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 6950d046eb6e ("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 13be2efc390a ("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: 6950d046eb6e ("rtc: cmos: Replace spin_lock_irqsave with spin_lock in hard IRQ") Signed-off-by: Mateusz Jończyk mat.jonczyk@o2.pl Reviewed-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Tested-by: Chris Bainbridge chris.bainbridge@gmail.com Reported-by: Chris Bainbridge chris.bainbridge@gmail.com Closes: https://lore.kernel.org/all/aDtJ92foPUYmGheF@debian.local/
---
Changes after DRAFT version of the patch: - rewrite commit message, - test this locally (also on top of 5.10.238 for the stable backport), - fix a grammar mistake in the comment. --- drivers/rtc/rtc-cmos.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c index 8172869bd3d7..0743c6acd6e2 100644 --- a/drivers/rtc/rtc-cmos.c +++ b/drivers/rtc/rtc-cmos.c @@ -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; }