s390/mcck: cleanup user process termination path

If a machine check interrupt hits while user process is
running __s390_handle_mcck() helper function is called
directly from the interrupt handler and terminates the
current process by calling make_task_dead() routine.

The make_task_dead() is not allowed to be called from
interrupt context which forces the machine check handler
switch to the kernel stack and enable local interrupts
first.

The __s390_handle_mcck() could also be called to service
pending work, but this time from the external interrupts
handler. It is the machine check handler that establishes
the work and schedules the external interrupt, therefore
the machine check interrupt itself should be disabled
while reading out the corresponding variable:

	local_mcck_disable();
	mcck = *this_cpu_ptr(&cpu_mcck);
	memset(this_cpu_ptr(&cpu_mcck), 0, sizeof(mcck));
	local_mcck_enable();

However, local_mcck_disable() does not have effect when
__s390_handle_mcck() is called directly form the machine
check handler, since the machine check interrupt is still
disabled. Therefore, it is not the opening bracket to the
following local_mcck_enable() function.

Simplify the user process termination flow by scheduling
the external interrupt and killing the affected process
from the interrupt context.

Assume a kernel-generated signal is always delivered and
ignore a value returned by do_send_sig_info() funciton.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/nmi.h

@@ -101,9 +101,8 @@ void nmi_alloc_mcesa_early(u64 *mcesad);
 int nmi_alloc_mcesa(u64 *mcesad);
 void nmi_free_mcesa(u64 *mcesad);
 
-void s390_handle_mcck(struct pt_regs *regs);
-void __s390_handle_mcck(void);
-int s390_do_machine_check(struct pt_regs *regs);
+void s390_handle_mcck(void);
+void s390_do_machine_check(struct pt_regs *regs);
 
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_S390_NMI_H */

arch/s390/kernel/entry.S

@@ -562,16 +562,6 @@ ENTRY(mcck_int_handler)
 	xc	__SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)
 	lgr	%r2,%r11		# pass pointer to pt_regs
 	brasl	%r14,s390_do_machine_check
-	cghi	%r2,0
-	je	.Lmcck_return
-	lg	%r1,__LC_KERNEL_STACK	# switch to kernel stack
-	mvc	STACK_FRAME_OVERHEAD(__PT_SIZE,%r1),0(%r11)
-	xc	__SF_BACKCHAIN(8,%r1),__SF_BACKCHAIN(%r1)
-	la	%r11,STACK_FRAME_OVERHEAD(%r1)
-	lgr	%r2,%r11
-	lgr	%r15,%r1
-	brasl	%r14,s390_handle_mcck
-.Lmcck_return:
 	lctlg	%c1,%c1,__PT_CR1(%r11)
 	lmg	%r0,%r10,__PT_R0(%r11)
 	mvc	__LC_RETURN_MCCK_PSW(16),__PT_PSW(%r11) # move return PSW

arch/s390/kernel/nmi.c

@@ -156,7 +156,7 @@ NOKPROBE_SYMBOL(s390_handle_damage);
  * Main machine check handler function. Will be called with interrupts disabled
  * and machine checks enabled.
  */
-void __s390_handle_mcck(void)
+void s390_handle_mcck(void)
 {
 	struct mcck_struct mcck;
 
@@ -192,23 +192,16 @@ void __s390_handle_mcck(void)
 	if (mcck.stp_queue)
 		stp_queue_work();
 	if (mcck.kill_task) {
-		local_irq_enable();
 		printk(KERN_EMERG "mcck: Terminating task because of machine "
 		       "malfunction (code 0x%016lx).\n", mcck.mcck_code);
 		printk(KERN_EMERG "mcck: task: %s, pid: %d.\n",
 		       current->comm, current->pid);
-		make_task_dead(SIGSEGV);
+		if (is_global_init(current))
+			panic("mcck: Attempting to kill init!\n");
+		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, current, PIDTYPE_PID);
 	}
 }
 
-void noinstr s390_handle_mcck(struct pt_regs *regs)
-{
-	trace_hardirqs_off();
-	pai_kernel_enter(regs);
-	__s390_handle_mcck();
-	pai_kernel_exit(regs);
-	trace_hardirqs_on();
-}
 /*
  * returns 0 if register contents could be validated
  * returns 1 otherwise
@@ -373,7 +366,7 @@ NOKPROBE_SYMBOL(s390_backup_mcck_info);
 /*
  * machine check handler.
  */
-int notrace s390_do_machine_check(struct pt_regs *regs)
+void notrace s390_do_machine_check(struct pt_regs *regs)
 {
 	static int ipd_count;
 	static DEFINE_SPINLOCK(ipd_lock);
@@ -503,16 +496,10 @@ int notrace s390_do_machine_check(struct pt_regs *regs)
 	}
 	clear_cpu_flag(CIF_MCCK_GUEST);
 
-	if (user_mode(regs) && mcck_pending) {
-		irqentry_nmi_exit(regs, irq_state);
-		return 1;
-	}
-
 	if (mcck_pending)
 		schedule_mcck_handler();
 
 	irqentry_nmi_exit(regs, irq_state);
-	return 0;
 }
 NOKPROBE_SYMBOL(s390_do_machine_check);
 

arch/s390/kernel/smp.c

@@ -522,7 +522,7 @@ static void smp_handle_ext_call(void)
 	if (test_bit(ec_call_function_single, &bits))
 		generic_smp_call_function_single_interrupt();
 	if (test_bit(ec_mcck_pending, &bits))
-		__s390_handle_mcck();
+		s390_handle_mcck();
 	if (test_bit(ec_irq_work, &bits))
 		irq_work_run();
 }