[Bug testsuite/31312] attach-many-short-lived-threads gives inconsistent results

["thiago.bauermann at linaro dot org" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31312

Thiago Jung Bauermann <thiago.bauermann at linaro dot org> changed:

           What    |Removed                     |Added
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                 CC|                            |thiago.bauermann at linaro dot org

--- Comment #16 from Thiago Jung Bauermann <thiago.bauermann at linaro dot org> ---
Hello,

(In reply to Tom Tromey from comment #12)
> > It then attempts to attach again.  At this point, the attach
> > times out and all subsequent gdb commands for the rest of the iterations
> > all time out.
> 
> It seems unusual to me that a single failure could somehow cause
> subsequent ones to fail.  Like, why would that be?

I also see this on aarch64-linux (sometimes), and I spent a bit of time
exploring the problem. I don't know yet what is going on, but I found two
interesting behaviours when trying to reproduce manually what the testcase is
doing¹:

1. Most of the time the attach fails with EPERM (which is the XFAIL case), but
occasionally GDB starts to use 100% of the CPU and never brings back the
prompt. At least in my case, this is why a single failure — e.g., "iter 8:
attach (timeout)" — causes all the subsequent ones to fail: GDB simply hangs
and the testcase can't make forward progress anymore.

2. Just now, the attach command did something surprising:

   (gdb) attach 2039552
   Attaching to process 2039552
   Cannot attach to lwp 2689792: Operation not permitted (1), process 2689792
is already traced by process 2039527

   PID 2039552 is the testcase inferior, and 2039527 is GDB. GDB didn't report
any success in attaching to the process.

I haven't digged deep enough to say anything about what exactly is going on
yet.

-- 
¹ that is, I'm running the attach-many-short-lived-threads in one terminal and
then repeatedly trying to attach to it from GDB in another terminal

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