#systemtap on 2020-06-20 — irc logs at freenode.irclog.whitequark.org

2015-11-12 23:18 fche changed the topic of #systemtap to: http://sourceware.org/systemtap; email systemtap@sourceware.org if answers here not timely, conversations may be logged

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03:39 <agentzh> fche: okay, i found fedora's kernel-debug package also has lockdep enabled. no need to fight against rawhide...

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07:06 <agentzh> fche: lockdep successfully caught a deadlock bug in my today's patch for adding back relay_flush() and also inode locking around it. it's indeed very powerful! thanks for the suggestion!

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07:07 <agentzh> one sad thing about lockdep is that loading and unloading ko modules would quickly leak until exceeding the max lockdep entry limit.

07:07 <agentzh> so i cannot load my test for long until a reboot is necessary.

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08:06 <agentzh> fche: the v6 patch is finally ready: https://gist.github.com/agentzh/932e7a32bb508da1d6cbe7ef65994000

08:08 <agentzh> hopefully it's good enough now...

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13:09 <fche> yo

13:12 <fche> it looks good to me

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16:58 <agentzh> fche: great thanks. will commit.

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20:27 <agentzh> fche: on fedora's kernel-debug, i found another ordering issue in the task finder & vma maps callbacks. this patch should fix it: https://gist.github.com/agentzh/a166220b1fefc09813db8e0859622923

20:27 <agentzh> will you have a look? thanks!

20:27 <agentzh> seems like kernel-debug is slow for many things, which actually helps exposing deep hidden issues.

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21:48 <fche> agentzh, dude, do you ever sleep?

21:48 <fche> just wondering

21:50 <agentzh> yes, i do :)

21:50 <agentzh> 7 hours

21:51 <fche> I'm not quite getting what the difference is before / after with the last patch

21:51 <fche> it seems stp_call_callbacks is called identically in both cases

21:51 <fche> and so is stp_call_mmap_callbacks_for_task

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21:54 <agentzh> fche: the process.begin probe's callback can be registered after vm tracker's?

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21:55 <agentzh> that's the goal.

21:55 <fche> nah I'm just trying to trace the control flow before & after the patch

21:56 <fche> ah farther down is the good new stuff

21:56 <fche> was focusing only on the top bits (__stp_utrace_task_finder_target_quiesce __stp_tf_quiesce_worker etc.)

21:59 <agentzh> yeah, seems like i'm still a bit confused with the ordering. i'll dig deeper.

21:59 <agentzh> that patch makes the simple test pass on kernel-debug but the test no longer passes with the default kernel of fedora.

21:59 <agentzh> something must still be wrong here.

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22:05 <agentzh> okay, seems like the order of the callbacks are already correct.

22:05 <agentzh> but the vma map's callback is not controllable here.

22:06 <agentzh> they are called multiple times by utrace, it seems

22:06 <agentzh> and only a later run will get the maps.

22:08 <agentzh> they run twice...

22:08 <agentzh> the process.begin handler runs in the first run but not vma maps', and then the 2nd time runs the vma one.

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22:27 <agentzh> there's a race condition reproduced on kernel-debug: stapio exec'ing to the target process (via stap -c CMD).

22:27 <agentzh> the first vma map callback is triggered before the process path changes.

22:27 <agentzh> so it finds nothing.

22:28 <agentzh> it reports stap_start_task_finder:1771: target path not matched: [/tmp/t_stap_LAN9nF3/a.out] != [/opt/stap-plus/libexec/systemtap/stapio]

22:28 <agentzh> seems like this is real race condition here.

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23:01 <fche> hm no harm with that one tho

23:01 <fche> I think

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