x86: get rid of calling do_notify_resume() when returning to kernel mode

If we end up calling do_notify_resume() with !user_mode(refs), it
does nothing (do_signal() explicitly bails out and we can't get there
with TIF_NOTIFY_RESUME in such situations).  Then we jump to
resume_userspace_sig, which rechecks the same thing and bails out
to resume_kernel, thus breaking the loop.

It's easier and cheaper to check *before* calling do_notify_resume()
and bail out to resume_kernel immediately.  And kill the check in
do_signal()...

Note that on amd64 we can't get there with !user_mode() at all - asm
glue takes care of that.
Acked-and-reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/x86/kernel/entry_32.S

@@ -316,7 +316,6 @@ ret_from_exception:
 	preempt_stop(CLBR_ANY)
 ret_from_intr:
 	GET_THREAD_INFO(%ebp)
-resume_userspace_sig:
 #ifdef CONFIG_VM86
 	movl PT_EFLAGS(%esp), %eax	# mix EFLAGS and CS
 	movb PT_CS(%esp), %al
@@ -615,9 +614,13 @@ work_notifysig:				# deal with pending signals and
 					# vm86-space
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_NONE)
+	movb PT_CS(%esp), %bl
+	andb $SEGMENT_RPL_MASK, %bl
+	cmpb $USER_RPL, %bl
+	jb resume_kernel
 	xorl %edx, %edx
 	call do_notify_resume
-	jmp resume_userspace_sig
+	jmp resume_userspace
 
 	ALIGN
 work_notifysig_v86:
@@ -630,9 +633,13 @@ work_notifysig_v86:
 #endif
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_NONE)
+	movb PT_CS(%esp), %bl
+	andb $SEGMENT_RPL_MASK, %bl
+	cmpb $USER_RPL, %bl
+	jb resume_kernel
 	xorl %edx, %edx
 	call do_notify_resume
-	jmp resume_userspace_sig
+	jmp resume_userspace
 END(work_pending)
 
 	# perform syscall exit tracing

arch/x86/kernel/signal.c

@@ -737,16 +737,6 @@ static void do_signal(struct pt_regs *regs)
 	siginfo_t info;
 	int signr;
 
-	/*
-	 * We want the common case to go fast, which is why we may in certain
-	 * cases get here from kernel mode. Just return without doing anything
-	 * if so.
-	 * X86_32: vm86 regs switched out by assembly code before reaching
-	 * here, so testing against kernel CS suffices.
-	 */
-	if (!user_mode(regs))
-		return;
-
 	signr = get_signal_to_deliver(&info, &ka, regs, NULL);
 	if (signr > 0) {
 		/* Whee! Actually deliver the signal.  */