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[Bug runtime/12642] utrace: taskfinder misses events when main thread does not go through at least one quiesce


http://sourceware.org/bugzilla/show_bug.cgi?id=12642

--- Comment #3 from David Smith <dsmith at redhat dot com> 2012-11-02 16:03:32 UTC ---
Depending on how you count, we've got 3 or 4 sets of utrace-like functionality
here, with different behaviors:

1) the original version of utrace, which is present in RHEL5, handled by
runtime/linux/task_finder.c. In this case we do send the main thread a
UTRACE_STOP, which causes it to stop and we attach correctly.

Here your testcase passes.

2) Version 2 of utrace, which is present in RHEL6 (also handled by
runtime/linux/task_finder.c). In this case we do send the main thread a
UTRACE_STOP - however the stop doesn't/can't interrupt a system call.

Here your testcase fails.

3) On new kernels without "real" utrace, we fake it with tracepoint handlers
and task_work_add().  This code is in runtime/linux/task_finder2.c and
runtime/stp_utrace.c. This code uses task_work_add() to run code when the task
is stopped.  This can't interrupt a system call.

Here your testcase fails.

4) The new dyninst runtime ("--runtime=dyninst") uses the dyninst library to
attach, which ends up using ptrace. Attaching to a running process isn't quite
there yet, but I think it should be possible to interrupt a system call.

So, for cases 2) and 3) above, we've still got some thinking to do.

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