[PATCH 7/7] Revert "rtc: cmos: Replace spin_lock_irqsave with spin_lock in hard IRQ"

Revert
commit 6950d046eb6e ("rtc: cmos: Replace spin_lock_irqsave with spin_lock in hard IRQ")
and add a comment.

As described in a previous
commit 66e4f4a9cc38 ("rtc: cmos: Use spin_lock_irqsave() in cmos_interrupt()")
from February 2020:

        cmos_interrupt() isn't always called from hardirq context, so
        we must use spin_lock_irqsave() & co.

Indeed, cmos_interrupt() is called from cmos_check_wkalrm(), which is
called from cmos_resume() - apparently not in an interrupt context.

A later
commit 6950d046eb6e ("rtc: cmos: Replace spin_lock_irqsave with spin_lock in hard IRQ")
did not take account of this and changed spin_lock_irqsave() to spin_lock().
This may cause a deadlock as quoted in the body of
commit 66e4f4a9cc38 ("rtc: cmos: Use spin_lock_irqsave() in cmos_interrupt()")
mentioned earlier.

Signed-off-by: Mateusz Jończyk <redacted>
Cc: Alessandro Zummo <redacted>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Ville Syrjälä <redacted>
Cc: Xiaofei Tan <redacted>
---
 drivers/rtc/rtc-cmos.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index c24465f7bed4..508fba8746a1 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c

@@ -741,10 +741,14 @@ static struct cmos_rtc	cmos_rtc;
 
 static irqreturn_t cmos_interrupt(int irq, void *p)
 {
+	unsigned long	flags;
 	u8		irqstat;
 	u8		rtc_control;
 
-	spin_lock(&rtc_lock);
+	/* cmos_interrupt() may be called from cmos_check_wkalrm() not in
+	 * interrupt context, so using spin_lock_irqsave() is required
+	 */
+	spin_lock_irqsave(&rtc_lock, flags);
 
 	/* When the HPET interrupt handler calls us, the interrupt
 	 * status is passed as arg1 instead of the irq number.  But

@@ -778,7 +782,7 @@ static irqreturn_t cmos_interrupt(int irq, void *p)
 			hpet_mask_rtc_irq_bit(RTC_AIE);
 		CMOS_READ(RTC_INTR_FLAGS);
 	}
-	spin_unlock(&rtc_lock);
+	spin_unlock_irqrestore(&rtc_lock, flags);
 
 	if (is_intr(irqstat)) {
 		rtc_update_irq(p, 1, irqstat);

-- 
2.25.1