Remove a potential cause of timing aberrations

It is possible to be preempted in the middle of reading
`globalclock64`. When this causes mismatched readings of the high
bits, we assumed the low bits were zero (as in the clock rollover
case). In the preemption case, this resulted in a time reading
that appears to be in the middle of another process's execution.

Correct this by re-reading the low bits when the high bits do not
match.

(If a short preemption occurs and the high bits match, it does not
matter the reading of the low bits occured before or after the
preemption, as it will still be consistent with the high bits.)

This bug likely affected `preemption_logger`, since that benchmark
rapidly reads timestamps in a tight loop and is thus highly likely
to be preempted mid-read.

0 files changed, 0 insertions, 0 deletions