Messages in this thread | | | Date | Tue, 17 Feb 2009 14:30:15 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH] ARM fix syscall trace return value |
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* Viktor Rosendahl (Viktor.Rosendahl@nokia.com) wrote: > 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 ? >
I had the same result as you with the ccnt-based clock I am currently developing, so I went back to a more "solid" and atomic atomic_add_return clock. But I noticed that we still had entry/exit with the same timestamps, so I was really unsure about what was happening, because there is no trace corruption and because I have never, ever, seen that kind of problem on any other architecture (x86, powerpc, mips...). So I fixed the syscall_trace exit parameter, which now makes sure there is a dependency on the return value. But I want to find out why the atomic add return failed to be atomic in that particular condition. I suspect there is a missing memory barrier in atomic.h.
Mathieu
> best regards, > > Viktor > >
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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