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    Subject[PATCH 3.16 243/410] usb: ohci: Proper handling of ed_rm_list to handle race condition between usb_kill_urb() and finish_unlinks()
    3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

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

    From: AMAN DEEP <aman.deep@samsung.com>

    commit 46408ea558df13b110e0866b99624384a33bdeba upstream.

    There is a race condition between finish_unlinks->finish_urb() function
    and usb_kill_urb() in ohci controller case. The finish_urb calls
    spin_unlock(&ohci->lock) before usb_hcd_giveback_urb() function call,
    then if during this time, usb_kill_urb is called for another endpoint,
    then new ed will be added to ed_rm_list at beginning for unlink, and
    ed_rm_list will point to newly added.

    When finish_urb() is completed in finish_unlinks() and ed->td_list
    becomes empty as in below code (in finish_unlinks() function):

    if (list_empty(&ed->td_list)) {
    *last = ed->ed_next;
    ed->ed_next = NULL;
    } else if (ohci->rh_state == OHCI_RH_RUNNING) {
    *last = ed->ed_next;
    ed->ed_next = NULL;
    ed_schedule(ohci, ed);
    }

    The *last = ed->ed_next will make ed_rm_list to point to ed->ed_next
    and previously added ed by usb_kill_urb will be left unreferenced by
    ed_rm_list. This causes usb_kill_urb() hang forever waiting for
    finish_unlink to remove added ed from ed_rm_list.

    The main reason for hang in this race condtion is addition and removal
    of ed from ed_rm_list in the beginning during usb_kill_urb and later
    last* is modified in finish_unlinks().

    As suggested by Alan Stern, the solution for proper handling of
    ohci->ed_rm_list is to remove ed from the ed_rm_list before finishing
    any URBs. Then at the end, we can add ed back to the list if necessary.

    This properly handle the updated ohci->ed_rm_list in usb_kill_urb().

    Fixes: 977dcfdc6031 ("USB: OHCI: don't lose track of EDs when a controller dies")
    Acked-by: Alan Stern <stern@rowland.harvard.edu>
    Signed-off-by: Aman Deep <aman.deep@samsung.com>
    Signed-off-by: Jeffy Chen <jeffy.chen@rock-chips.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    [bwh: Backported to 3.16: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    drivers/usb/host/ohci-q.c | 17 ++++++++++-------
    1 file changed, 10 insertions(+), 7 deletions(-)

    --- a/drivers/usb/host/ohci-q.c
    +++ b/drivers/usb/host/ohci-q.c
    @@ -966,6 +966,8 @@ skip_ed:
    * have modified this list. normally it's just prepending
    * entries (which we'd ignore), but paranoia won't hurt.
    */
    + *last = ed->ed_next;
    + ed->ed_next = NULL;
    modified = 0;

    /* unlink urbs as requested, but rescan the list after
    @@ -1024,20 +1026,21 @@ rescan_this:
    goto rescan_this;

    /*
    - * If no TDs are queued, take ED off the ed_rm_list.
    + * If no TDs are queued, ED is now idle.
    * Otherwise, if the HC is running, reschedule.
    - * If not, leave it on the list for further dequeues.
    + * If the HC isn't running, add ED back to the
    + * start of the list for later processing.
    */
    if (list_empty(&ed->td_list)) {
    - *last = ed->ed_next;
    - ed->ed_next = NULL;
    ed->state = ED_IDLE;
    } else if (ohci->rh_state == OHCI_RH_RUNNING) {
    - *last = ed->ed_next;
    - ed->ed_next = NULL;
    ed_schedule(ohci, ed);
    } else {
    - last = &ed->ed_next;
    + ed->ed_next = ohci->ed_rm_list;
    + ohci->ed_rm_list = ed;
    + /* Don't loop on the same ED */
    + if (last == &ohci->ed_rm_list)
    + last = &ed->ed_next;
    }

    if (modified)
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