Bug 468903

Created attachment 321720 [details]
test-clone.c test program

I've now tried the same test on my x86_64 system at home (somewhat faster
AMD Athlon 64 X2 4400+ dual core box), and I see the same failures.

I also tried it on kernel-2.6.27.4-51.fc10.x86_64 on my f10 beta
partition and 2.6.27 also gives the same error.

taking a look at it

Trying on 2.6.27.5-32.fc9.x86_64 on Fedora 9 on Intel Core 2 Duo. Doesn't fail. I am getting "DEF: ACCESS_CLONE_SAFE=1" anytime I run the test.

btw:
      struct timespec a_bit;
      memset((void *)&a_bit, 0, sizeof(a_bit));
      a_bit.tv_nsec = 20*1000; /* (a bit == 20 milliseconds :-) */
      nanosleep(&a_bit, NULL);
these are microseconds. Add another *1000 to get milliseconds.

I tried this:

while sleep 0.05; do echo -n .; ./test-clone >/dev/null 2>&1 || exit; done

and got a stream of dots, no failures.

Tested on kernel-2.6.27.4-51.fc10.x86_64.

At first I seemed to get failures under load, but I think they are false. I bumped up hang_count to 100 (and of course I have 20 microsecond delay fixed - replaced it by 10 millisecond), and now

while sleep 0.05; do echo -n .; ./test-clone2 >RES 2>&1 || exit; done

runs without failures while I have four "while true; do true; done" CPU hogging shells running on my 2 core machine.

I edited testcase a bit. For example, now it is C, not C++ (there was one instance of "char&"). I will attach the testcase.

Tom, can you run it on your machine(s)? If it runs ok but your original testcase fails, can it (original testcase) be fixed by correcting "microsecond" bug and using if (hang_count > 100) instead of if (hang_count > 10)?

Created attachment 323597 [details]
Updated test program test-clone2.c

Changes:
* after PTRACE_TRACEME, it is known practice to immediately do raise (SIGSTOP) afterwards. Removed SIGUSR1 usage for this.
* replaced nanosleep with usleep, fixing "microsecond" bug in the process.
* switched stdout into unbuffered mode, removed all fflush() calls.
* removed lone C++-ism present.
* other minor simplifications.

Unable to reproduce on vanilla 2.6.26.7 either

I'll check out the revised test when I get home, but unfortunately if it no
longer fails, it doesn't mean there isn't a bug, just that the test can't
reproduce it :-(. My real debugger really gets hung sometimes, and it is
definitely related to the stopped thread since the hangs don't happen without it.
Maybe I need more realistic activity in the threads themselves - waiting on a
mutex or something rather than merely sleeping. I'll play with it some more and
see what I can come up with.

> I'll check out the revised test when I get home, but unfortunately if it no
longer fails, it doesn't mean there isn't a bug, just that the test can't
reproduce it :-(

I fully agree that it doesn't rule out the bug. But I do need a testcase for it.

So far it is possible that what you saw was caused by having way too short (20 microseconds x5 times) period of waiting. IOW: the code was too eager to declare thread stuck when in fact it was not.

Please try to produce a testcase which works on vanilla kernel (with utrace disabled - the bug, if it exists, may also affect the version of utrace which is in mainline), but fails on utrace kernel (vanilla or Fedora one).

The .config of both kernels should be similar apart from CONFIG_UTRACE.

Yea, I'm still trying to reproduce the bug reliably. I may have to get
a complete trace of everything that happens when the real debugger
hangs then see if I can turn that into a test program. It may take
a while to figure out :-(.

I talking nonsense - there is no utrace in mainline yet. I meant "upstream utrace" (Roalnd's patchset against upstream kernel)

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