[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:

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--- Comment #19 from Thiago Jung Bauermann <thiago.bauermann at linaro dot org> ---
Created attachment 15415
  --> https://sourceware.org/bugzilla/attachment.cgi?id=15415&action=edit
Proposed fix. Needs to be cleaned up.

I investigated this problem some more, and I see three separate issues
revealed by attach-many-short-lived-threads.exp:

1. The issue I mentioned in comment #17 (which I have since confirmed is
   what is going on), where the linux_proc_attach_tgid_threads () never
   ends when there are zombie threads present in the inferior. Since
   attach-many-short-lived-threads.c constantly creates and finishes
   joinable threads, the chance of having zombie threads is high.

   From looking at the gdb.log files Carl provided, I believe he is
   seeing the same problem.

   The solution is to make GDB remember when it has already visited the
   /proc directory of a given LWP, and skip it in the following iterations.
   I implemented the attached patch to do that, and now I don't observe GDB
   hanging anymore in the aarch64-linux server in which I used to easily
   reproduce this problem. If Carl could test it on POWER10, it would be
   helpful. I'll clean up the code and post it on the mailing list.

2. Behaviour 2 which I described in comment #12. I'll repeat it here for
   completeness:

   (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.

   This is very rarely observed on my test system. I saw it only 3 times in
   thousands of testcase runs. I wasn't able to investigate it yet.

   I'll open a separate bugzilla about this.

3. This one isn't a bug, but an issue that arises from the way
   attach-many-short-lived-threads.c behaves: since it's constantly
   creating new threads it's impossible for GDB to know when it has
   attached to all of them so that it can finish the loop in
   linux_proc_attach_tgid_threads (). Because of this, even with the fix
   for issue #1 applied, the testcase fails once in a while — I left the
   test running in a loop overnight and it failed after about 2500
   iterations.

   The only way I can see to improve GDB's behaviour is to increase the
   number of iterations of the loop that checks for new threads. I suspect
   that the ability of the inferior to create new threads is proportional
   to the number of CPUs present in the system (my test machine has 160
   cores), so I will propose a patch that makes the number of iterations
   proportinal to the number of CPUs.

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