Date:   Thu, 30 Jul 2020 08:53:26 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org,
        linux-spi@...r.kernel.org
Subject: Re: [PATCH 2/3] driver core: Use rwsem for kill_device()
 serialization

On Wed, Jul 08, 2020 at 03:27:02PM +0200, Lukas Wunner wrote:
> kill_device() is currently serialized with driver probing by way of the
> device_lock().  We're about to serialize it with device_add() as well
> to prevent addition of children below a device which is going away.

Why?  Who does this?  Shouldn't the bus that is trying to do this know
this is happening?

> However the parent's device_lock() cannot be taken by device_add()
> lest deadlocks occur:  Addition of the parent may result in addition
> of children (as is the case with SPI controllers) and device_add()
> already takes the device_lock through the call to bus_probe_device() ->
> device_initial_probe() -> __device_attach().
> 
> Avoid such deadlocks by introducing an rw_semaphore whose specific
> purpose is to serialize kill_device() with other parts of the kernel.

Ugh, another lock, really?  :(

> Use an rw_semaphore instead of a mutex because the latter would preclude
> concurrent driver probing of multiple children below the same parent.
> The semaphore is acquired for writing when declaring a device dead and
> otherwise only acquired for reading.  It is private to the driver core,
> obviating the need to acquire a lock when calling kill_device(), as is
> currently done in nvdimm's nd_device_unregister() and device_del().
> 
> An alternative approach would be to convert the device_lock() itself to
> an rw_semaphore (instead of a mutex).
> 
> Signed-off-by: Lukas Wunner <lukas@...ner.de>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Geert Uytterhoeven <geert+renesas@...der.be>
> Cc: Pantelis Antoniou <pantelis.antoniou@...sulko.com>
> Cc: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> ---
>  drivers/base/base.h  |  2 ++
>  drivers/base/core.c  | 33 +++++++++++++++++++--------------
>  drivers/base/dd.c    |  8 ++++++++
>  drivers/nvdimm/bus.c |  8 +-------
>  4 files changed, 30 insertions(+), 21 deletions(-)
> 
> diff --git a/drivers/base/base.h b/drivers/base/base.h
> index 95c22c0f90360..7e71a1d262ef6 100644
> --- a/drivers/base/base.h
> +++ b/drivers/base/base.h
> @@ -79,6 +79,7 @@ struct driver_private {
>   * @async_driver - pointer to device driver awaiting probe via async_probe
>   * @device - pointer back to the struct device that this structure is
>   * associated with.
> + * @dead_sem - semaphore taken when declaring the device @dead.
>   * @dead - This device is currently either in the process of or has been
>   *	removed from the system. Any asynchronous events scheduled for this
>   *	device should exit without taking any action.
> @@ -94,6 +95,7 @@ struct device_private {
>  	struct list_head deferred_probe;
>  	struct device_driver *async_driver;
>  	struct device *device;
> +	struct rw_semaphore dead_sem;
>  	u8 dead:1;
>  };
>  #define to_device_private_parent(obj)	\
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 67d39a90b45c7..057da42b1a660 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -2526,6 +2526,7 @@ static int device_private_init(struct device *dev)
>  	klist_init(&dev->p->klist_children, klist_children_get,
>  		   klist_children_put);
>  	INIT_LIST_HEAD(&dev->p->deferred_probe);
> +	init_rwsem(&dev->p->dead_sem);
>  	return 0;
>  }
>  
> @@ -2780,21 +2781,27 @@ void put_device(struct device *dev)
>  }
>  EXPORT_SYMBOL_GPL(put_device);
>  
> +/**
> + * kill_device - declare device dead
> + * @dev: device in question
> + *
> + * Declare @dev dead to prevent it from binding to a driver.
> + * Return true if it was killed or false if it was already dead.
> + */
>  bool kill_device(struct device *dev)
>  {
> -	/*
> -	 * Require the device lock and set the "dead" flag to guarantee that
> -	 * the update behavior is consistent with the other bitfields near
> -	 * it and that we cannot have an asynchronous probe routine trying
> -	 * to run while we are tearing out the bus/class/sysfs from
> -	 * underneath the device.
> -	 */
> -	lockdep_assert_held(&dev->mutex);
> +	bool killed;
>  
> -	if (dev->p->dead)
> -		return false;
> -	dev->p->dead = true;
> -	return true;
> +	down_write(&dev->p->dead_sem);
> +	if (dev->p->dead) {
> +		killed = false;
> +	} else {
> +		dev->p->dead = true;
> +		killed = true;
> +	}
> +	up_write(&dev->p->dead_sem);
> +
> +	return killed;
>  }
>  EXPORT_SYMBOL_GPL(kill_device);

