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

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