----- On Jun 22, 2022, at 9:19 AM, Mathieu Desnoyers
mathieu.desnoy...@efficios.com wrote:
> ----- On Jun 22, 2022, at 3:45 AM, Minlan Wang wangmin...@szsandstone.com
> wrote:
>
> [...]
>
> Hi Minlan,
>
>> --
>> 1.8.3.1
>>
>>
>> And the lttng session is configured to trace these events:
>> kernel: syscall futex
>
> On the kernel side, in addition to the syscall futex, I would really like to
> see
> what
> happens in the scheduler, mainly the wait/wakeup tracepoints. This can be
> added
> by using:
>
> lttng enable-event -k 'sched_*'
>
> This should help us confirm whether we indeed have a situation where queued
> wake
> ups
> happen to wake up a wait happening only later, which is unexpected by the
> current liburcu
> userspace code.
>
> [...]
>
>> ---
>>
>> The babletrace output of this session is pretty big, 6 MB in size, i put it
>> in
>> the attachment trace_0622.tar.bz2.
>> Let my know if your mailbox can't handle such big attachment.
>
> It would be even better if you can share the binary trace, because then it's
> easy to
> load it in trace compass, cut away time ranges that don't matter, and lots of
> other
> useful stuff.
I just found the relevant snippet of documentation in futex(5):
FUTEX_WAIT
Returns 0 if the caller was woken up. Note that a wake-up can
also be caused by common futex usage patterns in unrelated code
that happened to have previously used the futex word's memory
location (e.g., typical futex-based implementations of Pthreads
mutexes can cause this under some conditions). Therefore, call‐
ers should always conservatively assume that a return value of 0
can mean a spurious wake-up, and use the futex word's value
(i.e., the user-space synchronization scheme) to decide whether
to continue to block or not.
I'm pretty sure this is what is happening here.
Thanks,
Mathieu
>
> Thanks,
>
> Mathieu
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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