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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Sebastian Andrzej Siewior <bigeasy@linutronix.de>
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Date:   Fri, 20 May 2022 14:32:24 -0500
In-Reply-To: <YodEWlfo4kFd8+mt@linutronix.de> (Sebastian Andrzej Siewior's
        message of "Fri, 20 May 2022 09:33:46 +0200")
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Subject: Re: [PATCH 00/16] ptrace: cleanups and calling do_cldstop with only
 siglock
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Sebastian Andrzej Siewior <bigeasy@linutronix.de> writes:

> On 2022-05-18 17:49:50 [-0500], Eric W. Biederman wrote:
>> 
>> For ptrace_stop to work on PREEMT_RT no spinlocks can be taken once
>> ptrace_freeze_traced has completed successfully.  Which fundamentally
>> means the lock dance of dropping siglock and grabbing tasklist_lock does
>> not work on PREEMPT_RT.  So I have worked through what is necessary so
>> that tasklist_lock does not need to be grabbed in ptrace_stop after
>> siglock is dropped.
> …
> It took me a while to realise that this is a follow-up I somehow assumed
> that you added a few patches on top. Might have been the yesterday's
> heat. b4 also refused to download this series because the v4 in this
> thread looked newer… Anyway. Both series applied:
>
> | =============================
> | WARNING: suspicious RCU usage
> | 5.18.0-rc7+ #16 Not tainted
> | -----------------------------
> | include/linux/ptrace.h:120 suspicious rcu_dereference_check() usage!
> |
> | other info that might help us debug this:
> |
> | rcu_scheduler_active = 2, debug_locks = 1
> | 2 locks held by ssdd/1734:
> |  #0: ffff88800eaa6918 (&sighand->siglock){....}-{2:2}, at: lock_parents_siglocks+0xf0/0x3b0
> |  #1: ffff88800eaa71d8 (&sighand->siglock/2){....}-{2:2}, at: lock_parents_siglocks+0x115/0x3b0
> |
> | stack backtrace:
> | CPU: 2 PID: 1734 Comm: ssdd Not tainted 5.18.0-rc7+ #16
> | Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.0-debian-1.16.0-4 04/01/2014
> | Call Trace:
> |  <TASK>
> |  dump_stack_lvl+0x45/0x5a
> |  unlock_parents_siglocks+0xb6/0xc0
> |  ptrace_stop+0xb9/0x390
> |  get_signal+0x51c/0x8d0
> |  arch_do_signal_or_restart+0x31/0x750
> |  exit_to_user_mode_prepare+0x157/0x220
> |  irqentry_exit_to_user_mode+0x5/0x50
> |  asm_sysvec_apic_timer_interrupt+0x12/0x20
>
> That is ptrace_parent() in unlock_parents_siglocks().

How odd.  I thought I had the appropriate lockdep config options enabled
in my test build to catch things like this.  I guess not.

Now I am trying to think how to tell it that holding the appropriate
iglock makes this ok.

Eric