Re: Race condition between driver_probe_device and device_shutdownâ

[Ming Lei]

Cc pm list because it is related with PM.

Hi Greg,

On Mon, May 21, 2012 at 3:51 AM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:

>
> And how can that happen with a real bus?  Don't we have a lock

The races may be triggered when one device is just probed(triggered
by plug) or released(triggered by unplug) at the same time of running
reboot/poweroff.

> somewhere per-bus that should be protecting this type of thing (sorry,
> can't dig through the code at the moment, on the road...)

device_shutdown is called with only holding reboot_mutex, so I think no
any protection on dev->driver there.

>
> How come no one has ever hit them in the past 10 years?  What am I
> missing here?

The window is so small that maybe it is very very difficult to trigger
the races, :-)
But looks Wedson is luck enough to observe it.

>> Looks the above makes sense to serialize .shutdown with
>> .probe and .release.
>
> Let me look at the code when I get back in a few days, but I really
> thought we already had a lock protecting all of this...

Also the previous patch don't cover the .runtime_resume races with
.probe or .release, so the right fix may be below:

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 346be8b..cbc8bd2 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1820,6 +1820,11 @@ void device_shutdown(void)
 		list_del_init(&dev->kobj.entry);
 		spin_unlock(&devices_kset->list_lock);

+		/*hold lock[s] to avoid races with .probe/.release*/
+		if (dev->parent)
+			device_lock(dev->parent);
+		device_lock(dev);
+
 		/* Don't allow any more runtime suspends */
 		pm_runtime_get_noresume(dev);
 		pm_runtime_barrier(dev);
@@ -1831,6 +1836,9 @@ void device_shutdown(void)
 			dev_dbg(dev, "shutdown\n");
 			dev->driver->shutdown(dev);
 		}
+		device_unlock(dev);
+		if (dev->parent)
+			device_unlock(dev->parent);
 		put_device(dev);

 		spin_lock(&devices_kset->list_lock);

Another candidate fix is to register a reboot notifier in driver core to prevent
driver from being bound or unbound from start of reboot/shutdown, but looks
not easy as the way of holding device locks.


Thanks,
--
Ming Lei
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