s390/etr,stp: fix possible deadlock on machine check

The first level machine check handler for etr and stp machine checks may
call queue_work() while in nmi context. This may deadlock e.g. if the
machine check happened when the interrupted context did hold a lock, that
also will be acquired by queue_work().
Therefore split etr and stp machine check handling into first and second
level handling. The second level handling will then issue the queue_work()
call in process context which avoids the potential deadlock.

Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/include/asm/etr.h

···
 #define ETR_PTFF_SGS	0x43	/* set gross steering rate */
 
 /* Functions needed by the machine check handler */
-void etr_switch_to_local(void);
-void etr_sync_check(void);
+int etr_switch_to_local(void);
+int etr_sync_check(void);
+void etr_queue_work(void);
 
 /* notifier for syncs */
 extern struct atomic_notifier_head s390_epoch_delta_notifier;
···
 } __attribute__ ((packed));
 
 /* Functions needed by the machine check handler */
-void stp_sync_check(void);
-void stp_island_check(void);
+int stp_sync_check(void);
+int stp_island_check(void);
+void stp_queue_work(void);
 
 #endif /* __S390_ETR_H */

arch/s390/kernel/nmi.c

···
 	int kill_task;
 	int channel_report;
 	int warning;
+	unsigned int etr_queue : 1;
+	unsigned int stp_queue : 1;
 	unsigned long long mcck_code;
 };
 
···
 		if (xchg(&mchchk_wng_posted, 1) == 0)
 			kill_cad_pid(SIGPWR, 1);
 	}
+	if (mcck.etr_queue)
+		etr_queue_work();
+	if (mcck.stp_queue)
+		stp_queue_work();
 	if (mcck.kill_task) {
 		local_irq_enable();
 		printk(KERN_EMERG "mcck: Terminating task because of machine "
···
 	if (mci->ed && mci->ec) {
 		/* External damage */
 		if (S390_lowcore.external_damage_code & (1U << ED_ETR_SYNC))
-			etr_sync_check();
+			mcck->etr_queue |= etr_sync_check();
 		if (S390_lowcore.external_damage_code & (1U << ED_ETR_SWITCH))
-			etr_switch_to_local();
+			mcck->etr_queue |= etr_switch_to_local();
 		if (S390_lowcore.external_damage_code & (1U << ED_STP_SYNC))
-			stp_sync_check();
+			mcck->stp_queue |= stp_sync_check();
 		if (S390_lowcore.external_damage_code & (1U << ED_STP_ISLAND))
-			stp_island_check();
+			mcck->stp_queue |= stp_island_check();
+		if (mcck->etr_queue || mcck->stp_queue)
+			set_cpu_flag(CIF_MCCK_PENDING);
 	}
 	if (mci->se)
 		/* Storage error uncorrected */

arch/s390/kernel/time.c

···
  * Switch to local machine check. This is called when the last usable
  * ETR port goes inactive. After switch to local the clock is not in sync.
  */
-void etr_switch_to_local(void)
+int etr_switch_to_local(void)
 {
 	if (!etr_eacr.sl)
-		return;
+		return 0;
 	disable_sync_clock(NULL);
 	if (!test_and_set_bit(ETR_EVENT_SWITCH_LOCAL, &etr_events)) {
 		etr_eacr.es = etr_eacr.sl = 0;
 		etr_setr(&etr_eacr);
-		queue_work(time_sync_wq, &etr_work);
+		return 1;
 	}
+	return 0;
 }
 
 /*
···
  * After a ETR sync check the clock is not in sync. The machine check
  * is broadcasted to all cpus at the same time.
  */
-void etr_sync_check(void)
+int etr_sync_check(void)
 {
 	if (!etr_eacr.es)
-		return;
+		return 0;
 	disable_sync_clock(NULL);
 	if (!test_and_set_bit(ETR_EVENT_SYNC_CHECK, &etr_events)) {
 		etr_eacr.es = 0;
 		etr_setr(&etr_eacr);
-		queue_work(time_sync_wq, &etr_work);
+		return 1;
 	}
+	return 0;
+}
+
+void etr_queue_work(void)
+{
+	queue_work(time_sync_wq, &etr_work);
 }
 
 /*
···
  * After a STP sync check the clock is not in sync. The machine check
  * is broadcasted to all cpus at the same time.
  */
-void stp_sync_check(void)
+int stp_sync_check(void)
 {
 	disable_sync_clock(NULL);
-	queue_work(time_sync_wq, &stp_work);
+	return 1;
 }
 
 /*
···
  * have matching CTN ids and have a valid stratum-1 configuration
  * but the configurations do not match.
  */
-void stp_island_check(void)
+int stp_island_check(void)
 {
 	disable_sync_clock(NULL);
-	queue_work(time_sync_wq, &stp_work);
+	return 1;
 }
 
+void stp_queue_work(void)
+{
+	queue_work(time_sync_wq, &stp_work);
+}
 
 static int stp_sync_clock(void *data)
 {