Commit 00a39d86 authored by Mateusz Jończyk's avatar Mateusz Jończyk Committed by Alexandre Belloni
Browse files

rtc: cmos: use spin_lock_irqsave in cmos_interrupt 



cmos_interrupt() can be called in a non-interrupt context, such as in
an ACPI event handler (which runs in an interrupt thread). Therefore,
usage of spin_lock(&rtc_lock) is insecure. Use spin_lock_irqsave() /
spin_unlock_irqrestore() instead.

Before a misguided
commit 6950d046 ("rtc: cmos: Replace spin_lock_irqsave with spin_lock in hard IRQ")
the cmos_interrupt() function used spin_lock_irqsave(). That commit
changed it to spin_lock() and broke locking, which was partially fixed in
commit 13be2efc ("rtc: cmos: Disable irq around direct invocation of cmos_interrupt()")

That second commit did not take account of the ACPI fixed event handler
pathway, however. It introduced local_irq_disable() workarounds in
cmos_check_wkalrm(), which can cause problems on PREEMPT_RT kernels
and are now unnecessary.

Add an explicit comment so that this change will not be reverted by
mistake.

Cc: stable@vger.kernel.org
Fixes: 6950d046 ("rtc: cmos: Replace spin_lock_irqsave with spin_lock in hard IRQ")
Signed-off-by: default avatarMateusz Jończyk <mat.jonczyk@o2.pl>
Reviewed-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: default avatarChris Bainbridge <chris.bainbridge@gmail.com>
Reported-by: default avatarChris Bainbridge <chris.bainbridge@gmail.com>
Closes: https://lore.kernel.org/all/aDtJ92foPUYmGheF@debian.local/
Link: https://lore.kernel.org/r/20250607210608.14835-1-mat.jonczyk@o2.pl


Signed-off-by: default avatarAlexandre Belloni <alexandre.belloni@bootlin.com>
parent b1248da0

drivers/rtc/rtc-cmos.c

+6 −4
Original line number Diff line number Diff line
@@ -692,8 +692,12 @@ static irqreturn_t cmos_interrupt(int irq, void *p) 
{

	u8		irqstat;

	u8		rtc_control;

	unsigned long	flags;



	spin_lock(&rtc_lock);

	/* We cannot use spin_lock() here, as cmos_interrupt() is also called

	 * in a non-irq context.

	 */

	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
@@ -727,7 +731,7 @@ 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);
@@ -1295,9 +1299,7 @@ static void cmos_check_wkalrm(struct device *dev) 
	 * ACK the rtc irq here

	 */

	if (t_now >= cmos->alarm_expires && cmos_use_acpi_alarm()) {

		local_irq_disable();

		cmos_interrupt(0, (void *)cmos->rtc);

		local_irq_enable();

		return;

	}