On 10/21/2013 08:21 PM, Daniel Merrill wrote:
More follow up on this, we went ahead and put some logging in shadow.c
which from what we could find is where the signal is "kicking" the thread.
From the logging it looks like the only signals we get (while attached to
GDB) are SIGSTOP, SIGTRAP, SIGRT32 and SIGKILL(upon exiting the debugger).
I'm assuming the SIGSTOP, SIGTRAP and SIGKILL are normal from the
debugger. It looks like shadow.c looks for SIGTRAP and SIGSTOP and sets an
XNDEBUG state on the thread which I assume allows it to restart the
suspend?
XNDEBUG marks a thread which is ptraced, this has implications when
managing the system timer while the app is single-stepped/stopped by a
debugger.
SIGRT32 I believe comes from our calls to t_delete. I'm guessing
this is what's causing the suspends to fail? Anyway, I appreciate any
additional insight anyone can offer. Thanks again for all the help.
t_delete() will cause t_suspend() to unblock if sent to the suspended
task, due to receiving SIGRT32/SIGCANCEL from the linux side, which is
how the NPTL deals with async cancellation internally (t_delete() ->
pthread_cancel() -> t(g)kill(SIGCANCEL)).
Internally, XNBREAK will be raised for that task, causing -EINTR to be
propagated back. However, since there is SIGCANCEL pending for the task,
the NPTL handler should run on the way back to the call site in
t_suspend(), and the task should never return from this handler.
In short, receiving EINTR from t_suspend() is unexpected, particularly
when unblocked by SIGCANCEL. I could not reproduce this issue based on
the simple test, running over GDB (7.5.1).
A few questions more:
- regardless of t_delete(), is the problem about one or multiple threads
unblocking unexpectedly from t_suspend(0), when single-stepping a
distinct thread over GDB?
- I'm testing with Xenomai 2.6.3. Which version have you been using, on
which cpu/platform, using which I-pipe release in the kernel (check
/proc/xenomai/{version, hal}?
- Also could you write a simple test code illustrating the issue so that
I could try reproducing it? Typically, would this be reproducible on
your setup with a single task running t_suspend(0), while ptracing the
main routine in parallel?
TIA,
--
Philippe.
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