meta-comment, this should have been device_kill() :(

>  
> @@ -2817,9 +2824,7 @@ void device_del(struct device *dev)
>  	struct kobject *glue_dir = NULL;
>  	struct class_interface *class_intf;
>  
> -	device_lock(dev);
>  	kill_device(dev);
> -	device_unlock(dev);
>  
>  	if (dev->fwnode && dev->fwnode->dev == dev)
>  		dev->fwnode->dev = NULL;
> diff --git a/drivers/base/dd.c b/drivers/base/dd.c
> index 31c668651e824..9353d811cce83 100644
> --- a/drivers/base/dd.c
> +++ b/drivers/base/dd.c
> @@ -817,6 +817,7 @@ static void __device_attach_async_helper(void *_dev, async_cookie_t cookie)
>  	};
>  
>  	device_lock(dev);
> +	down_read(&dev->p->dead_sem);
>  
>  	/*
>  	 * Check if device has already been removed or claimed. This may
> @@ -838,6 +839,7 @@ static void __device_attach_async_helper(void *_dev, async_cookie_t cookie)
>  	if (dev->parent)
>  		pm_runtime_put(dev->parent);
>  out_unlock:
> +	up_read(&dev->p->dead_sem);
>  	device_unlock(dev);
>  
>  	put_device(dev);
> @@ -848,6 +850,7 @@ static int __device_attach(struct device *dev, bool allow_async)
>  	int ret = 0;
>  
>  	device_lock(dev);
> +	down_read(&dev->p->dead_sem);

Ick, see, now two locks for the same device structure?  I really don't
like that as the complexity involved is now double.


>  	if (dev->p->dead) {
>  		goto out_unlock;
>  	} else if (dev->driver) {
> @@ -893,6 +896,7 @@ static int __device_attach(struct device *dev, bool allow_async)
>  			pm_runtime_put(dev->parent);
>  	}
>  out_unlock:
> +	up_read(&dev->p->dead_sem);
>  	device_unlock(dev);
>  	return ret;
>  }
> @@ -967,6 +971,7 @@ int device_driver_attach(struct device_driver *drv, struct device *dev)
>  	int ret = 0;
>  
>  	__device_driver_lock(dev, dev->parent);
> +	down_read(&dev->p->dead_sem);
>  
>  	/*
>  	 * If device has been removed or someone has already successfully
> @@ -975,6 +980,7 @@ int device_driver_attach(struct device_driver *drv, struct device *dev)
>  	if (!dev->p->dead && !dev->driver)
>  		ret = driver_probe_device(drv, dev);
>  
> +	up_read(&dev->p->dead_sem);
>  	__device_driver_unlock(dev, dev->parent);
>  
>  	return ret;
> @@ -987,6 +993,7 @@ static void __driver_attach_async_helper(void *_dev, async_cookie_t cookie)
>  	int ret = 0;
>  
>  	__device_driver_lock(dev, dev->parent);
> +	down_read(&dev->p->dead_sem);
>  
>  	drv = dev->p->async_driver;
>  
> @@ -997,6 +1004,7 @@ static void __driver_attach_async_helper(void *_dev, async_cookie_t cookie)
>  	if (!dev->p->dead && !dev->driver)
>  		ret = driver_probe_device(drv, dev);
>  
> +	up_read(&dev->p->dead_sem);
>  	__device_driver_unlock(dev, dev->parent);
>  
>  	dev_dbg(dev, "driver %s async attach completed: %d\n", drv->name, ret);
> diff --git a/drivers/nvdimm/bus.c b/drivers/nvdimm/bus.c
> index 09087c38fabdc..35e069c69386a 100644
> --- a/drivers/nvdimm/bus.c
> +++ b/drivers/nvdimm/bus.c
> @@ -559,8 +559,6 @@ EXPORT_SYMBOL(nd_device_register);
>  
>  void nd_device_unregister(struct device *dev, enum nd_async_mode mode)
>  {
> -	bool killed;
> -
>  	switch (mode) {
>  	case ND_ASYNC:
>  		/*
> @@ -584,11 +582,7 @@ void nd_device_unregister(struct device *dev, enum nd_async_mode mode)
>  		 * or otherwise let the async path handle it if the
>  		 * unregistration was already queued.
>  		 */
> -		nd_device_lock(dev);
> -		killed = kill_device(dev);
> -		nd_device_unlock(dev);
> -
> -		if (!killed)
> +		if (!kill_device(dev))

So, why are you pushing this down into the driver core, can't this be
done in whatever crazy bus wants to do this, like is done here?

That will save us the extra lock complexity in the driver core.

What bus wants/needs this now?

thanks,

greg k-h

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