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    Subject[PATCH 5.3 083/148] xen/xenbus: fix self-deadlock after killing user process
    Date
    From: Juergen Gross <jgross@suse.com>

    commit a8fabb38525c51a094607768bac3ba46b3f4a9d5 upstream.

    In case a user process using xenbus has open transactions and is killed
    e.g. via ctrl-C the following cleanup of the allocated resources might
    result in a deadlock due to trying to end a transaction in the xenbus
    worker thread:

    [ 2551.474706] INFO: task xenbus:37 blocked for more than 120 seconds.
    [ 2551.492215] Tainted: P OE 5.0.0-29-generic #5
    [ 2551.510263] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    [ 2551.528585] xenbus D 0 37 2 0x80000080
    [ 2551.528590] Call Trace:
    [ 2551.528603] __schedule+0x2c0/0x870
    [ 2551.528606] ? _cond_resched+0x19/0x40
    [ 2551.528632] schedule+0x2c/0x70
    [ 2551.528637] xs_talkv+0x1ec/0x2b0
    [ 2551.528642] ? wait_woken+0x80/0x80
    [ 2551.528645] xs_single+0x53/0x80
    [ 2551.528648] xenbus_transaction_end+0x3b/0x70
    [ 2551.528651] xenbus_file_free+0x5a/0x160
    [ 2551.528654] xenbus_dev_queue_reply+0xc4/0x220
    [ 2551.528657] xenbus_thread+0x7de/0x880
    [ 2551.528660] ? wait_woken+0x80/0x80
    [ 2551.528665] kthread+0x121/0x140
    [ 2551.528667] ? xb_read+0x1d0/0x1d0
    [ 2551.528670] ? kthread_park+0x90/0x90
    [ 2551.528673] ret_from_fork+0x35/0x40

    Fix this by doing the cleanup via a workqueue instead.

    Reported-by: James Dingwall <james@dingwall.me.uk>
    Fixes: fd8aa9095a95c ("xen: optimize xenbus driver for multiple concurrent xenstore accesses")
    Cc: <stable@vger.kernel.org> # 4.11
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/xen/xenbus/xenbus_dev_frontend.c | 20 ++++++++++++++++++--
    1 file changed, 18 insertions(+), 2 deletions(-)

    --- a/drivers/xen/xenbus/xenbus_dev_frontend.c
    +++ b/drivers/xen/xenbus/xenbus_dev_frontend.c
    @@ -55,6 +55,7 @@
    #include <linux/string.h>
    #include <linux/slab.h>
    #include <linux/miscdevice.h>
    +#include <linux/workqueue.h>

    #include <xen/xenbus.h>
    #include <xen/xen.h>
    @@ -116,6 +117,8 @@ struct xenbus_file_priv {
    wait_queue_head_t read_waitq;

    struct kref kref;
    +
    + struct work_struct wq;
    };

    /* Read out any raw xenbus messages queued up. */
    @@ -300,14 +303,14 @@ static void watch_fired(struct xenbus_wa
    mutex_unlock(&adap->dev_data->reply_mutex);
    }

    -static void xenbus_file_free(struct kref *kref)
    +static void xenbus_worker(struct work_struct *wq)
    {
    struct xenbus_file_priv *u;
    struct xenbus_transaction_holder *trans, *tmp;
    struct watch_adapter *watch, *tmp_watch;
    struct read_buffer *rb, *tmp_rb;

    - u = container_of(kref, struct xenbus_file_priv, kref);
    + u = container_of(wq, struct xenbus_file_priv, wq);

    /*
    * No need for locking here because there are no other users,
    @@ -333,6 +336,18 @@ static void xenbus_file_free(struct kref
    kfree(u);
    }

    +static void xenbus_file_free(struct kref *kref)
    +{
    + struct xenbus_file_priv *u;
    +
    + /*
    + * We might be called in xenbus_thread().
    + * Use workqueue to avoid deadlock.
    + */
    + u = container_of(kref, struct xenbus_file_priv, kref);
    + schedule_work(&u->wq);
    +}
    +
    static struct xenbus_transaction_holder *xenbus_get_transaction(
    struct xenbus_file_priv *u, uint32_t tx_id)
    {
    @@ -650,6 +665,7 @@ static int xenbus_file_open(struct inode
    INIT_LIST_HEAD(&u->watches);
    INIT_LIST_HEAD(&u->read_buffers);
    init_waitqueue_head(&u->read_waitq);
    + INIT_WORK(&u->wq, xenbus_worker);

    mutex_init(&u->reply_mutex);
    mutex_init(&u->msgbuffer_mutex);

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