linux-kernel - Re: [PATCH v1] driver core: check for dead devices before onlining/offlining

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Message-ID: <20200124090052.GA2958140@kroah.com>
Date:   Fri, 24 Jan 2020 10:00:52 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Saravana Kannan <saravanak@...gle.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v1] driver core: check for dead devices before
 onlining/offlining

On Mon, Jan 20, 2020 at 11:49:09AM +0100, David Hildenbrand wrote:
> We can have rare cases where the removal of a device races with
> somebody trying to online it (esp. via sysfs). We can simply check
> if the device is already removed or getting removed under the dev->lock.
> 
> E.g., right now, if memory block devices are removed (remove_memory()),
> we do a:
> 
> remove_memory() -> lock_device_hotplug() -> mem_hotplug_begin() ->
> lock_device() -> dev->dead = true
> 
> Somebody coming via sysfs (/sys/devices/system/memory/memoryX/online)
> triggers a:
> 
> lock_device_hotplug_sysfs() -> device_online() -> lock_device() ...
> 
> So if we made it just before the lock_device_hotplug_sysfs() but get
> delayed until remove_memory() released all locks, we will continue
> taking locks and trying to online the device - which is then a zombie
> device.
> 
> Note that at least the memory onlining path seems to be protected by
> checking if all memory sections are still present (something we can then
> get rid of). We do have other sysfs attributes
> (e.g., /sys/devices/system/memory/memoryX/valid_zones) that don't do any
> such locking yet and might race with memory removal in a similar way. For
> these users, we can then do a
> 
> device_lock(dev);
> if (!device_is_dead(dev)) {
> 	/* magic /*
> }
> device_unlock(dev);
> 
> Introduce and use device_is_dead() right away.
> 
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> Cc: Saravana Kannan <saravanak@...gle.com>
> Cc: Heikki Krogerus <heikki.krogerus@...ux.intel.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Michal Hocko <mhocko@...nel.org>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
> 
> Am I missing any obvious mechanism in the device core that handles
> something like this already? (especially also for other sysfs attributes?)

So is a sysfs attribute causing the device itself to go away?  We have
problems with that in the past, look at how the scsi layer handled it, I
think there's a specific call you should be making instead of trying to
rely on this "dead" flag.

thanks,

greg k-h