Re: [PATCH RT 2/2 v4] preempt-rt/x86: Delay calling signals in int3

[Oleg Nesterov <oleg@redhat.com>]

On 02/03, Steven Rostedt wrote:
>
> If
> we can solve this in a clean way using the existing signal
> infrastructure, I'm all for that.

I am not sure, I know almost nothing about rt and about this
low-level stuff. But please look at my attempt below.

So. it is very simple. The patch simply changes force_sig_info() to
check in_atomic(), if it is true we offload the sending to
do_notify_resume(). Of course, I do not know if we can rely on this
check in rt kernels.

Note:

	- The patch adds the new code under CONFIG_PREEMPT_RT_FULL,
	  it should probably check X86_64 or defined(TIF_NOTIFY_RESUME)
	  as well.

	- I think we can later move task->forced_info into restart_block's
	  union.

	- We could modify get_signal_to_deliver() instead of the
	  arch-dependant do_notify_resume(). In this case we do not
	  need TIF_NOTIFY_RESUME, TIF_SIGPENDING is enough.

What do you think?

Oleg.
---

 arch/x86/kernel/signal.c |    9 +++++++++
 include/linux/sched.h    |    4 ++++
 kernel/signal.c          |   31 +++++++++++++++++++++++++++++--
 3 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 46a01bd..22cb8ff 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -816,6 +816,15 @@ do_notify_resume(struct pt_regs *regs, void *unused, __u32 thread_info_flags)
 		mce_notify_process();
 #endif /* CONFIG_X86_64 && CONFIG_X86_MCE */
 
+#ifdef CONFIG_PREEMPT_RT_FULL
+	if (unlikely(current->forced_info.si_signo)) {
+		struct task_struct *t = current;
+		force_sig_info(t->forced_info.si_signo,
+					&t->forced_info, t);
+		t->forced_info.si_signo = 0;
+	}
+#endif
+
 	/* deal with pending signal delivery */
 	if (thread_info_flags & _TIF_SIGPENDING)
 		do_signal(regs);
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2234985..942c545 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1407,6 +1407,10 @@ struct task_struct {
 	sigset_t blocked, real_blocked;
 	sigset_t saved_sigmask;	/* restored if set_restore_sigmask() was used */
 	struct sigpending pending;
+#ifdef CONFIG_PREEMPT_RT_FULL
+	/* TODO: move me into ->restart_block ? */
+	struct siginfo forced_info;
+#endif
 
 	unsigned long sas_ss_sp;
 	size_t sas_ss_size;
diff --git a/kernel/signal.c b/kernel/signal.c
index c73c428..5c0b61a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1228,8 +1228,8 @@ int do_send_sig_info(int sig, struct siginfo *info, struct task_struct *p,
  * We don't want to have recursive SIGSEGV's etc, for example,
  * that is why we also clear SIGNAL_UNKILLABLE.
  */
-int
-force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
+static int
+do_force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
 {
 	unsigned long int flags;
 	int ret, blocked, ignored;
@@ -1254,6 +1254,33 @@ force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
 	return ret;
 }
 
+int force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
+{
+#ifdef CONFIG_PREEMPT_RT_FULL
+	if (in_atomic()) {
+		if (WARN_ON_ONCE(t != current))
+			return 0;
+		if (WARN_ON_ONCE(t->forced_info.si_signo))
+			return 0;
+
+		if (is_si_special(info)) {
+			WARN_ON_ONCE(info != SEND_SIG_PRIV);
+			t->forced_info.si_signo = sig;
+			t->forced_info.si_errno = 0;
+			t->forced_info.si_code = SI_KERNEL;
+			t->forced_info.si_pid = 0;
+			t->forced_info.si_uid = 0;
+		} else {
+			t->forced_info = *info;
+		}
+
+		set_tsk_thread_flag(t, TIF_NOTIFY_RESUME);
+		return 0;
+	}
+#endif
+	return do_force_sig_info(sig, info, t);
+}
+
 /*
  * Nuke all other threads in the group.
  */