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    Subject[PATCH] mce: fix RCU lockdep from mce_log()
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    Date
    Hi,

    Please review this patch, I am not very familiar with MCE/RCU so I'm not sure that this is the correct fix (otherwise consider it a bug report :)).
    This does "fix" the message though and I can use MCE normally.

    Thanks,
    Davidlohr


    From: Davidlohr Bueso <dave@gnu.org>

    Based on the following message:

    ===================================================
    [ INFO: suspicious rcu_dereference_check() usage. ]
    ---------------------------------------------------
    arch/x86/kernel/cpu/mcheck/mce.c:1628 invoked rcu_dereference_check() without protection!

    other info that might help us debug this:

    rcu_scheduler_active = 1, debug_locks = 1
    no locks held by mcelog/2350.

    stack backtrace:
    Pid: 2350, comm: mcelog Tainted: G W 2.6.37-rc1+ #7
    Call Trace:
    [<ffffffff8108e6d4>] lockdep_rcu_dereference+0xa4/0xc0
    [<ffffffff810189e9>] mce_poll+0xa9/0xd0
    [<ffffffff81160585>] do_sys_poll+0x275/0x550
    [<ffffffff8115f0e0>] ? __pollwait+0x0/0xf0
    [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
    [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
    [<ffffffff8130431c>] ? rcu_read_lock_held+0x2c/0x30
    [<ffffffff8130549a>] ? radix_tree_lookup_element+0xda/0x100
    [<ffffffff81121f08>] ? __do_fault+0x128/0x470
    [<ffffffff81100bdb>] ? filemap_fault+0xdb/0x4e0
    [<ffffffff810ffe75>] ? unlock_page+0x25/0x30
    [<ffffffff81069ddf>] ? sigprocmask+0x3f/0x100
    [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
    [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
    [<ffffffff8108ff4d>] ? trace_hardirqs_on+0xd/0x10
    [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
    [<ffffffff811608a7>] sys_ppoll+0x47/0x190
    [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
    [<ffffffff81835d39>] ? trace_hardirqs_on_thunk+0x3a/0x3f
    [<ffffffff810030eb>] system_call_fastpath+0x16/0x1b

    At this point the arch/x86/kernel/cpu/mcheck/mce.c:1628 invoked rcu_dereference_check() without protection!

    other info that might help us debug this:

    rcu_scheduler_active = 1, debug_locks = 1
    no locks held by mcelog/2350.

    stack backtrace:
    Pid: 2350, comm: mcelog Tainted: G W 2.6.37-rc1+ #7
    Call Trace:
    [<ffffffff8108e6d4>] lockdep_rcu_dereference+0xa4/0xc0
    [<ffffffff810189e9>] mce_poll+0xa9/0xd0
    [<ffffffff81160585>] do_sys_poll+0x275/0x550
    [<ffffffff8115f0e0>] ? __pollwait+0x0/0xf0
    [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
    [<ffffffff8115f1d0>] ? pollwake+0x0/0x60
    [<ffffffff8130431c>] ? rcu_read_lock_held+0x2c/0x30
    [<ffffffff8130549a>] ? radix_tree_lookup_element+0xda/0x100
    [<ffffffff81121f08>] ? __do_fault+0x128/0x470
    [<ffffffff81100bdb>] ? filemap_fault+0xdb/0x4e0
    [<ffffffff810ffe75>] ? unlock_page+0x25/0x30
    [<ffffffff81069ddf>] ? sigprocmask+0x3f/0x100
    [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
    [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
    [<ffffffff8108ff4d>] ? trace_hardirqs_on+0xd/0x10
    [<ffffffff8183672b>] ? _raw_spin_unlock_irq+0x2b/0x60
    [<ffffffff811608a7>] sys_ppoll+0x47/0x190
    [<ffffffff8108fefd>] ? trace_hardirqs_on_caller+0x13d/0x180
    [<ffffffff81835d39>] ? trace_hardirqs_on_thunk+0x3a/0x3f
    [<ffffffff810030eb>] system_call_fastpath+0x16/0x1b

    At this point the lockdep_is_held(&mce_read_mutex) call is failing.
    So check if the mce_read_mutex is held before derefencing instead of using rcu_dereference_check_mce()

    Signed-off-by: Davidlohr Bueso <dave@gnu.org>
    ---
    arch/x86/kernel/cpu/mcheck/mce.c | 8 ++++++--
    1 files changed, 6 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
    index 7a35b72..6f95b2c 100644
    --- a/arch/x86/kernel/cpu/mcheck/mce.c
    +++ b/arch/x86/kernel/cpu/mcheck/mce.c
    @@ -1625,8 +1625,12 @@ out:
    static unsigned int mce_poll(struct file *file, poll_table *wait)
    {
    poll_wait(file, &mce_wait, wait);
    - if (rcu_dereference_check_mce(mcelog.next))
    - return POLLIN | POLLRDNORM;
    +
    + if (mutex_is_locked(&mce_read_mutex)) {
    + if (rcu_dereference_index_check(mcelog.next,
    + rcu_read_lock_sched_held()))
    + return POLLIN | POLLRDNORM;
    + }
    if (!mce_apei_read_done && apei_check_mce())
    return POLLIN | POLLRDNORM;
    return 0;
    --
    1.7.1




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