On Mon, 13 Jul 2020 01:18:14 -0500
Scott Cheloha <[email protected]> wrote:
> To review, the update protocol is:
>
> 1. Set tk_generation to zero; the update has begun.
>
> 2. Memory barrier. The side effects of step (1) are "visible" to
> other CPUs before those of step (3).
>
> 3. Update the other tk_* members from the active timehands.
>
> 4. Memory barrier. The side effects of step (3) are "visible" before
> those of step (5).
>
> 5. Set tk_generation to th_generation; the update is over. The
> timekeep page is consistent once more.
>
> --
>
> As far as I can tell, tk_generation always changes after an update,
> during step (5).
There are 2 values, th0.th_generation and th1.th_generation. When the
kernel updates tk_generation, it switches between these 2 values, but
the values might be equal.
>From /sys/kern/kern_tc.c tc_windup():
tho = timehands;
th = tho->th_next;
ogen = th->th_generation;
th->th_generation = 0;
membar_producer();
... /* update th */
if (++ogen == 0)
ogen = 1;
membar_producer();
th->th_generation = ogen;
timehands is a circular list, where th0->th_next == th1 and
th1->th_next == th0. Now suppose that we call tc_windup() with,
timehands == th0
th0.th_generation == 2177
th1.th_generation == 2176
We assign tho = th0, th = th1, and ogen = 2176. After we update the
clock, we increment ogen and set th1.th_generation = 2177. When we
update the user timekeep page, we change tk_generation from 2177, the
old th0.th_generation, to 2177, the new th1.th_generation; but libc
sees the same value 2177 and can't detect the update.
We update the clock 100 times per second (in a 100 Hz kernel), but
were incrementing tk_generation only 50 times per second.
I run the diff from my first mail (copied below). It moves apart the
initial values of th_generation, so they don't become equal:
th0.th_generation = UINT_MAX / 2 /* changed from 1 */
th1.th_generation = 0 /* not changed */
It might be better to change how tc_windup() sets ogen. --George
Index: kern/kern_tc.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_tc.c,v
retrieving revision 1.62
diff -u -p -r1.62 kern_tc.c
--- kern/kern_tc.c 6 Jul 2020 13:33:09 -0000 1.62
+++ kern/kern_tc.c 13 Jul 2020 02:59:58 -0000
@@ -98,7 +98,8 @@ static struct timehands th0 = {
.th_counter = &dummy_timecounter,
.th_scale = UINT64_MAX / 1000000,
.th_offset = { .sec = 1, .frac = 0 },
- .th_generation = 1,
+ /* Keep apart generations of th0, th1, for user timekeep. */
+ .th_generation = UINT_MAX / 2,
.th_next = &th1
};
