arm64: signal: Use ARCH_RT_DELAYS_SIGNAL_SEND.

The software breakpoint is handled via do_debug_exception() which
disables preemption. On PREEMPT_RT spinlock_t become sleeping locks and
must not be acquired with disabled preemption.

Use ARCH_RT_DELAYS_SIGNAL_SEND so the signal (from send_user_sigtrap())
is sent delayed in return to userland.

Cc: stable-rt@vger.kernel.org
Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20211012084421.35136-1-zhe.he@windriver.com

arch/arm64/include/asm/signal.h

@@ -22,4 +22,8 @@ static inline void __user *arch_untagged_si_addr(void __user *addr,
 }
 #define arch_untagged_si_addr arch_untagged_si_addr
 
+#if defined(CONFIG_PREEMPT_RT)
+#define ARCH_RT_DELAYS_SIGNAL_SEND
+#endif
+
 #endif

arch/arm64/kernel/signal.c

@@ -928,6 +928,14 @@ void do_notify_resume(struct pt_regs *regs, unsigned long thread_flags)
 		} else {
 			local_daif_restore(DAIF_PROCCTX);
 
+#ifdef ARCH_RT_DELAYS_SIGNAL_SEND
+			if (unlikely(current->forced_info.si_signo)) {
+				struct task_struct *t = current;
+				force_sig_info(&t->forced_info);
+				t->forced_info.si_signo = 0;
+			}
+#endif
+
 			if (thread_flags & _TIF_UPROBE)
 				uprobe_notify_resume(regs);