
-                return wait_task_zombie(wo, p);
+                /*
+                 * Allow access to stopped/continued state via zombie by
+                 * falling through.  Clearing of notask_error is complex.
+                 *
+                 * When !@ptrace:
+                 *
+                 * If WEXITED is set, notask_error should naturally be
+                 * cleared.  If not, subset of WSTOPPED|WCONTINUED is set,
+                 * so, if there are live subthreads, there are events to
+                 * wait for.  If all subthreads are dead, it's still safe
+                 * to clear - this function will be called again in finite
+                 * amount time once all the subthreads are released and
+                 * will then return without clearing.
+                 *
+                 * When @ptrace:
+                 *
+                 * Stopped state is per-task and thus can't change once the
+                 * target task dies.  Only continued and exited can happen.
+                 * Clear notask_error if WCONTINUED | WEXITED.
+                 */
+                if (likely(!ptrace) || (wo->wo_flags & (WCONTINUED | WEXITED)))
+                        wo->notask_error = 0;
+        } else {
+                /*
+                 * If @p is ptraced by a task in its real parent's group,
+                 * hide group stop/continued state when looking at @p as
+                 * the real parent; otherwise, a single stop can be
+                 * reported twice as group and ptrace stops.
+                 *
+                 * If a ptracer wants to distinguish the two events for its
+                 * own children, it should create a separate process which
+                 * takes the role of real parent.
+                 */
+                if (likely(!ptrace) && task_ptrace(p) &&
+                    same_thread_group(p->parent, p->real_parent))
+                        return 0;
+
+                /*
+                 * @p is alive and it's gonna stop, continue or exit, so
+                 * there always is something to wait for.
+                 */
+                wo->notask_error = 0;
+        }
 
         /*
-         * It's stopped or running now, so it might
-         * later continue, exit, or stop again.
+         * Wait for stopped.  Depending on @ptrace, different stopped state
+         * is used and the two don't interact with each other.
          */
-        wo->notask_error = 0;
-
         if (task_stopped_code(p, ptrace))
                 return wait_task_stopped(wo, ptrace, p);
 
+        /*
+         * Wait for continued.  There's only one continued state and the
+         * ptracer can consume it which can confuse the real parent.  Don't
+         * use WCONTINUED from ptracer.  You don't need or want it.
+         */
         return wait_task_continued(wo, p);
 }
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 0fc1eed28d27..512bd017218d 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -37,35 +37,33 @@ void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
         child->parent = new_parent;
 }
 
-/*
- * Turn a tracing stop into a normal stop now, since with no tracer there
- * would be no way to wake it up with SIGCONT or SIGKILL.  If there was a
- * signal sent that would resume the child, but didn't because it was in
- * TASK_TRACED, resume it now.
- * Requires that irqs be disabled.
- */
-static void ptrace_untrace(struct task_struct *child)
-{
-        spin_lock(&child->sighand->siglock);
-        if (task_is_traced(child)) {
-                /*
-                 * If the group stop is completed or in progress,
-                 * this thread was already counted as stopped.
-                 */
-                if (child->signal->flags & SIGNAL_STOP_STOPPED ||
-                    child->signal->group_stop_count)
-                        __set_task_state(child, TASK_STOPPED);
-                else
-                        signal_wake_up(child, 1);
-        }
-        spin_unlock(&child->sighand->siglock);
-}
-
-/*
- * unptrace a task: move it back to its original parent and
- * remove it from the ptrace list.
+/**
+ * __ptrace_unlink - unlink ptracee and restore its execution state
+ * @child: ptracee to be unlinked
  *
- * Must be called with the tasklist lock write-held.
+ * Remove @child from the ptrace list, move it back to the original parent,
+ * and restore the execution state so that it conforms to the group stop
+ * state.
+ *
+ * Unlinking can happen via two paths - explicit PTRACE_DETACH or ptracer
+ * exiting.  For PTRACE_DETACH, unless the ptracee has been killed between
+ * ptrace_check_attach() and here, it's guaranteed to be in TASK_TRACED.
+ * If the ptracer is exiting, the ptracee can be in any state.
+ *
+ * After detach, the ptracee should be in a state which conforms to the
+ * group stop.  If the group is stopped or in the process of stopping, the
+ * ptracee should be put into TASK_STOPPED; otherwise, it should be woken
+ * up from TASK_TRACED.
+ *
+ * If the ptracee is in TASK_TRACED and needs to be moved to TASK_STOPPED,
+ * it goes through TRACED -> RUNNING -> STOPPED transition which is similar
+ * to but in the opposite direction of what happens while attaching to a
+ * stopped task.  However, in this direction, the intermediate RUNNING
+ * state is not hidden even from the current ptracer and if it immediately
+ * re-attaches and performs a WNOHANG wait(2), it may fail.
+ *
+ * CONTEXT:
+ * write_lock_irq(tasklist_lock)
  */
 void __ptrace_unlink(struct task_struct *child)
 {
@@ -75,8 +73,27 @@ void __ptrace_unlink(struct task_struct *child)
         child->parent = child->real_parent;
         list_del_init(&child->ptrace_entry);
 
