Re: [PATCH v5] devcoredump : Serialize devcd_del work

[Mukesh Ojha]

Thanks @greg.

Hi @johannes,

Could you review this patch?

-Mukesh

On 6/27/2022 6:41 PM, Greg KH wrote:
> On Fri, May 27, 2022 at 07:33:40PM +0530, Mukesh Ojha wrote:
> > 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/2e1f81e2-428c-f11f-ce92-eb11048cb271@xxxxxxxxxxx/
> > Signed-off-by: Mukesh Ojha <quic_mojha@xxxxxxxxxxx>
> > ---
>
> I need an ack from the devcoredump maintainer before I can take this...
>
> thanks,
>
> greg k-h