preempt-rt/x86: Handle sending signals from do_trap() by gdb

Index: linux-rt.git/arch/x86/kernel/traps.c
===================================================================
@@ -87,9 +87,21 @@ static inline void conditional_sti(struc
 		local_irq_enable();
 }
 
-static inline void preempt_conditional_sti(struct pt_regs *regs)
+static inline void conditional_sti_ist(struct pt_regs *regs)
 {
+#ifdef CONFIG_X86_64
+	/*
+	 * X86_64 uses a per CPU stack on the IST for certain traps
+	 * like int3. The task can not be preempted when using one
+	 * of these stacks, thus preemption must be disabled, otherwise
+	 * the stack can be corrupted if the task is scheduled out,
+	 * and another task comes in and uses this stack.
+	 *
+	 * On x86_32 the task keeps its own stack and it is OK if the
+	 * task schedules out.
+	 */
 	inc_preempt_count();
+#endif
 	if (regs->flags & X86_EFLAGS_IF)
 		local_irq_enable();
 }

@@ -100,11 +112,13 @@ static inline void conditional_cli(struc
 		local_irq_disable();
 }
 
-static inline void preempt_conditional_cli(struct pt_regs *regs)
+static inline void conditional_cli_ist(struct pt_regs *regs)
 {
 	if (regs->flags & X86_EFLAGS_IF)
 		local_irq_disable();
+#ifdef CONFIG_X86_64
 	dec_preempt_count();
+#endif
 }
 
 static void __kprobes

@@ -222,9 +236,9 @@ dotraplinkage void do_stack_segment(stru
 	if (notify_die(DIE_TRAP, "stack segment", regs, error_code,
 			12, SIGBUS) == NOTIFY_STOP)
 		return;
-	preempt_conditional_sti(regs);
+	conditional_sti_ist(regs);
 	do_trap(12, SIGBUS, "stack segment", regs, error_code, NULL);
-	preempt_conditional_cli(regs);
+	conditional_cli_ist(regs);
 }
 
 dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)

@@ -316,9 +330,9 @@ dotraplinkage void __kprobes do_int3(str
 		return;
 #endif
 
-	preempt_conditional_sti(regs);
+	conditional_sti_ist(regs);
 	do_trap(3, SIGTRAP, "int3", regs, error_code, NULL);
-	preempt_conditional_cli(regs);
+	conditional_cli_ist(regs);
 }
 
 #ifdef CONFIG_X86_64

@@ -412,12 +426,12 @@ dotraplinkage void __kprobes do_debug(st
 		return;
 
 	/* It's safe to allow irq's after DR6 has been saved */
-	preempt_conditional_sti(regs);
+	conditional_sti_ist(regs);
 
 	if (regs->flags & X86_VM_MASK) {
 		handle_vm86_trap((struct kernel_vm86_regs *) regs,
 				error_code, 1);
-		preempt_conditional_cli(regs);
+		conditional_cli_ist(regs);
 		return;
 	}
 

@@ -436,7 +450,7 @@ dotraplinkage void __kprobes do_debug(st
 	si_code = get_si_code(tsk->thread.debugreg6);
 	if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
 		send_sigtrap(tsk, regs, error_code, si_code);
-	preempt_conditional_cli(regs);
+	conditional_cli_ist(regs);
 
 	return;
 }

Index: linux-rt.git/arch/x86/kernel/signal.c
===================================================================
@@ -820,6 +820,15 @@ do_notify_resume(struct pt_regs *regs, v
 		mce_notify_process();
 #endif /* CONFIG_X86_64 && CONFIG_X86_MCE */
 
+#ifdef ARCH_RT_DELAYS_SIGNAL_SEND
+	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);

Index: linux-rt.git/include/linux/sched.h
===================================================================
@@ -1405,6 +1405,10 @@ 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;

Index: linux-rt.git/kernel/signal.c
===================================================================
@@ -1273,8 +1273,8 @@ int do_send_sig_info(int sig, struct sig
  * 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;

@@ -1299,6 +1299,39 @@ force_sig_info(int sig, struct siginfo *
 	return ret;
 }
 
+int force_sig_info(int sig, struct siginfo *info, struct task_struct *t)
+{
+/*
+ * On some archs, PREEMPT_RT has to delay sending a signal from a trap
+ * since it can not enable preemption, and the signal code's spin_locks
+ * turn into mutexes. Instead, it must set TIF_NOTIFY_RESUME which will
+ * send the signal on exit of the trap.
+ */
+#ifdef ARCH_RT_DELAYS_SIGNAL_SEND
+	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.
  */

Index: linux-rt.git/arch/x86/include/asm/signal.h
===================================================================
@@ -31,6 +31,19 @@ typedef struct {
 	unsigned long sig[_NSIG_WORDS];
 } sigset_t;
 
+/*
+ * Because some traps use the IST stack, we must keep
+ * preemption disabled while calling do_trap(), but do_trap()
+ * may call force_sig_info() which will grab the signal spin_locks
+ * for the task, which in PREEMPT_RT_FULL are mutexes.
+ * By defining ARCH_RT_DELAYS_SIGNAL_SEND the force_sig_info() will
+ * set TIF_NOTIFY_RESUME and set up the signal to be sent on exit
+ * of the trap.
+ */
+#if defined(CONFIG_PREEMPT_RT_FULL) && defined(CONFIG_X86_64)
+#define ARCH_RT_DELAYS_SIGNAL_SEND
+#endif
+
 #else
 /* Here we must cater to libcs that poke about in kernel headers.  */