Re: [PATCH 00/16] ptrace: cleanups and calling do_cldstop with only siglock

[Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>]

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().

Sebastian