[PATCH 1/2] rtc: cmos: Disable ACPI RTC event before handling it

[Gregory Price <gourry@gourry.net>]

Users of RTC_UIE_ON can stop receiving update interrupts when rtc-cmos
handles alarms through ACPI. In one 50-run test, 40 runs passed and 10
timed out after receiving between one and four of five expected events.

The ACPI ff_rt_clk counter stopped at 218, exactly matching the number
of events delivered to userspace, and reported:

    218     STS disabled     unmasked

rtc_handler() runs in the threaded ACPI SCI handler and calls
cmos_interrupt(), which schedules rtc->irqwork. The work can run before
rtc_handler() returns:

    ACPI SCI thread          rtc->irqwork

    cmos_interrupt()
      schedule_work()
                             __rtc_set_alarm()
                               rtc_wake_on()
    acpi_clear_event()
    acpi_disable_event()

The SCI thread then disables the event that the worker enabled for the
next alarm. That alarm sets its status while delivery remains disabled,
so no update interrupt reaches userspace.

Clear and disable the current ACPI RTC event before cmos_interrupt()
schedules the work. The worker can then enable the next event without
the handler undoing it. With this ordering, 150 runs delivered all 750
events without a stall.

Fixes: 311ee9c151ad ("rtc: cmos: allow using ACPI for RTC alarm instead of HPET")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Gregory Price (Meta) <gourry@gourry.net>
---
 drivers/rtc/rtc-cmos.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/rtc/rtc-cmos.c b/drivers/rtc/rtc-cmos.c
index f89ab58f5048a..462799667aa18 100644
--- a/drivers/rtc/rtc-cmos.c
+++ b/drivers/rtc/rtc-cmos.c
@@ -758,7 +758,9 @@ static u32 rtc_handler(void *context)
 	unsigned char rtc_intr;
 	unsigned long flags;
 
-
+	/* Quiesce this event before rtc->irqwork can arm the next one. */
+	acpi_clear_event(ACPI_EVENT_RTC);
+	acpi_disable_event(ACPI_EVENT_RTC, 0);
 	/*
 	 * Always update rtc irq when ACPI is used as RTC Alarm.
 	 * Or else, ACPI SCI is enabled during suspend/resume only,
@@ -781,8 +783,6 @@ static u32 rtc_handler(void *context)
 	}
 
 	pm_wakeup_hard_event(dev);
-	acpi_clear_event(ACPI_EVENT_RTC);
-	acpi_disable_event(ACPI_EVENT_RTC, 0);
 	return ACPI_INTERRUPT_HANDLED;
 }
 
-- 
2.53.0-Meta