linux-kernel - Re: [PATCH] devcoredump : Serialize devcd

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Message-ID: <2a14c4f64ff3e029a76c85d064146e6c303c96bb.camel@sipsolutions.net>
Date:   Fri, 22 Apr 2022 15:41:51 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Mukesh Ojha <quic_mojha@...cinc.com>, linux-kernel@...r.kernel.org
Cc:     tglx@...utronix.de, sboyd@...nel.org, rafael@...nel.org
Subject: Re: [PATCH] devcoredump : Serialize devcd_del work

On Tue, 2022-04-19 at 15:57 +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 to serialize the behaviour.
> 
>  	cpu0(X)			                      cpu1(Y)
> 
>     dev_coredump() uevent sent to userspace
>     device_add()  =========================> userspace 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
> 

Wouldn't it be easier to simply schedule this before adding it to sysfs
and sending the uevent?

johannes