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    Subject[PREEMPT_RT] [PATCH] scheduling with irqs disabled: strace/0x00000000/2011
    Date
    Hi,

    While running strace on some testcase running on -rt kernel (2.6.20-rt8 and
    2.6.21-rc6-rt0), I have seen the following BUG:

    BUG: scheduling with irqs disabled: strace/0x00000000/2011
    caller is rt_spin_lock_slowlock+0x102/0x1af

    Call Trace:
    [<ffffffff8026d828>] dump_trace+0xbd/0x3d8
    [<ffffffff8026db87>] show_trace+0x44/0x6d
    [<ffffffff8026ddc8>] dump_stack+0x13/0x15
    [<ffffffff80264dc6>] schedule+0x87/0x10b
    [<ffffffff80265b06>] rt_spin_lock_slowlock+0x102/0x1af
    [<ffffffff802661af>] rt_spin_lock+0x1f/0x21
    [<ffffffff8029af0c>] force_sig_info+0x26/0xb5
    [<ffffffff8029b018>] force_sig_specific+0x11/0x13
    [<ffffffff80298659>] ptrace_attach+0xdf/0x10b
    [<ffffffff802986d7>] sys_ptrace+0x52/0xb8
    [<ffffffff8025f42c>] tracesys+0x151/0x1be
    [<00000034ecec71c9>]

    ---------------------------
    | preempt count: 00000000 ]
    | 0-level deep critical section nesting:
    ----------------------------------------

    In ptrace_attach, this is what happens:

    task_lock
    local_irq_disable
    write_lock(tasklist_lock) Using trylocks.
    Some work
    __ptrace_link
    Send SIGSTOP to target thread
    write_unlock_irq(tasklist_lock)
    task_unlock

    On -rt, write_unlock_irq doesn't do local_irq_enable. Even if it did, we are
    calling it after sending SIGSTOP to target thread. To fix the problem, I
    think we should call write_unlock(tasklist_lock) and local_irq_enable()
    instead of write_unlock_irq. Also, we should call them before calling
    force_sig_specific(). I think there is no need to hold the tasklist lock
    while calling force_sig_specific(). Is my understanding correct?
    Alternatively, can we remove the call to local_irq_disable() in -rt kernel?

    For the non-rt kernel too, I think we should do
    write_unlock_irq(tasklist_lock) before sending SIGSTOP.

    The following patch solves the problem for me.

    Thanks and regards,
    Sripathi.

    Signed-off-by: Sripathi Kodi <sripathik@in.ibm.com>

    --- linux-2.6.21-rc6-rt0-org/kernel/ptrace.c 2007-04-06 08:06:56.000000000
    +0530
    +++ linux-2.6.21-rc6-rt0/kernel/ptrace.c 2007-04-19 18:18:40.000000000 +0530
    @@ -205,10 +205,16 @@ repeat:

    __ptrace_link(task, current);

    + write_unlock(&tasklist_lock);
    + local_irq_enable();
    +
    force_sig_specific(SIGSTOP, task);
    + goto out2;

    bad:
    - write_unlock_irq(&tasklist_lock);
    + write_unlock(&tasklist_lock);
    + local_irq_enable();
    +out2:
    task_unlock(task);
    out:
    return retval;
    -
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