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    Subject[PATCH] Exiting queue and task might race to free cic
    Date
    Hi Jens

    Looking at the bug reported here
    http://thread.gmane.org/gmane.linux.kernel/722539
    it looks like an exiting queue can race with an exiting task.

    When a queue exits the queue lock is taken and cfq_exit_queue() would free all
    the cic's associated with the queue.

    But when a task exits, cfq_exit_io_context() gets cic one by one and then
    locks the associated queue to call __cfq_exit_single_io_context. It looks like
    between getting a cic from the ioc and locking the queue, the queue might have
    exited on another cpu. Isn't this possible?

    If possible, either verifying whether cic->key is still not null or q->flags
    does not have QUEUE_FLAG_DEAD set would fix this.

    Thanks
    Nikanth Karthikesan

    Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>

    ---
    diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    index 6a062ee..b9b627a 100644
    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -1318,7 +1318,12 @@ static void cfq_exit_single_io_context(struct
    io_context *ioc,
    unsigned long flags;

    spin_lock_irqsave(q->queue_lock, flags);
    - __cfq_exit_single_io_context(cfqd, cic);
    + /*
    + * cic might have been already exited when an exiting task
    + * races with an exiting queue.
    + */
    + if (likely(cic->key))
    + __cfq_exit_single_io_context(cfqd, cic);
    spin_unlock_irqrestore(q->queue_lock, flags);
    }
    }
    Or this would also work

    diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    index 6a062ee..7a068bd 100644
    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -1318,7 +1318,11 @@ static void cfq_exit_single_io_context(struct
    io_context *ioc,
    unsigned long flags;

    spin_lock_irqsave(q->queue_lock, flags);
    - __cfq_exit_single_io_context(cfqd, cic);
    + /*
    + * Make sure the queue is not dead.
    + */
    + if (likely(!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)))
    + __cfq_exit_single_io_context(cfqd, cic);
    spin_unlock_irqrestore(q->queue_lock, flags);
    }
    }



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