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[Bug tapsets/25290] process(EXE).begin may occasionally miss already-running target processes from EXE


https://sourceware.org/bugzilla/show_bug.cgi?id=25290

--- Comment #2 from agentzh <agentzh at gmail dot com> ---
Okay, with fche's guidance on IRC, I've narrowed it down to a race condition
that the utrace process.begin callback would be invoked when the stap sessions
state is still STAP_SESSION_STARTING instead of STAP_SESSION_RUNNING
occasionally.

As per fche's suggestion on IRC, I'd propose the following patch to fix this:

https://pastebin.com/WumwHdAT

I've tested this patch myself on 3.10 kernels from CentOS, 4.15 kernels from
Ubuntu and 5.0 kernels from Fedora. And it's quite stable now. Also tested the
process(EXE).end and process(EXE).thread.{begin,end} probes. All look fine now.

Feedback welcome! Thanks!

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