[PATCH v2 1/1] sparc64: Fix comparator problem with timer interrupts

[Tony Rodriguez]

On SPARC64 the check:

    return ((long)(new_tick - (orig_tick + adj))) > 0L;

Is safe only if retries make forward progress. The comparator can
take effect with a latency, so the moment when counter == comparator
may be missed, which can cause delays or hangs on some SPARC64 systems.

For clarity:
    exp = orig_tick + adj   /* expected comparator value */

The current check requires new_tick to be strictly greater than exp;
equality (new_tick == exp) is treated as not yet passed and the caller
will retry.

By contrast, using:

    return ((long)(new_tick - (orig_tick + adj))) >= 0L;

causes the caller to stop retrying and assume the timer is scheduled;
both equality and greater-than are accepted (new_tick == exp or
new_tick > exp).

Signed-off-by: Tony Rodriguez <unixpro1970@xxxxxxxxx>
---
 arch/sparc/kernel/time_64.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/sparc/kernel/time_64.c b/arch/sparc/kernel/time_64.c
index 87b267043ccd..783b60e547c4 100644
--- a/arch/sparc/kernel/time_64.c
+++ b/arch/sparc/kernel/time_64.c
@@ -146,7 +146,7 @@ static int tick_add_compare(unsigned long adj)
 			     : "=r" (new_tick));
 	new_tick &= ~TICKCMP_IRQ_BIT;

-	return ((long)(new_tick - (orig_tick+adj))) > 0L;
+	return ((long)(new_tick - (orig_tick+adj))) >= 0L;
 }

 static unsigned long tick_add_tick(unsigned long adj)
@@ -277,7 +277,7 @@ static int stick_add_compare(unsigned long adj)
 			     : "=r" (new_tick));
 	new_tick &= ~TICKCMP_IRQ_BIT;

-	return ((long)(new_tick - (orig_tick+adj))) > 0L;
+	return ((long)(new_tick - (orig_tick+adj))) >= 0L;
 }

 static unsigned long stick_get_frequency(void)
@@ -411,7 +411,7 @@ static int hbtick_add_compare(unsigned long adj)

 	val2 = __hbird_read_stick() & ~TICKCMP_IRQ_BIT;

-	return ((long)(val2 - val)) > 0L;
+	return ((long)(val2 - val)) >= 0L;
 }

 static unsigned long hbtick_get_frequency(void)
--
2.53.0