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    Subject[PATCH 3.5 22/62] workqueue: ensure @task is valid across kthread_stop()
    Date
    3.5.7.32 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Lai Jiangshan <laijs@cn.fujitsu.com>

    commit 5bdfff96c69a4d5ab9c49e60abf9e070ecd2acbb upstream.

    When a kworker should die, the kworkre is notified through WORKER_DIE
    flag instead of kthread_should_stop(). This, IIRC, is primarily to
    keep the test synchronized inside worker_pool lock. WORKER_DIE is
    first set while holding pool->lock, the lock is dropped and
    kthread_stop() is called.

    Unfortunately, this means that there's a slight chance that the target
    kworker may see WORKER_DIE before kthread_stop() finishes and exits
    and frees the target task before or during kthread_stop().

    Fix it by pinning the target task before setting WORKER_DIE and
    putting it after kthread_stop() is done.

    tj: Improved patch description and comment. Moved pinning above
    WORKER_DIE for better signify what it's protecting.

    Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
    Signed-off-by: Tejun Heo <tj@kernel.org>
    [ luis: backported to 3.5: adjusted context ]
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    kernel/workqueue.c | 7 +++++++
    1 file changed, 7 insertions(+)

    diff --git a/kernel/workqueue.c b/kernel/workqueue.c
    index 89743ae..d19727f 100644
    --- a/kernel/workqueue.c
    +++ b/kernel/workqueue.c
    @@ -1472,12 +1472,19 @@ static void destroy_worker(struct worker *worker)
    if (worker->flags & WORKER_IDLE)
    gcwq->nr_idle--;

    + /*
    + * Once WORKER_DIE is set, the kworker may destroy itself at any
    + * point. Pin to ensure the task stays until we're done with it.
    + */
    + get_task_struct(worker->task);
    +
    list_del_init(&worker->entry);
    worker->flags |= WORKER_DIE;

    spin_unlock_irq(&gcwq->lock);

    kthread_stop(worker->task);
    + put_task_struct(worker->task);
    kfree(worker);

    spin_lock_irq(&gcwq->lock);
    --
    1.9.0


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