Messages in this thread Patch in this message | | | Subject | Re: [PATCH v1] driver core: check for dead devices before onlining/offlining | From | David Hildenbrand <> | Date | Fri, 24 Jan 2020 18:14:09 +0100 |
| |
On 20.01.20 11:49, 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. >
So, I just added the following:
diff --git a/drivers/base/core.c b/drivers/base/core.c index 01cd06eeb513..49c4d8671073 100644 --- a/drivers/base/core.c +++ b/drivers/base/core.c @@ -1567,6 +1567,7 @@ static ssize_t online_store(struct device *dev, struct device_attribute *attr, if (ret < 0) return ret;
+ msleep(10000); ret = lock_device_hotplug_sysfs(); if (ret) return ret; Then triggered echo 1 > /sys/devices/system/memory/memory51/online And quickly afterwards unplugged the DIMM. Good news is that we get (after 10 seconds) sh: echo: write error: No such device
Reason is that unplug will not finish before all sysfs attributes have been exited by other threads. Therefore, the device_hotplug_lock will remain held by the removing thread. The thread stuck in the sysfs attribute will fail to trylock the device_hotplug_lock and return.
Other sysfs attributes that don't do a lock_device_hotplug_sysfs() might have to do a trylock/lock on the device_lock to synchronize properly.
Summary: This patch is not necessary.
-- Thanks,
David / dhildenb
| |