-        if (task_is_traced(child))
-                ptrace_untrace(child);
+        spin_lock(&child->sighand->siglock);
+
+        /*
+         * Reinstate GROUP_STOP_PENDING if group stop is in effect and
+         * @child isn't dead.
+         */
+        if (!(child->flags & PF_EXITING) &&
+            (child->signal->flags & SIGNAL_STOP_STOPPED ||
+             child->signal->group_stop_count))
+                child->group_stop |= GROUP_STOP_PENDING;
+
+        /*
+         * If transition to TASK_STOPPED is pending or in TASK_TRACED, kick
+         * @child in the butt.  Note that @resume should be used iff @child
+         * is in TASK_TRACED; otherwise, we might unduly disrupt
+         * TASK_KILLABLE sleeps.
+         */
+        if (child->group_stop & GROUP_STOP_PENDING || task_is_traced(child))
+                signal_wake_up(child, task_is_traced(child));
+
+        spin_unlock(&child->sighand->siglock);
 }
 
 /*
@@ -95,16 +112,14 @@ int ptrace_check_attach(struct task_struct *child, int kill)
          */
         read_lock(&tasklist_lock);
         if ((child->ptrace & PT_PTRACED) && child->parent == current) {
-                ret = 0;
                 /*
                  * child->sighand can't be NULL, release_task()
                  * does ptrace_unlink() before __exit_signal().
                  */
                 spin_lock_irq(&child->sighand->siglock);
-                if (task_is_stopped(child))
-                        child->state = TASK_TRACED;
-                else if (!task_is_traced(child) && !kill)
-                        ret = -ESRCH;
+                WARN_ON_ONCE(task_is_stopped(child));
+                if (task_is_traced(child) || kill)
+                        ret = 0;
                 spin_unlock_irq(&child->sighand->siglock);
         }
         read_unlock(&tasklist_lock);
@@ -168,6 +183,7 @@ bool ptrace_may_access(struct task_struct *task, unsigned int mode)
 
 static int ptrace_attach(struct task_struct *task)
 {
+        bool wait_trap = false;
         int retval;
 
         audit_ptrace(task);
@@ -207,12 +223,42 @@ static int ptrace_attach(struct task_struct *task)
         __ptrace_link(task, current);
         send_sig_info(SIGSTOP, SEND_SIG_FORCED, task);
 
+        spin_lock(&task->sighand->siglock);
+
+        /*
+         * If the task is already STOPPED, set GROUP_STOP_PENDING and
+         * TRAPPING, and kick it so that it transits to TRACED.  TRAPPING
+         * will be cleared if the child completes the transition or any
+         * event which clears the group stop states happens.  We'll wait
+         * for the transition to complete before returning from this
+         * function.
+         *
+         * This hides STOPPED -> RUNNING -> TRACED transition from the
+         * attaching thread but a different thread in the same group can
+         * still observe the transient RUNNING state.  IOW, if another
+         * thread's WNOHANG wait(2) on the stopped tracee races against
+         * ATTACH, the wait(2) may fail due to the transient RUNNING.
+         *
+         * The following task_is_stopped() test is safe as both transitions
+         * in and out of STOPPED are protected by siglock.
+         */
+        if (task_is_stopped(task)) {
+                task->group_stop |= GROUP_STOP_PENDING | GROUP_STOP_TRAPPING;
+                signal_wake_up(task, 1);
+                wait_trap = true;
+        }
+
+        spin_unlock(&task->sighand->siglock);
+
         retval = 0;
 unlock_tasklist:
         write_unlock_irq(&tasklist_lock);
 unlock_creds:
         mutex_unlock(&task->signal->cred_guard_mutex);
 out:
+        if (wait_trap)
+                wait_event(current->signal->wait_chldexit,
+                           !(task->group_stop & GROUP_STOP_TRAPPING));
         return retval;
 }
 
@@ -315,8 +361,6 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
         if (child->ptrace) {
                 child->exit_code = data;
                 dead = __ptrace_detach(current, child);
-                if (!child->exit_state)
-                        wake_up_state(child, TASK_TRACED | TASK_STOPPED);
         }
         write_unlock_irq(&tasklist_lock);
 
diff --git a/kernel/signal.c b/kernel/signal.c
index dc17929ab78a..c15e9792b088 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -124,7 +124,7 @@ static inline int has_pending_signals(sigset_t *signal, sigset_t *blocked)
 
 static int recalc_sigpending_tsk(struct task_struct *t)
 {
-        if (t->signal->group_stop_count > 0 ||
+        if ((t->group_stop & GROUP_STOP_PENDING) ||
             PENDING(&t->pending, &t->blocked) ||
             PENDING(&t->signal->shared_pending, &t->blocked)) {
                 set_tsk_thread_flag(t, TIF_SIGPENDING);
@@ -223,6 +223,83 @@ static inline void print_dropped_signal(int sig)
                                 current->comm, current->pid, sig);
 }
 
