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commit c33f27196d9cb072d48bf5389bb64ad99a0d735a
Author: Damien Zammit <damien@zamaudio.com>
Date: Sun Sep 21 09:14:04 2025 +0000
mach_clock: Fix monotonic clock sometimes going backwards
Between reading mtime and reading hpclock_read_counter,
there may be an interrupt that updates mtime, therefore
we need a check to perform the clock read process again
in this case.
TESTED: on UP using:
```
\#include <stdio.h>
\#include <time.h>
int main()
{
struct timespec ts, now;
int i;
int cnt = 0;
clock_gettime(CLOCK_MONOTONIC, &ts);
for (i = 0; i < 10000000; i++) {
clock_gettime(CLOCK_MONOTONIC, &now);
if ((now.tv_nsec < ts.tv_nsec) && (now.tv_sec <= ts.tv_sec)) {
printf("BACKWARDS\n");
cnt++;
printf(" %u %09lu\n %u %09lu\n\n",
ts.tv_sec, ts.tv_nsec,
now.tv_sec, now.tv_nsec
);
}
ts = now;
}
printf("went backwards %d out of %d times\n", cnt, i);
return 0;
}
```
Before the change, some backward transitions were detected.
After this change, none were detected.
Message-ID: <20250921091345.2183347-1-damien@zamaudio.com>
diff --git a/kern/mach_clock.c b/kern/mach_clock.c
index 3be0fb74..1c2ba0cb 100644
--- a/kern/mach_clock.c
+++ b/kern/mach_clock.c
@@ -132,24 +132,30 @@ MACRO_END
#define read_mapped_time(time) \
MACRO_BEGIN \
+ uint32_t _last_hpc; \
do { \
+ _last_hpc = last_hpc_read; \
(time)->seconds = mtime->time_value.seconds; \
__sync_synchronize(); \
(time)->nanoseconds = mtime->time_value.nanoseconds; \
__sync_synchronize(); \
- } while ((time)->seconds != mtime->check_seconds64); \
- time_value64_add_hpc(time); \
+ } while (((time)->seconds != mtime->check_seconds64) \
+ || (_last_hpc != last_hpc_read)); \
+ time_value64_add_hpc(time, _last_hpc); \
MACRO_END
-#define read_mapped_uptime(uptime) \
+#define read_mapped_uptime(uptime) \
MACRO_BEGIN \
+ uint32_t _last_hpc; \
do { \
+ _last_hpc = last_hpc_read; \
(uptime)->seconds = mtime->uptime_value.seconds; \
__sync_synchronize(); \
(uptime)->nanoseconds = mtime->uptime_value.nanoseconds;\
__sync_synchronize(); \
- } while ((uptime)->seconds != mtime->check_upseconds64); \
- time_value64_add_hpc(uptime); \
+ } while (((uptime)->seconds != mtime->check_upseconds64) \
+ || (_last_hpc != last_hpc_read)); \
+ time_value64_add_hpc(uptime, _last_hpc); \
MACRO_END
def_simple_lock_irq_data(static, timer_lock) /* lock for ... */
@@ -443,11 +449,11 @@ clock_boottime_update(const struct time_value64 *new_time)
* Add the time value since last clock interrupt in nanosecond.
*/
static void
-time_value64_add_hpc(time_value64_t *value)
+time_value64_add_hpc(time_value64_t *value, uint32_t last_hpc)
{
uint32_t now = hpclock_read_counter();
/* Time since last clock interrupt in nanosecond. */
- int64_t ns = (now - last_hpc_read) * hpclock_get_counter_period_nsec();
+ int64_t ns = (now - last_hpc) * hpclock_get_counter_period_nsec();
/* Limit the value of ns under the period of a clock interrupt. */
if (ns >= tick * 1000)
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