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SubjectRe: [PATCH] driver core: fix shutdown races with probe/remove(v2)
On Mon, Jun 11, 2012 at 01:13:20PM +0800, Ming Lei wrote:
> Firstly, .shutdown callback may touch a uninitialized hardware
> if dev->driver is set and .probe is not completed.
>
> Secondly, device_shutdown() may dereference a null pointer to cause
> oops when dev->driver is cleared after it is checked in
> device_shutdown().
>
> So just try to hold device lock and its parent lock(if it has) to
> fix the races.
>
> Cc: Alan Stern <stern@rowland.harvard.edu>
> Cc: stable@vger.kernel.org

Why stable? Are there known systems that crash right now without this
change? I don't think we ever heard back from the original poster about
this issue as to what exactly was going wrong.


> Signed-off-by: Ming Lei <ming.lei@canonical.com>
> ---
> v2:
> - take Alan's suggestion to use device_trylock to avoid
> hanging during shutdown by buggy device or driver
> - hold parent reference counter
>
> drivers/base/core.c | 32 ++++++++++++++++++++++++++++++++
> 1 file changed, 32 insertions(+)
>
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index 346be8b..f2fc989 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -1796,6 +1796,16 @@ out:
> }
> EXPORT_SYMBOL_GPL(device_move);
>
> +static int __try_lock(struct device *dev)
> +{
> + int i = 0;
> +
> + while (!device_trylock(dev) && i++ < 100)
> + msleep(10);
> +
> + return i < 100;
> +}

That's a totally arbritary time, why does this work and other times do
not? And what is this returning, if the lock was grabbed successfully?
What's with the __ naming?

I really don't like this at all.


> +
> /**
> * device_shutdown - call ->shutdown() on each device to shutdown.
> */
> @@ -1810,8 +1820,11 @@ void device_shutdown(void)
> * devices offline, even as the system is shutting down.
> */
> while (!list_empty(&devices_kset->list)) {
> + int nonlocked;
> +
> dev = list_entry(devices_kset->list.prev, struct device,
> kobj.entry);
> + get_device(dev->parent);

Why grab the parent reference?

> get_device(dev);
> /*
> * Make sure the device is off the kset list, in the
> @@ -1820,6 +1833,18 @@ void device_shutdown(void)
> list_del_init(&dev->kobj.entry);
> spin_unlock(&devices_kset->list_lock);
>
> + /* hold lock to avoid race with .probe/.release */
> + if (dev->parent && !__try_lock(dev->parent))
> + nonlocked = 2;
> + else if (!__try_lock(dev))
> + nonlocked = 1;
> + else
> + nonlocked = 0;

Ick ick ick. Why can't we just grab the lock to try to only call these
callbacks one at a time? What is causing the big problem here that I am
missing?

> +
> + if (nonlocked)
> + dev_err(dev, "can't hold %slock for shutdown\n",
> + nonlocked == 1 ? "" : "parent ");

What can anyone do with this message? I sure wouldn't know what to do
with it, do you? If so, what?

greg k-h



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