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Subject[PATCH 4/7] signal: use GROUP_STOP_PENDING to stop once for a single group stop
Date
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: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
---
fs/exec.c | 1 +
include/linux/sched.h | 3 +++
kernel/signal.c | 36 +++++++++++++++++++++---------------
3 files changed, 25 insertions(+), 15 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index c62efcb..03cafa3 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1653,6 +1653,7 @@ static int zap_process(struct task_struct *start, int exit_code)

t = start;
do {
+ task_clear_group_stop(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 4790cf9..f97ba1b 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1759,8 +1759,11 @@ 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(struct task_struct *task);
+
#ifdef CONFIG_PREEMPT_RCU

#define RCU_READ_UNLOCK_BLOCKED (1 << 0) /* blocked while in RCU read-side. */
diff --git a/kernel/signal.c b/kernel/signal.c
index 89aad36..7e85e42 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);
@@ -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(struct task_struct *task)
+void task_clear_group_stop(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(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(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(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(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 @@ 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,
@@ -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:
--
1.7.1


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