In-Reply-To: <[email protected]>
On Fri, 24 Mar 2006 09:22:51 -0800, Ray Lee wrote:
> On 3/24/06, Andreas Mohr <[email protected]> wrote:
> > + loops += bclock;
> [...]
> > - } while ((now-bclock) < loops);
> > + } while (now < loops);
>
> Erm, aren't you introducing an overflow problem here?
>
> if loops is 2^32-1, bclock is 1, the old version would execute the
> proper number of times, the new one will blow out in one tick.
Yes, but the old version has a bug too. I don't have 2.6.16-mm1, but
in 2.6.16 it's in arch/i386/kernel/timers/timer_tsc.c and
arch/x86_64/lib/delay.c:
static void delay_tsc(unsigned long loops)
{
unsigned long bclock, now;
rdtscl(bclock);
do
{
rep_nop();
rdtscl(now);
} while ((now-bclock) < loops);
}
If (loops == 100000) and (bclock == 2^32-1) the loop will terminate
immediately when the low part of the TSC overflows because (now-bclock)
is a large number. That can't be right...
I'm running a system with this applied now. I think there are still
problems if someone uses huge delays, though. What keeps someone from
trying to delay for > 2^31 cycles?
i386 delay_tsc() will truncate delays when the TSC is within 'loops'
of overflow. We must be able to handle TSC overflow both before and
after 'end', i.e. [1], [2] and [3] below.
zero
|
case A (end < start) start [1] | [2] end [3]
|
case B (end > start) start [1] end [2] | [3]
Signed-off-by: Chuck Ebbert <[email protected]>
--- 2.6.16-d2.orig/arch/i386/kernel/timers/timer_tsc.c
+++ 2.6.16-d2/arch/i386/kernel/timers/timer_tsc.c
@@ -170,14 +170,22 @@ unsigned long long sched_clock(void)
static void delay_tsc(unsigned long loops)
{
- unsigned long bclock, now;
+ unsigned long start, end, now;
- rdtscl(bclock);
- do
- {
- rep_nop();
- rdtscl(now);
- } while ((now-bclock) < loops);
+ rdtscl(start);
+ end = start + loops;
+
+ if (unlikely(end < start)) {
+ do {
+ rep_nop();
+ rdtscl(now);
+ } while (now > start || now < end);
+ } else {
+ do {
+ rep_nop();
+ rdtscl(now);
+ } while (now > start && now < end);
+ }
}
#ifdef CONFIG_HPET_TIMER
--
Chuck
"Penguins don't come from next door, they come from the Antarctic!"
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
