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From: "agentzh at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: systemtap@sourceware.org
Subject: [Bug runtime/26144] New: task finder: vma maps callback may run
 after other task finder callbacks
Date: Sun, 21 Jun 2020 06:51:49 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D26144

            Bug ID: 26144
           Summary: task finder: vma maps callback may run after other
                    task finder callbacks
           Product: systemtap
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: runtime
          Assignee: systemtap at sourceware dot org
          Reporter: agentzh at gmail dot com
  Target Milestone: ---

I've noted on fedora's kernel-debug package's kernels, it's easy to reprodu=
ce
the case where __stp_utrace_task_finder_target_quiesce() fails to run the
callbacks because the current context does not allow sleeping. Then the
callbacks would be postponed using stp_task_work_add() which in turn calls
task_work_add(). The task_work_add() always adds the callbacks in the rever=
sed
order, which leads to the incorrect result of running vma maps callback aft=
er
other callbacks (like process.begin's).

Below is the logs demonstrating this bad case:

https://gist.github.com/agentzh/c2e99a87c6aae652bb2eb6d39c20c077

It seems the correct fix would be combining all the callbacks, including the
vma maps and other task finder callbacks, into a single utrace attach callb=
ack.
If we always handle all callbacks for each target task as a single entity, =
it
would be possible to ensure the execution order of these callbacks, even in=
 the
case of non-sleeping contexts and task_work_add(). Any better ideas would be
highly appreciated. Thanks!

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