Messages in this thread | | | Subject | Re: [PATCH] ARM fix syscall trace return value | From | Viktor Rosendahl <> | Date | Tue, 17 Feb 2009 21:22:03 +0200 |
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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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