Messages in this thread | | | From | Jaccon Bastiaansen <> | Date | Mon, 20 Jun 2022 23:24:47 +0200 | Subject | Clocksource: should the read() function of a clocksource be NMI safe? |
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Hello all,
The ktime_get_*_fast_ns() functions in kernel/time/timekeeping.c are listed as NMI safe.
If I am correct, the implementation of these functions uses the read() function of the currently selected clocksource.
But a potential lockup can occur if this read() function uses a raw spinlock. A running read() function can be interrupted by an NMI which also calls the read() function. The NMI handler will then spin forever in an attempt to lock an already locked raw spinlock.
Is my reasoning correct? Should the read() function of a clocksource always be NMI safe? This requirement is not documented and I see that several clocksource drivers (such as drivers/clocksource/i8253.c) are not NMI safe (they lock a raw spinlock in their read() function).
Regards, Jaccon Bastiaansen
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