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SubjectRe: [PATCH v5] devcoredump : Serialize devcd_del work
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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@quicinc.com/
>> Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
>> ---
>
> I need an ack from the devcoredump maintainer before I can take this...
>
> thanks,
>
> greg k-h

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