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Date: Tue, 4 Mar 2008 21:57:44 +0300 From: Oleg Nesterov <oleg@...sign.ru> To: Andrew Morton <akpm@...ux-foundation.org>, Roland McGrath <roland@...hat.com> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Davide Libenzi <davidel@...ilserver.org>, Ingo Molnar <mingo@...e.hu>, Jiri Kosina <jkosina@...e.cz>, Linus Torvalds <torvalds@...ux-foundation.org>, Pavel Emelyanov <xemul@...nvz.org>, linux-kernel@...r.kernel.org 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@...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(¤t->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(¤t->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; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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