Messages in this thread | | | From | Russell King <> | Subject | Re: gettimeofday question | Date | Mon, 19 Mar 2001 21:34:06 +0000 (GMT) |
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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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