+/**
+ * task_clear_group_stop_trapping - clear group stop trapping bit
+ * @task: target task
+ *
+ * If GROUP_STOP_TRAPPING is set, a ptracer is waiting for us.  Clear it
+ * and wake up the ptracer.  Note that we don't need any further locking.
+ * @task->siglock guarantees that @task->parent points to the ptracer.
+ *
+ * CONTEXT:
+ * Must be called with @task->sighand->siglock held.
+ */
+static void task_clear_group_stop_trapping(struct task_struct *task)
+{
+        if (unlikely(task->group_stop & GROUP_STOP_TRAPPING)) {
+                task->group_stop &= ~GROUP_STOP_TRAPPING;
+                __wake_up_sync(&task->parent->signal->wait_chldexit,
+                               TASK_UNINTERRUPTIBLE, 1);
+        }
+}
+
+/**
+ * task_clear_group_stop_pending - clear pending group stop
+ * @task: target task
+ *
+ * Clear group stop states for @task.
+ *
+ * CONTEXT:
+ * Must be called with @task->sighand->siglock held.
+ */
+void task_clear_group_stop_pending(struct task_struct *task)
+{
+        task->group_stop &= ~(GROUP_STOP_PENDING | GROUP_STOP_CONSUME |
+                              GROUP_STOP_DEQUEUED);
+}
+
+/**
+ * task_participate_group_stop - participate in a group stop
+ * @task: task participating in a group stop
+ *
+ * @task has GROUP_STOP_PENDING set and is participating in a group stop.
+ * Group stop states are cleared and the group stop count is consumed if
+ * %GROUP_STOP_CONSUME was set.  If the consumption completes the group
+ * stop, the appropriate %SIGNAL_* flags are set.
+ *
+ * CONTEXT:
+ * Must be called with @task->sighand->siglock held.
+ *
+ * RETURNS:
+ * %true if group stop completion should be notified to the parent, %false
+ * otherwise.
+ */
+static bool task_participate_group_stop(struct task_struct *task)
+{
+        struct signal_struct *sig = task->signal;
+        bool consume = task->group_stop & GROUP_STOP_CONSUME;
+
+        WARN_ON_ONCE(!(task->group_stop & GROUP_STOP_PENDING));
+
+        task_clear_group_stop_pending(task);
+
+        if (!consume)
+                return false;
+
+        if (!WARN_ON_ONCE(sig->group_stop_count == 0))
+                sig->group_stop_count--;
+
+        /*
+         * Tell the caller to notify completion iff we are entering into a
+         * fresh group stop.  Read comment in do_signal_stop() for details.
+         */
+        if (!sig->group_stop_count && !(sig->flags & SIGNAL_STOP_STOPPED)) {
+                sig->flags = SIGNAL_STOP_STOPPED;
+                return true;
+        }
+        return false;
+}
+
 /*
  * allocate a new signal queue record
  * - this may be called without locks if and only if t == current, otherwise an
@@ -527,7 +604,7 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, siginfo_t *info)
                  * is to alert stop-signal processing code when another
                  * processor has come along and cleared the flag.
                  */
-                tsk->signal->flags |= SIGNAL_STOP_DEQUEUED;
+                current->group_stop |= GROUP_STOP_DEQUEUED;
         }
         if ((info->si_code & __SI_MASK) == __SI_TIMER && info->si_sys_private) {
                 /*
@@ -727,34 +804,14 @@ static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns)
         } else if (sig == SIGCONT) {
                 unsigned int why;
                 /*
-                 * Remove all stop signals from all queues,
-                 * and wake all threads.
+                 * Remove all stop signals from all queues, wake all threads.
                  */
                 rm_from_queue(SIG_KERNEL_STOP_MASK, &signal->shared_pending);
                 t = p;
                 do {
-                        unsigned int state;
+                        task_clear_group_stop_pending(t);
                         rm_from_queue(SIG_KERNEL_STOP_MASK, &t->pending);
-                        /*
-                         * If there is a handler for SIGCONT, we must make
-                         * sure that no thread returns to user mode before
-                         * we post the signal, in case it was the only
-                         * thread eligible to run the signal handler--then
-                         * it must not do anything between resuming and
-                         * running the handler.  With the TIF_SIGPENDING
-                         * flag set, the thread will pause and acquire the
-                         * siglock that we hold now and until we've queued
-                         * the pending signal.
-                         *
-                         * Wake up the stopped thread _after_ setting
-                         * TIF_SIGPENDING
-                         */
-                        state = __TASK_STOPPED;
-                        if (sig_user_defined(t, SIGCONT) && !sigismember(&t->blocked, SIGCONT)) {
-                                set_tsk_thread_flag(t, TIF_SIGPENDING);
-                                state |= TASK_INTERRUPTIBLE;
-                        }
-                        wake_up_state(t, state);
+                        wake_up_state(t, __TASK_STOPPED);
                 } while_each_thread(p, t);
 
                 /*
@@ -780,13 +837,6 @@ static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns)
                         signal->flags = why | SIGNAL_STOP_CONTINUED;
                         signal->group_stop_count = 0;
                         signal->group_exit_code = 0;
-                } 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.
-                         */
-                        signal->flags &= ~SIGNAL_STOP_DEQUEUED;
                 }
         }
 
@@ -875,6 +925,7 @@ static void complete_signal(int sig, struct task_struct *p, int group)
                         signal->group_stop_count = 0;
                         t = p;
                         do {
+                                task_clear_group_stop_pending(t);
                                 sigaddset(&t->pending.signal, SIGKILL);
                                 signal_wake_up(t, 1);
                         } while_each_thread(p, t);
@@ -1109,6 +1160,7 @@ int zap_other_threads(struct task_struct *p)
         p->signal->group_stop_count = 0;
 
         while_each_thread(p, t) {
+                task_clear_group_stop_pending(t);
                 count++;
 
                 /* Don't bother with already dead threads */
@@ -1536,16 +1588,30 @@ int do_notify_parent(struct task_struct *tsk, int sig)
         return ret;
 }
 
