Eli Carter writes:
> What are you seeing that I'm missing?
Ok, after sitting down and thinking again about this problem, its not
the 9.9999ms case, but the 10.000000001 case:
First time:
- interrupts disabled
- read jiffies
- read counter
- jiffies_p != jiffies_t
- set jiffies_p = jiffies_t
- set counter_p = counter
- correction of (latch - counter) applied -> almost a full 10ms
- interrupts enabled
- counter rolls over
- jiffies updated
- counter is at, or near maximum
- time returned we'll call "0".
10.0000001ms later:
- interrupts disabled
- read jiffies
- counter rolls over
- read counter
- jiffies_p != jiffies_t
- set jiffies_p = jiffies_t
- set counter_p = counter
- correction of (latch - counter) applied -> almost nothing
- interrupts enabled
- jiffies updated
- time returned - "0" + almost nothing
Next read immediately after:
- interrupts disabled
- read jiffies
- read counter
- jiffies_p != jiffies_t
- set jiffies_p = jiffies_t
- set counter_p = counter
- correction of (latch - counter) applied -> slightly more than
almost nothing
- interrupts enabled
- time returned - "10ms" + slightly more than almost nothing
Like I say, this requires good timing to create, so may not be too much of
a problem, but it does seem to be a problem that could occur.
I'm wondering if something like the following will plug this hole:
read_lock_xtime_and_ints();
jiffies_1 = jiffies;
counter_1 = counter;
read_unlock_xtime_and_ints();
read_lock_xtime_and_ints();
jiffies_2 = jiffies;
counter_2 = counter;
read_unlock_xtime_and_ints();
if (jiffies_1 != jiffies_2) {
/*
* we rolled over while reading counter_1. Therefore
* we can't trust it. Use *_2 instead. Note that we
* would have received an interrupt between read_unlock
* and read_lock.
*/
jiffies_1 = jiffies_2;
counter_1 = counter_1;
} else {
/*
* we didn't roll over while reading counter_1
* we can safely use counter_1 as is. Neither
* did we receive a timer interrupt between the
* read_unlock and read_lock.
*/
}
/* apply standard counter correction factor */
The only thing I haven't looked at is whether xtime would be updated.
--
Russell King (rmk@arm.linux.org.uk) The developer of ARM Linux
http://www.arm.linux.org.uk/personal/aboutme.html
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This archive was generated by hypermail 2b29 : Fri Mar 23 2001 - 21:00:13 EST