Re: [PATCH] ARM fix syscall trace return value

[Viktor Rosendahl]

On Tue, 2009-02-17 at 19:18 +0100, ext Mathieu Desnoyers wrote: 
> Hi Russell,
> 
> I am currently finding core bugs in the Linux kernel implementation of
> the ARM architecture. :-( e.g. return value not being sent to the
> syscall_trace function upon exit (upon which LTTng depends). (patch
> below)
> 
> This is _very_ silly because there is no dependency on the syscall being
> executed, and the syscall_entry/syscall_exit events are recorded at the
> _exact_ same time. Yes, I mean the _exact_ same time : using a clock
> which consists of atomic_add_return monotonic increments, it seems like
> ARM is able to return the _same_ value of an atomic increment return
> *twice* !! I think the atomic.h primitives are broken and that they
> allow concurrent modification of a given atomic variable by the pipeline.
> It sounds weird, and I hope I am not crazy (just getting into the ARM
> world..). ;) Any thoughts ? I'll try adding some barriers to see if it
> helps.

Hi Mathieu,

I am currently investigating a very similar behavior,
(syscall_entry/syscall_exit events having the exact same time in lttng).

However, I am using the CCNT (together with trace-clock-32-to-64.c) for
timestamping. This is, if I understand you correctly, a different clock
than the one you are using, not using atomic_add_return(). Thus, I
suspect that the reason for getting the exact same time for entry/exit
events might be something else than the clocks being broken. 

I have to admit that I cannot explain how it can happen though. Could it
be some weird problem in the lttng trace recording ?

best regards,

Viktor


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