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Message-ID: <11ff242b-195c-de75-d3d2-3e7b17a2ef3c@quicinc.com>
Date:   Fri, 22 Apr 2022 15:33:08 +0530
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Subject: Re: [PATCH] devcoredump : Serialize devcd_del work
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To:     <linux-kernel@vger.kernel.org>,
        Greg KH <gregkh@linuxfoundation.org>
CC:     <tglx@linutronix.de>, <sboyd@kernel.org>, <rafael@kernel.org>,
        <johannes@sipsolutions.net>
References: <1650364077-22694-1-git-send-email-quic_mojha@quicinc.com>
From:   Mukesh Ojha <quic_mojha@quicinc.com>
In-Reply-To: <1650364077-22694-1-git-send-email-quic_mojha@quicinc.com>
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Hi All,

Request you all the review comments on the fix of the described problem?

-Mukesh


On 4/19/2022 3:57 PM, 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
>                                                       debug_object_fixup()
>                                                        timer_fixup_assert_init()
>                                                         timer_setup()
>                                                           do_init_timer()   ==> reinitialized the
>                                                                                   timer to
>                                                                                   timer->entry.pprev=NULL
> 
>                                                    timer_pending()
>                                                     !hlist_unhashed_lockless(&timer->entry)
>                                                       !h->pprev  ==> del_timer checks
>                                                                    and finds it to be NULL
>   								  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>
> ---
>   drivers/base/devcoredump.c | 11 +++++++++--
>   1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/base/devcoredump.c b/drivers/base/devcoredump.c
> index f4d794d..316f566 100644
> --- a/drivers/base/devcoredump.c
> +++ b/drivers/base/devcoredump.c
> @@ -25,6 +25,7 @@ struct devcd_entry {
>   	struct device devcd_dev;
>   	void *data;
>   	size_t datalen;
> +	struct mutex mutex;
>   	struct module *owner;
>   	ssize_t (*read)(char *buffer, loff_t offset, size_t count,
>   			void *data, size_t datalen);
> @@ -84,7 +85,9 @@ static ssize_t devcd_data_write(struct file *filp, struct kobject *kobj,
>   	struct device *dev = kobj_to_dev(kobj);
>   	struct devcd_entry *devcd = dev_to_devcd(dev);
>   
> +	mutex_lock(&devcd->mutex);
>   	mod_delayed_work(system_wq, &devcd->del_wk, 0);
> +	mutex_unlock(&devcd->mutex);
>   
>   	return count;
>   }
> @@ -112,7 +115,9 @@ static int devcd_free(struct device *dev, void *data)
>   {
>   	struct devcd_entry *devcd = dev_to_devcd(dev);
>   
> +	mutex_lock(&devcd->mutex);
>   	flush_delayed_work(&devcd->del_wk);
> +	mutex_unlock(&devcd->mutex);
>   	return 0;
>   }
>   
> @@ -278,13 +283,14 @@ void dev_coredumpm(struct device *dev, struct module *owner,
>   	devcd->read = read;
>   	devcd->free = free;
>   	devcd->failing_dev = get_device(dev);
> -
> +	mutex_init(&devcd->mutex);
>   	device_initialize(&devcd->devcd_dev);
>   
>   	dev_set_name(&devcd->devcd_dev, "devcd%d",
>   		     atomic_inc_return(&devcd_count));
>   	devcd->devcd_dev.class = &devcd_class;
>   
> +	mutex_lock(&devcd->mutex);
>   	if (device_add(&devcd->devcd_dev))
>   		goto put_device;
>   
> @@ -301,10 +307,11 @@ void dev_coredumpm(struct device *dev, struct module *owner,
>   
>   	INIT_DELAYED_WORK(&devcd->del_wk, devcd_del);
>   	schedule_delayed_work(&devcd->del_wk, DEVCD_TIMEOUT);
> -
> +	mutex_unlock(&devcd->mutex);
>   	return;
>    put_device:
>   	put_device(&devcd->devcd_dev);
> +	mutex_unlock(&devcd->mutex);
>    put_module:
>   	module_put(owner);
>    free: