lkml.org 
[lkml]   [2008]   [Mar]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[RFC,PATCH 1/2] 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 oddity is that if the child is ptraced by another
process, CLD_CONTINUED may be delivered to ->real_parent after ptrace_detach().
Hopefully these problems are minor.

The patch asks for the futher obvious cleanups, I'll send them separately.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

include/linux/sched.h | 6 ++++++
kernel/signal.c | 29 +++++++++++++++++++----------
2 files changed, 25 insertions(+), 10 deletions(-)

--- 25/include/linux/sched.h~1_SIGCONT_IMPL 2008-03-02 15:35:23.000000000 +0300
+++ 25/include/linux/sched.h 2008-03-03 17:06:53.000000000 +0300
@@ -555,6 +555,12 @@ 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)
--- 25/kernel/signal.c~1_SIGCONT_IMPL 2008-03-02 15:35:23.000000000 +0300
+++ 25/kernel/signal.c 2008-03-03 17:06:53.000000000 +0300
@@ -596,10 +596,8 @@ static void handle_stop_signal(int sig,
* 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;
@@ -636,25 +634,23 @@ static void handle_stop_signal(int sig,
* 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;
}
}

@@ -1758,6 +1754,19 @@ int get_signal_to_deliver(siginfo_t *inf

relock:
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;



\
 
 \ /
  Last update: 2008-03-04 19:59    [W:0.043 / U:0.444 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site