-static void do_notify_parent_cldstop(struct task_struct *tsk, int why)
+/**
+ * do_notify_parent_cldstop - notify parent of stopped/continued state change
+ * @tsk: task reporting the state change
+ * @for_ptracer: the notification is for ptracer
+ * @why: CLD_{CONTINUED|STOPPED|TRAPPED} to report
+ *
+ * Notify @tsk's parent that the stopped/continued state has changed.  If
+ * @for_ptracer is %false, @tsk's group leader notifies to its real parent.
+ * If %true, @tsk reports to @tsk->parent which should be the ptracer.
+ *
+ * CONTEXT:
+ * Must be called with tasklist_lock at least read locked.
+ */
+static void do_notify_parent_cldstop(struct task_struct *tsk,
+                                     bool for_ptracer, int why)
 {
         struct siginfo info;
         unsigned long flags;
         struct task_struct *parent;
         struct sighand_struct *sighand;
 
-        if (task_ptrace(tsk))
+        if (for_ptracer) {
                 parent = tsk->parent;
-        else {
+        } else {
                 tsk = tsk->group_leader;
                 parent = tsk->real_parent;
         }
@@ -1621,6 +1687,15 @@ static int sigkill_pending(struct task_struct *tsk)
 }
 
 /*
+ * Test whether the target task of the usual cldstop notification - the
+ * real_parent of @child - is in the same group as the ptracer.
+ */
+static bool real_parent_is_ptracer(struct task_struct *child)
+{
+        return same_thread_group(child->parent, child->real_parent);
+}
+
+/*
  * This must be called with current->sighand->siglock held.
  *
  * This should be the path for all ptrace stops.
@@ -1631,10 +1706,12 @@ static int sigkill_pending(struct task_struct *tsk)
  * If we actually decide not to stop at all because the tracer
  * is gone, we keep current->exit_code unless clear_code.
  */
-static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info)
+static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
         __releases(&current->sighand->siglock)
         __acquires(&current->sighand->siglock)
 {
+        bool gstop_done = false;
+
         if (arch_ptrace_stop_needed(exit_code, info)) {
                 /*
                  * The arch code has something special to do before a
@@ -1655,21 +1732,49 @@ static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info)
         }
 
         /*
-         * If there is a group stop in progress,
-         * we must participate in the bookkeeping.
+         * If @why is CLD_STOPPED, we're trapping to participate in a group
+         * stop.  Do the bookkeeping.  Note that if SIGCONT was delievered
+         * while siglock was released for the arch hook, PENDING could be
+         * clear now.  We act as if SIGCONT is received after TASK_TRACED
+         * is entered - ignore it.
          */
-        if (current->signal->group_stop_count > 0)
-                --current->signal->group_stop_count;
+        if (why == CLD_STOPPED && (current->group_stop & GROUP_STOP_PENDING))
+                gstop_done = task_participate_group_stop(current);
 
         current->last_siginfo = info;
         current->exit_code = exit_code;
 
-        /* Let the debugger run.  */
-        __set_current_state(TASK_TRACED);
+        /*
+         * TRACED should be visible before TRAPPING is cleared; otherwise,
+         * the tracer might fail do_wait().
+         */
+        set_current_state(TASK_TRACED);
+
+        /*
+         * We're committing to trapping.  Clearing GROUP_STOP_TRAPPING and
+         * transition to TASK_TRACED should be atomic with respect to
+         * siglock.  This hsould be done after the arch hook as siglock is
+         * released and regrabbed across it.
+         */
+        task_clear_group_stop_trapping(current);
+
         spin_unlock_irq(&current->sighand->siglock);
         read_lock(&tasklist_lock);
         if (may_ptrace_stop()) {
-                do_notify_parent_cldstop(current, CLD_TRAPPED);
+                /*
+                 * 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 && !real_parent_is_ptracer(current))
+                        do_notify_parent_cldstop(current, false, why);
+
                 /*
                  * Don't want to allow preemption here, because
                  * sys_ptrace() needs this task to be inactive.
@@ -1684,7 +1789,16 @@ static void ptrace_stop(int exit_code, int clear_code, siginfo_t *info)
                 /*
                  * 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
+                 * GROUP_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);
+
                 __set_current_state(TASK_RUNNING);
                 if (clear_code)
                         current->exit_code = 0;
@@ -1728,7 +1842,7 @@ void ptrace_notify(int exit_code)
 
         /* Let the debugger run.  */
         spin_lock_irq(&current->sighand->siglock);
-        ptrace_stop(exit_code, 1, &info);
+        ptrace_stop(exit_code, CLD_TRAPPED, 1, &info);
         spin_unlock_irq(&current->sighand->siglock);
 }
 
@@ -1741,66 +1855,115 @@ void ptrace_notify(int exit_code)
 static int do_signal_stop(int signr)
 {
         struct signal_struct *sig = current->signal;
-        int notify;
 
-        if (!sig->group_stop_count) {
+        if (!(current->group_stop & GROUP_STOP_PENDING)) {
+                unsigned int gstop = GROUP_STOP_PENDING | GROUP_STOP_CONSUME;
                 struct task_struct *t;
 
-                if (!likely(sig->flags & SIGNAL_STOP_DEQUEUED) ||
+                /* signr will be recorded in task->group_stop for retries */
+                WARN_ON_ONCE(signr & ~GROUP_STOP_SIGMASK);
+
+                if (!likely(current->group_stop & GROUP_STOP_DEQUEUED) ||
                     unlikely(signal_group_exit(sig)))
                         return 0;
                 /*
-                 * There is no group stop already in progress.
-                 * We must initiate one now.
+                 * There is no group stop already in progress.  We must
+                 * initiate one now.
+                 *
+                 * While ptraced, a task may be resumed while group stop is
+                 * still in effect and then receive a stop signal and
+                 * initiate another group stop.  This deviates from the
+                 * usual behavior as two consecutive stop signals can't
+                 * cause two group stops when !ptraced.  That is why we
+                 * also check !task_is_stopped(t) below.
+                 *
+                 * The condition can be distinguished by testing whether
+                 * SIGNAL_STOP_STOPPED is already set.  Don't generate
+                 * group_exit_code in such case.
+                 *
+                 * This is not necessary for SIGNAL_STOP_CONTINUED because
+                 * an intervening stop signal is required to cause two
+                 * continued events regardless of ptrace.
                  */
-                sig->group_exit_code = signr;
+                if (!(sig->flags & SIGNAL_STOP_STOPPED))
+                        sig->group_exit_code = signr;
+                else
+                        WARN_ON_ONCE(!task_ptrace(current));
 
+                current->group_stop &= ~GROUP_STOP_SIGMASK;
+                current->group_stop |= signr | gstop;
                 sig->group_stop_count = 1;
-                for (t = next_thread(current); t != current; t = next_thread(t))
+                for (t = next_thread(current); t != current;
+                     t = next_thread(t)) {
+                        t->group_stop &= ~GROUP_STOP_SIGMASK;
                         /*
                          * Setting state to TASK_STOPPED for a group
                          * stop is always done with the siglock held,
                          * so this check has no races.
                          */
-                        if (!(t->flags & PF_EXITING) &&
-                            !task_is_stopped_or_traced(t)) {
+                        if (!(t->flags & PF_EXITING) && !task_is_stopped(t)) {
+                                t->group_stop |= signr | gstop;
                                 sig->group_stop_count++;
                                 signal_wake_up(t, 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.  When ptraced, every thread reports itself.
-         */
-        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;
+retry:
+        if (likely(!task_ptrace(current))) {
+                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.
+                 */
+                if (task_participate_group_stop(current))
+                        notify = CLD_STOPPED;
+
                 __set_current_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 */
+                schedule();
+
+                spin_lock_irq(&current->sighand->siglock);
+        } else {
+                ptrace_stop(current->group_stop & GROUP_STOP_SIGMASK,
+                            CLD_STOPPED, 0, NULL);
+                current->exit_code = 0;
         }
-        spin_unlock_irq(&current->sighand->siglock);
 
-        if (notify) {
-                read_lock(&tasklist_lock);
-                do_notify_parent_cldstop(current, notify);
-                read_unlock(&tasklist_lock);
+        /*
+         * GROUP_STOP_PENDING could be set if another group stop has
+         * started since being woken up or ptrace wants us to transit
+         * between TASK_STOPPED and TRACED.  Retry group stop.
+         */
+        if (current->group_stop & GROUP_STOP_PENDING) {
+                WARN_ON_ONCE(!(current->group_stop & GROUP_STOP_SIGMASK));
+                goto retry;
         }
 
-        /* Now we don't run again until woken by SIGCONT or SIGKILL */
-        do {
-                schedule();
-        } while (try_to_freeze());
+        /* PTRACE_ATTACH might have raced with task killing, clear trapping */
+        task_clear_group_stop_trapping(current);
+
+        spin_unlock_irq(&current->sighand->siglock);
 
         tracehook_finish_jctl();
-        current->exit_code = 0;
 
         return 1;
 }
@@ -1814,7 +1977,7 @@ static int ptrace_signal(int signr, siginfo_t *info,
         ptrace_signal_deliver(regs, cookie);
 
         /* Let the debugger run.  */
-        ptrace_stop(signr, 0, info);
+        ptrace_stop(signr, CLD_TRAPPED, 0, info);
 
         /* We're back.  Did the debugger cancel the sig?  */
         signr = current->exit_code;
@@ -1869,18 +2032,36 @@ relock:
          * the CLD_ si_code into SIGNAL_CLD_MASK bits.
          */
         if (unlikely(signal->flags & SIGNAL_CLD_MASK)) {
-                int why = (signal->flags & SIGNAL_STOP_CONTINUED)
-                                ? CLD_CONTINUED : CLD_STOPPED;
+                struct task_struct *leader;
+                int why;
+
+                if (signal->flags & SIGNAL_CLD_CONTINUED)
+                        why = CLD_CONTINUED;
+                else
+                        why = CLD_STOPPED;
+
                 signal->flags &= ~SIGNAL_CLD_MASK;
 
-                why = tracehook_notify_jctl(why, CLD_CONTINUED);
                 spin_unlock_irq(&sighand->siglock);
 
-                if (why) {
-                        read_lock(&tasklist_lock);
-                        do_notify_parent_cldstop(current->group_leader, why);
-                        read_unlock(&tasklist_lock);
-                }
+                /*
+                 * Notify the parent that we're continuing.  This event is
+                 * always per-process and doesn't make whole lot of sense
+                 * for ptracers, who shouldn't consume the state via
+                 * wait(2) either, but, for backward compatibility, notify
+                 * 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);
+
+                leader = current->group_leader;
+                if (task_ptrace(leader) && !real_parent_is_ptracer(leader))
+                        do_notify_parent_cldstop(leader, true, why);
+
+                read_unlock(&tasklist_lock);
+
                 goto relock;
         }
 
@@ -1897,8 +2078,8 @@ relock:
                 if (unlikely(signr != 0))
                         ka = return_ka;
                 else {
-                        if (unlikely(signal->group_stop_count > 0) &&
-                            do_signal_stop(0))
+                        if (unlikely(current->group_stop &
+                                     GROUP_STOP_PENDING) && do_signal_stop(0))
                                 goto relock;
 
                         signr = dequeue_signal(current, &current->blocked,
@@ -2045,17 +2226,19 @@ void exit_signals(struct task_struct *tsk)
                 if (!signal_pending(t) && !(t->flags & PF_EXITING))
                         recalc_sigpending_and_wake(t);
 
-        if (unlikely(tsk->signal->group_stop_count) &&
-                        !--tsk->signal->group_stop_count) {
-                tsk->signal->flags = SIGNAL_STOP_STOPPED;
-                group_stop = tracehook_notify_jctl(CLD_STOPPED, CLD_STOPPED);
-        }
+        if (unlikely(tsk->group_stop & GROUP_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, group_stop);
+                do_notify_parent_cldstop(tsk, false, group_stop);
                 read_unlock(&tasklist_lock);
         }
 }