[PATCH v2 07/13] parisc: Replace regular spinlock with spin_trylock on panic path

["Guilherme G. Piccoli" <gpiccoli@igalia.com>]

The panic notifiers' callbacks execute in an atomic context, with
interrupts/preemption disabled, and all CPUs not running the panic
function are off, so it's very dangerous to wait on a regular
spinlock, there's a risk of deadlock.

Refactor the panic notifier of parisc/power driver to make use
of spin_trylock - for that, we've added a second version of the
soft-power function. Also, some comments were reorganized and
trailing white spaces, useless header inclusion and blank lines
were removed.

Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Acked-by: Helge Deller <deller@gmx.de> # parisc
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>

---

V2:
- Added Helge's ACK - thanks!

 arch/parisc/include/asm/pdc.h |  1 +
 arch/parisc/kernel/firmware.c | 27 +++++++++++++++++++++++----
 drivers/parisc/power.c        | 17 ++++++++++-------
 3 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/arch/parisc/include/asm/pdc.h b/arch/parisc/include/asm/pdc.h
index b643092d4b98..7a106008e258 100644
--- a/arch/parisc/include/asm/pdc.h
+++ b/arch/parisc/include/asm/pdc.h
@@ -83,6 +83,7 @@ int pdc_do_firm_test_reset(unsigned long ftc_bitmap);
 int pdc_do_reset(void);
 int pdc_soft_power_info(unsigned long *power_reg);
 int pdc_soft_power_button(int sw_control);
+int pdc_soft_power_button_panic(int sw_control);
 void pdc_io_reset(void);
 void pdc_io_reset_devices(void);
 int pdc_iodc_getc(void);
diff --git a/arch/parisc/kernel/firmware.c b/arch/parisc/kernel/firmware.c
index 6a7e315bcc2e..0e2f70b592f4 100644
--- a/arch/parisc/kernel/firmware.c
+++ b/arch/parisc/kernel/firmware.c
@@ -1232,15 +1232,18 @@ int __init pdc_soft_power_info(unsigned long *power_reg)
 }
 
 /*
- * pdc_soft_power_button - Control the soft power button behaviour
- * @sw_control: 0 for hardware control, 1 for software control 
+ * pdc_soft_power_button{_panic} - Control the soft power button behaviour
+ * @sw_control: 0 for hardware control, 1 for software control
  *
  *
  * This PDC function places the soft power button under software or
  * hardware control.
- * Under software control the OS may control to when to allow to shut 
- * down the system. Under hardware control pressing the power button 
+ * Under software control the OS may control to when to allow to shut
+ * down the system. Under hardware control pressing the power button
  * powers off the system immediately.
+ *
+ * The _panic version relies in spin_trylock to prevent deadlock
+ * on panic path.
  */
 int pdc_soft_power_button(int sw_control)
 {
@@ -1254,6 +1257,22 @@ int pdc_soft_power_button(int sw_control)
 	return retval;
 }
 
+int pdc_soft_power_button_panic(int sw_control)
+{
+	int retval;
+	unsigned long flags;
+
+	if (!spin_trylock_irqsave(&pdc_lock, flags)) {
+		pr_emerg("Couldn't enable soft power button\n");
+		return -EBUSY; /* ignored by the panic notifier */
+	}
+
+	retval = mem_pdc_call(PDC_SOFT_POWER, PDC_SOFT_POWER_ENABLE, __pa(pdc_result), sw_control);
+	spin_unlock_irqrestore(&pdc_lock, flags);
+
+	return retval;
+}
+
 /*
  * pdc_io_reset - Hack to avoid overlapping range registers of Bridges devices.
  * Primarily a problem on T600 (which parisc-linux doesn't support) but
diff --git a/drivers/parisc/power.c b/drivers/parisc/power.c
index 456776bd8ee6..8512884de2cf 100644
--- a/drivers/parisc/power.c
+++ b/drivers/parisc/power.c
@@ -37,7 +37,6 @@
 #include <linux/module.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
-#include <linux/notifier.h>
 #include <linux/panic_notifier.h>
 #include <linux/reboot.h>
 #include <linux/sched/signal.h>
@@ -175,16 +174,21 @@ static void powerfail_interrupt(int code, void *x)
 
 
 
-/* parisc_panic_event() is called by the panic handler.
- * As soon as a panic occurs, our tasklets above will not be
- * executed any longer. This function then re-enables the 
- * soft-power switch and allows the user to switch off the system
+/*
+ * parisc_panic_event() is called by the panic handler.
+ *
+ * As soon as a panic occurs, our tasklets above will not
+ * be executed any longer. This function then re-enables
+ * the soft-power switch and allows the user to switch off
+ * the system. We rely in pdc_soft_power_button_panic()
+ * since this version spin_trylocks (instead of regular
+ * spinlock), preventing deadlocks on panic path.
  */
 static int parisc_panic_event(struct notifier_block *this,
 		unsigned long event, void *ptr)
 {
 	/* re-enable the soft-power switch */
-	pdc_soft_power_button(0);
+	pdc_soft_power_button_panic(0);
 	return NOTIFY_DONE;
 }
 
@@ -193,7 +197,6 @@ static struct notifier_block parisc_panic_block = {
 	.priority	= INT_MAX,
 };
 
-
 static int __init power_init(void)
 {
 	unsigned long ret;
-- 
2.37.1