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    SubjectRe: workqueue, pci: INFO: possible recursive locking detected
    On 07/19/2013 04:57 PM, Srivatsa S. Bhat wrote:
    > On 07/19/2013 07:17 AM, Lai Jiangshan wrote:
    >> On 07/19/2013 04:23 AM, Srivatsa S. Bhat wrote:
    >>>
    >>> ---
    >>>
    >>> kernel/workqueue.c | 6 ++++++
    >>> 1 file changed, 6 insertions(+)
    >>>
    >>>
    >>> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
    >>> index f02c4a4..07d9a67 100644
    >>> --- a/kernel/workqueue.c
    >>> +++ b/kernel/workqueue.c
    >>> @@ -4754,7 +4754,13 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
    >>> {
    >>> struct work_for_cpu wfc = { .fn = fn, .arg = arg };
    >>>
    >>> +#ifdef CONFIG_LOCKDEP
    >>> + static struct lock_class_key __key;
    >>
    >> Sorry, this "static" should be removed.
    >>
    >
    > That didn't help either :-( Because it makes lockdep unhappy,
    > since the key isn't persistent.
    >
    > This is the patch I used:
    >
    > ---
    >
    > diff --git a/kernel/workqueue.c b/kernel/workqueue.c
    > index f02c4a4..7967e3b 100644
    > --- a/kernel/workqueue.c
    > +++ b/kernel/workqueue.c
    > @@ -4754,7 +4754,13 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
    > {
    > struct work_for_cpu wfc = { .fn = fn, .arg = arg };
    >
    > +#ifdef CONFIG_LOCKDEP
    > + struct lock_class_key __key;
    > + INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
    > + lockdep_init_map(&wfc.work.lockdep_map, "&wfc.work", &__key, 0);
    > +#else
    > INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
    > +#endif
    > schedule_work_on(cpu, &wfc.work);
    > flush_work(&wfc.work);
    > return wfc.ret;
    >
    >
    > And here are the new warnings:
    >
    >
    > Block layer SCSI generic (bsg) driver version 0.4 loaded (major 252)
    > io scheduler noop registered
    > io scheduler deadline registered
    > io scheduler cfq registered (default)
    > BUG: key ffff881039557b98 not in .data!
    > ------------[ cut here ]------------
    > WARNING: CPU: 8 PID: 1 at kernel/lockdep.c:2987 lockdep_init_map+0x168/0x170()

    Sorry again.

    From 0096b9dac2282ec03d59a3f665b92977381a18ad Mon Sep 17 00:00:00 2001
    From: Lai Jiangshan <laijs@cn.fujitsu.com>
    Date: Mon, 22 Jul 2013 19:08:51 +0800
    Subject: [PATCH] [PATCH] workqueue: allow the function of work_on_cpu() can
    call work_on_cpu()

    If the @fn call work_on_cpu() again, the lockdep will complain:

    > [ INFO: possible recursive locking detected ]
    > 3.11.0-rc1-lockdep-fix-a #6 Not tainted
    > ---------------------------------------------
    > kworker/0:1/142 is trying to acquire lock:
    > ((&wfc.work)){+.+.+.}, at: [<ffffffff81077100>] flush_work+0x0/0xb0
    >
    > but task is already holding lock:
    > ((&wfc.work)){+.+.+.}, at: [<ffffffff81075dd9>] process_one_work+0x169/0x610
    >
    > other info that might help us debug this:
    > Possible unsafe locking scenario:
    >
    > CPU0
    > ----
    > lock((&wfc.work));
    > lock((&wfc.work));
    >
    > *** DEADLOCK ***

    It is false-positive lockdep report. In this sutiation,
    the two "wfc"s of the two work_on_cpu() are different,
    they are both on stack. flush_work() can't be deadlock.

    To fix this, we need to avoid the lockdep checking in this case,
    But we don't want to change the flush_work(), so we use
    completion instead of flush_work() in the work_on_cpu().

    Reported-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
    ---
    kernel/workqueue.c | 5 ++++-
    1 files changed, 4 insertions(+), 1 deletions(-)

    diff --git a/kernel/workqueue.c b/kernel/workqueue.c
    index f02c4a4..b021a45 100644
    --- a/kernel/workqueue.c
    +++ b/kernel/workqueue.c
    @@ -4731,6 +4731,7 @@ struct work_for_cpu {
    long (*fn)(void *);
    void *arg;
    long ret;
    + struct completion done;
    };

    static void work_for_cpu_fn(struct work_struct *work)
    @@ -4738,6 +4739,7 @@ static void work_for_cpu_fn(struct work_struct *work)
    struct work_for_cpu *wfc = container_of(work, struct work_for_cpu, work);

    wfc->ret = wfc->fn(wfc->arg);
    + complete(&wfc->done);
    }

    /**
    @@ -4755,8 +4757,9 @@ long work_on_cpu(int cpu, long (*fn)(void *), void *arg)
    struct work_for_cpu wfc = { .fn = fn, .arg = arg };

    INIT_WORK_ONSTACK(&wfc.work, work_for_cpu_fn);
    + init_completion(&wfc.done);
    schedule_work_on(cpu, &wfc.work);
    - flush_work(&wfc.work);
    + wait_for_completion(&wfc.done);
    return wfc.ret;
    }
    EXPORT_SYMBOL_GPL(work_on_cpu);
    --
    1.7.4.4

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