devcoredump : Serialize devcd_del work

In following scenario(diagram), when one thread X running dev_coredumpm()
adds devcd device to the framework which sends uevent notification to
userspace and another thread Y reads this uevent and call to
devcd_data_write() which eventually try to delete the queued timer that
is not initialized/queued yet.

So, debug object reports some warning and in the meantime, timer is
initialized and queued from X path. and from Y path, it gets reinitialized
again and timer->entry.pprev=NULL and try_to_grab_pending() stucks.

To fix this, introduce mutex and a boolean flag to serialize the behaviour.

 	cpu0(X)			                cpu1(Y)

    dev_coredump() uevent sent to user space
    device_add()  ======================> user space process Y reads the
                                          uevents writes to devcd fd
                                          which results into writes to

                                         devcd_data_write()
                                           mod_delayed_work()
                                             try_to_grab_pending()
                                               del_timer()
                                                 debug_assert_init()
   INIT_DELAYED_WORK()
   schedule_delayed_work()
                                                   debug_object_fixup()
                                                     timer_fixup_assert_init()
                                                       timer_setup()
                                                         do_init_timer()
                                                       /*
                                                        Above call reinitializes
                                                        the timer to
                                                        timer->entry.pprev=NULL
                                                        and this will be checked
                                                        later in timer_pending() call.
                                                       */
                                                 timer_pending()
                                                  !hlist_unhashed_lockless(&timer->entry)
                                                    !h->pprev
                                                /*
                                                  del_timer() checks h->pprev and finds
                                                  it to be NULL due to which
                                                  try_to_grab_pending() stucks.
                                                */

Link: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Mukesh Ojha <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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