powerpc64 head -r344018 stuck sleeping problems: th->th_scale * tc_delta(th) overflows unsigned 64 bits sometimes [patched failed]

[Konstantin Belousov]

On Sat, Mar 02, 2019 at 07:38:20AM +1100, Bruce Evans wrote:
> On Fri, 1 Mar 2019, Konstantin Belousov wrote:
> > +		scale = th->th_scale;
> > +		delta = tc_delta(th);
> > +		if (__predict_false(th->th_scale_bits + fls(delta) > 63)) {
> 
> Better, but shouldn't be changed (and the bug that causes the large intervals
> remains unlocated), and if it is changed then it should use:
> 
>  		if (delta >= th->th_large_delta)
> 
> > @@ -1464,6 +1483,11 @@ tc_windup(struct bintime *new_boottimebin)
> > 	scale += (th->th_adjustment / 1024) * 2199;
> > 	scale /= th->th_counter->tc_frequency;
> > 	th->th_scale = scale * 2;
> > +#ifdef _LP64
> > +	th->th_scale_bits = ffsl(th->th_scale);
> > +#else
> > +	th->th_scale_bits = ffsll(th->th_scale);
> > +#endif
> 
>  	th->th_large_delta = ((uint64_t)1 << 63) / scale;

So I am able to reproduce it with some surprising ease on HPET running
on Haswell.

I looked at the generated code for libc which still uses ffsll() on 32bit,
due to the ABI issues.  At least clang generates two BSF instructions for
this code, so I think that forking vdso_timehands ABI for this is not
reasonable right now.

diff --git a/lib/libc/sys/__vdso_gettimeofday.c b/lib/libc/sys/__vdso_gettimeofday.c
index 3749e0473af..3c3c71207bd 100644
--- a/lib/libc/sys/__vdso_gettimeofday.c
+++ b/lib/libc/sys/__vdso_gettimeofday.c
@@ -32,6 +32,8 @@ __FBSDID("$FreeBSD$");
 #include <sys/time.h>
 #include <sys/vdso.h>
 #include <errno.h>
+#include <limits.h>
+#include <strings.h>
 #include <time.h>
 #include <machine/atomic.h>
 #include "libc_private.h"
@@ -62,7 +64,8 @@ binuptime(struct bintime *bt, struct vdso_timekeep *tk, int abs)
 {
 	struct vdso_timehands *th;
 	uint32_t curr, gen;
-	u_int delta;
+	uint64_t scale, x;
+	u_int delta, scale_bits;
 	int error;
 
 	do {
@@ -78,7 +81,19 @@ binuptime(struct bintime *bt, struct vdso_timekeep *tk, int abs)
 			continue;
 		if (error != 0)
 			return (error);
-		bintime_addx(bt, th->th_scale * delta);
+		scale = th->th_scale;
+#ifdef _LP64
+		scale_bits = ffsl(scale);
+#else
+		scale_bits = ffsll(scale);
+#endif
+		if (__predict_false(scale_bits + fls(delta) > 63)) {
+			x = (scale >> 32) * delta;
+			scale &= UINT_MAX;
+			bt->sec += x >> 32;
+			bintime_addx(bt, x << 32);
+		}
+		bintime_addx(bt, scale * delta);
 		if (abs)
 			bintime_add(bt, &th->th_boottime);
 
diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
index 2656fb4d22f..0a11c726e3c 100644
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -72,6 +72,7 @@ struct timehands {
 	struct timecounter	*th_counter;
 	int64_t			th_adjustment;
 	uint64_t		th_scale;
+	uint64_t		th_large_delta;
 	u_int	 		th_offset_count;
 	struct bintime		th_offset;
 	struct bintime		th_bintime;
@@ -355,13 +356,22 @@ void
 binuptime(struct bintime *bt)
 {
 	struct timehands *th;
-	u_int gen;
+	uint64_t scale, x;
+	u_int delta, gen;
 
 	do {
 		th = timehands;
 		gen = atomic_load_acq_int(&th->th_generation);
 		*bt = th->th_offset;
-		bintime_addx(bt, th->th_scale * tc_delta(th));
+		scale = th->th_scale;
+		delta = tc_delta(th);
+		if (__predict_false(th->th_large_delta <= delta)) {
+			x = (scale >> 32) * delta;
+			scale &= UINT_MAX;
+			bt->sec += x >> 32;
+			bintime_addx(bt, x << 32);
+		}
+		bintime_addx(bt, scale * delta);
 		atomic_thread_fence_acq();
 	} while (gen == 0 || gen != th->th_generation);
 }
@@ -388,13 +398,22 @@ void
 bintime(struct bintime *bt)
 {
 	struct timehands *th;
-	u_int gen;
+	uint64_t scale, x;
+	u_int delta, gen;
 
 	do {
 		th = timehands;
 		gen = atomic_load_acq_int(&th->th_generation);
 		*bt = th->th_bintime;
-		bintime_addx(bt, th->th_scale * tc_delta(th));
+		scale = th->th_scale;
+		delta = tc_delta(th);
+		if (__predict_false(th->th_large_delta <= delta)) {
+			x = (scale >> 32) * delta;
+			scale &= UINT_MAX;
+			bt->sec += x >> 32;
+			bintime_addx(bt, x << 32);
+		}
+		bintime_addx(bt, scale * delta);
 		atomic_thread_fence_acq();
 	} while (gen == 0 || gen != th->th_generation);
 }
@@ -1464,6 +1483,7 @@ tc_windup(struct bintime *new_boottimebin)
 	scale += (th->th_adjustment / 1024) * 2199;
 	scale /= th->th_counter->tc_frequency;
 	th->th_scale = scale * 2;
+	th->th_large_delta = ((uint64_t)1 << 63) / scale;
 
 	/*
 	 * Now that the struct timehands is again consistent, set the new