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SubjectRe: lockdep trace from posix timers
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On Tue, 2012-07-24 at 16:36 -0400, Dave Jones wrote:

> ======================================================
> [ INFO: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected ]
> 3.5.0+ #122 Not tainted
> ------------------------------------------------------
> trinity-child2/5327 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
> blocked: (tasklist_lock){.+.+..}, instance: ffffffff81c05098, at: [<ffffffff8109762b>] posix_cpu_timer_del+0x2b/0xe0
>
> and this task is already holding:
> blocked: (&(&new_timer->it_lock)->rlock){-.-...}, instance: ffff880143bce170, at: [<ffffffff81093d49>] __lock_timer+0x89/0x1f0
> which would create a new lock dependency:
> (&(&new_timer->it_lock)->rlock){-.-...} -> (tasklist_lock){.+.+..}
>
> but this new dependency connects a HARDIRQ-irq-safe lock:

> to a HARDIRQ-irq-unsafe lock:
> (&(&p->alloc_lock)->rlock){+.+...}

> other info that might help us debug this:
>
> Chain exists of:
> &(&new_timer->it_lock)->rlock --> tasklist_lock --> &(&p->alloc_lock)->rlock
>
> Possible interrupt unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock(&(&p->alloc_lock)->rlock);
> local_irq_disable();
> lock(&(&new_timer->it_lock)->rlock);
> lock(tasklist_lock);
> <Interrupt>
> lock(&(&new_timer->it_lock)->rlock);
>
> *** DEADLOCK ***
>
> 1 lock on stack by trinity-child2/5327:
> #0: blocked: (&(&new_timer->it_lock)->rlock){-.-...}, instance: ffff880143bce170, at: [<ffffffff81093d49>] __lock_timer+0x89/0x1f0


> the dependencies between the lock to be acquired and HARDIRQ-irq-unsafe lock:

> [<ffffffff810da83d>] lock_acquire+0xad/0x220
> [<ffffffff816895f6>] _raw_spin_lock+0x46/0x80
> [<ffffffff812d5f2e>] keyctl_session_to_parent+0xde/0x490
> [<ffffffff812d634d>] sys_keyctl+0x6d/0x1a0
> [<ffffffff8169336d>] system_call_fastpath+0x1a/0x1f

> stack backtrace:
> Pid: 5327, comm: trinity-child2 Not tainted 3.5.0+ #122
> Call Trace:
> [<ffffffff810d8194>] check_usage+0x4e4/0x500
> [<ffffffff81023729>] ? native_sched_clock+0x19/0x80
> [<ffffffff810d59a8>] ? trace_hardirqs_off_caller+0x28/0xd0
> [<ffffffff81023729>] ? native_sched_clock+0x19/0x80
> [<ffffffff810d820b>] check_irq_usage+0x5b/0xe0
> [<ffffffff810d93da>] __lock_acquire+0xd8a/0x1ae0
> [<ffffffff810d8956>] ? __lock_acquire+0x306/0x1ae0
> [<ffffffff810d59a8>] ? trace_hardirqs_off_caller+0x28/0xd0
> [<ffffffff810da2a5>] ? lock_release_non_nested+0x175/0x320
> [<ffffffff810da83d>] lock_acquire+0xad/0x220
> [<ffffffff8109762b>] ? posix_cpu_timer_del+0x2b/0xe0
> [<ffffffff81689b59>] _raw_read_lock+0x49/0x80
> [<ffffffff8109762b>] ? posix_cpu_timer_del+0x2b/0xe0
> [<ffffffff81093d95>] ? __lock_timer+0xd5/0x1f0
> [<ffffffff8109762b>] posix_cpu_timer_del+0x2b/0xe0
> [<ffffffff81094786>] sys_timer_delete+0x26/0x100
> [<ffffffff8169336d>] system_call_fastpath+0x1a/0x1f


So we have:


sys_keyctl()
keyctl_session_to_parent()
write_lock_irq(&tasklist_lock)
task_lock(parent) parent->alloc_lock

VS

sys_timer_delete()
lock_timer() timer->it_lock
posix_cpu_timer_del()
read_lock(&tasklist_lock)


Creating:

timer->it_lock -> tasklist_lock -> task->alloc_lock

And since it_lock is IRQ-safe and alloc_lock isn't, you've got the IRQ
inversion deadlock reported.

The task_lock() in keyctl_session_to_parent() comes from Al who didn't
think it necessary to write a changelog in d35abdb2.

David, Al, anybody want to have a go at fixing this?


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