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Commit 39efa3ef authored by Tejun Heo's avatar Tejun Heo
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signal: Use GROUP_STOP_PENDING to stop once for a single group stop 



Currently task->signal->group_stop_count is used to decide whether to
stop for group stop.  However, if there is a task in the group which
is taking a long time to stop, other tasks which are continued by
ptrace would repeatedly stop for the same group stop until the group
stop is complete.

Conversely, if a ptraced task is in TASK_TRACED state, the debugger
won't get notified of group stops which is inconsistent compared to
the ptraced task in any other state.

This patch introduces GROUP_STOP_PENDING which tracks whether a task
is yet to stop for the group stop in progress.  The flag is set when a
group stop starts and cleared when the task stops the first time for
the group stop, and consulted whenever whether the task should
participate in a group stop needs to be determined.  Note that now
tasks in TASK_TRACED also participate in group stop.

This results in the following behavior changes.

* For a single group stop, a ptracer would see at most one stop
  reported.

* A ptracee in TASK_TRACED now also participates in group stop and the
  tracer would get the notification.  However, as a ptraced task could
  be in TASK_STOPPED state or any ptrace trap could consume group
  stop, the notification may still be missing.  These will be
  addressed with further patches.

* A ptracee may start a group stop while one is still in progress if
  the tracer let it continue with stop signal delivery.  Group stop
  code handles this correctly.

Oleg:

* Spotted that a task might skip signal check even when its
  GROUP_STOP_PENDING is set.  Fixed by updating
  recalc_sigpending_tsk() to check GROUP_STOP_PENDING instead of
  group_stop_count.

* Pointed out that task->group_stop should be cleared whenever
  task->signal->group_stop_count is cleared.  Fixed accordingly.

* Pointed out the behavior inconsistency between TASK_TRACED and
  RUNNING and the last behavior change.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarOleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
parent e5c1902e

fs/exec.c

+1 −0
Original line number Diff line number Diff line
@@ -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);

include/linux/sched.h

+3 −0
Original line number Diff line number Diff line
@@ -1775,8 +1775,11 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t * 
/*

 * task->group_stop flags

 */

#define GROUP_STOP_PENDING	(1 << 16) /* task should stop for group stop */

#define GROUP_STOP_CONSUME	(1 << 17) /* consume group stop count */



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. */

kernel/signal.c

+21 −15
Original line number Diff line number Diff line
@@ -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);
@@ -232,19 +232,19 @@ static inline void print_dropped_signal(int sig) 
 * CONTEXT:

 * Must be called with @task->sighand->siglock held.

 */

static void task_clear_group_stop_pending(struct task_struct *task)

void task_clear_group_stop_pending(struct task_struct *task)

{

	task->group_stop &= ~GROUP_STOP_CONSUME;

	task->group_stop &= ~(GROUP_STOP_PENDING | GROUP_STOP_CONSUME);

}



/**

 * task_participate_group_stop - participate in a group stop

 * @task: task participating in a group stop

 *

 * @task 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.

 * @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.
@@ -254,6 +254,8 @@ 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)
@@ -765,6 +767,9 @@ static int prepare_signal(int sig, struct task_struct *p, int from_ancestor_ns) 
		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
@@ -906,6 +911,7 @@ 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);
@@ -1139,6 +1145,7 @@ 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 */
@@ -1690,7 +1697,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info) 
	 * If there is a group stop in progress,

	 * we must participate in the bookkeeping.

	 */

	if (current->signal->group_stop_count > 0)

	if (current->group_stop & GROUP_STOP_PENDING)

		task_participate_group_stop(current);



	current->last_siginfo = info;
@@ -1775,8 +1782,8 @@ static int do_signal_stop(int signr) 
	struct signal_struct *sig = current->signal;

	int notify = 0;



	if (!sig->group_stop_count) {

		unsigned int gstop = GROUP_STOP_CONSUME;

	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) ||
@@ -1796,8 +1803,7 @@ static int do_signal_stop(int signr) 
			 * 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 = gstop;

				sig->group_stop_count++;

				signal_wake_up(t, 0);
@@ -1926,8 +1932,8 @@ int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka, 
		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,
@@ -2073,7 +2079,7 @@ 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) &&

	if (unlikely(tsk->group_stop & GROUP_STOP_PENDING) &&

	    task_participate_group_stop(tsk))

		group_stop = CLD_STOPPED;

out: