It appears the thread ids are reused. That is, if thread 2 exits, and
another starts, then thread 2 is reused.
Is there anyway to detect this during instrumentation, or is this a
limitation of the Valgrind tools?
When trying to separate the work of each thread, having
VG_(get_running_tid)() report non-unique ID's is troublesome.
Mike
On Wed, Jun 21, 2017 at 4:28 PM Philippe Waroquiers <
philippe.waroqui...@skynet.be> wrote:
> On Wed, 2017-06-21 at 19:17 +0000, Mike Lui wrote:
> > I posted this on the valgrind-dev mailing list a while back but didn't
> > receive a response. Hopefully some users have some input.
> >
> >
> > I'm working on a project that leverages Callgrind to generate VEX IR
> > traces. I'm using Valgrind 3.12.0.
> > I also use Callgrind's infrastructure to detect when Valgrind switches
> > thread contexts, however I'm getting unexpected behavior.
> >
> >
> > It looks like the best place to detect a thread context switch
> > in Callgrind is in CLG_(setup_bbcc) in bbcc.c (line 561):
> >
> >
> > /* This is needed because thread switches can not reliable be
> > tracked
> > * with callback CLG_(run_thread) only: we have otherwise no way to
> > get
> > * the thread ID after a signal handler returns.
> > * This could be removed again if that bug is fixed in Valgrind.
> > * This is in the hot path but hopefully not to costly.
> > */
> > tid = VG_(get_running_tid)();
> > #if 1
> > /* CLG_(switch_thread) is a no-op when tid is equal to
> > CLG_(current_tid).
> > * As this is on the hot path, we only call CLG_(switch_thread)(tid)
> > * if tid differs from the CLG_(current_tid).
> > */
> > if (UNLIKELY(tid != CLG_(current_tid)))
> > CLG_(switch_thread)(tid);
> >
> >
> > The above is called every instrumented basic block.
> > I've noticed strange behavior, where a thread switch would not always
> > be detected.
> > I detected the unexpected behavior with the following modifications:
> >
> >
> > To investigate further, I modified the above:
> > - if (UNLIKELY(tid != CLG_(current_tid)))
> >
> > + if (UNLIKELY(tid != CLG_(current_tid))) {
> > CLG_(switch_thread)(tid);
> > + VG_(printf)("Thread switched to: %d\n", tid);
> > + }
> >
> Adding --trace-sched=yes might help to understand what happens.
> Also, --trace-signals=yes might be useful if the problem is signal
> related.
>
> Philippe
>
>
>
>
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