[PATCH 0/2] (Was: ptrace() hangs on attempt to seize/attach stopped & frozen task)

From: Oleg Nesterov
Date: Thu Nov 19 2015 - 13:46:27 EST

On 11/09, Andrey Ryabinin wrote:
> So, the ptrace() hangs if we try to attach to stopped task from freezing cgroup.

OK, lets make it killable at least, TASK_KILLABLE looks better in
any case.

I am starting to think again that we should remove JOBCTL_TRAPPING,
but this needs a bit more discussion and probably testing.

Anyway I agree with Tejun, the freezer doesn't look nice too.

Hmm. I need to re-check but it seems we have another problem...
ptrace_stop() does task_participate_group_stop() and this (in particular)
clears JOBCTL_STOP_PENDING. This is only correct if the task is still traced,
but this is not necessarily true and we can in fact clear JOBCTL_STOP_PENDING
set by __ptrace_unlink() called by the exiting tracer.


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