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author | Peter Hilber <peter.hilber@opensynergy.com> | 2023-12-18 08:38:40 +0100 |
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committer | Thomas Gleixner <tglx@linutronix.de> | 2024-02-19 12:18:51 +0100 |
commit | 87a41130881995f82f7adbafbfeddaebfb35f0ef (patch) | |
tree | 67e66493accd03aed7bcacce07cf1e6dbbc38ca8 /drivers/net/netconsole.c | |
parent | 84dccadd3e2a3f1a373826ad71e5ced5e76b0c00 (diff) | |
download | linux-87a41130881995f82f7adbafbfeddaebfb35f0ef.tar.gz linux-87a41130881995f82f7adbafbfeddaebfb35f0ef.tar.bz2 linux-87a41130881995f82f7adbafbfeddaebfb35f0ef.zip |
timekeeping: Fix cross-timestamp interpolation corner case decision
The cycle_between() helper checks if parameter test is in the open interval
(before, after). Colloquially speaking, this also applies to the counter
wrap-around special case before > after. get_device_system_crosststamp()
currently uses cycle_between() at the first call site to decide whether to
interpolate for older counter readings.
get_device_system_crosststamp() has the following problem with
cycle_between() testing against an open interval: Assume that, by chance,
cycles == tk->tkr_mono.cycle_last (in the following, "cycle_last" for
brevity). Then, cycle_between() at the first call site, with effective
argument values cycle_between(cycle_last, cycles, now), returns false,
enabling interpolation. During interpolation,
get_device_system_crosststamp() will then call cycle_between() at the
second call site (if a history_begin was supplied). The effective argument
values are cycle_between(history_begin->cycles, cycles, cycles), since
system_counterval.cycles == interval_start == cycles, per the assumption.
Due to the test against the open interval, cycle_between() returns false
again. This causes get_device_system_crosststamp() to return -EINVAL.
This failure should be avoided, since get_device_system_crosststamp() works
both when cycles follows cycle_last (no interpolation), and when cycles
precedes cycle_last (interpolation). For the case cycles == cycle_last,
interpolation is actually unneeded.
Fix this by changing cycle_between() into timestamp_in_interval(), which
now checks against the closed interval, rather than the open interval.
This changes the get_device_system_crosststamp() behavior for three corner
cases:
1. Bypass interpolation in the case cycles == tk->tkr_mono.cycle_last,
fixing the problem described above.
2. At the first timestamp_in_interval() call site, cycles == now no longer
causes failure.
3. At the second timestamp_in_interval() call site, history_begin->cycles
== system_counterval.cycles no longer causes failure.
adjust_historical_crosststamp() also works for this corner case,
where partial_history_cycles == total_history_cycles.
These behavioral changes should not cause any problems.
Fixes: 2c756feb18d9 ("time: Add history to cross timestamp interface supporting slower devices")
Signed-off-by: Peter Hilber <peter.hilber@opensynergy.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20231218073849.35294-3-peter.hilber@opensynergy.com
Diffstat (limited to 'drivers/net/netconsole.c')
0 files changed, 0 insertions, 0 deletions