Hi, On Fri, Sep 4, 2015 at 9:20 AM, Jiang Liu <jiang.liu@xxxxxxxxxxxxxxx> wrote: > On 2015/9/4 4:08, Rafael J. Wysocki wrote: >> Hi Tejun, >> >> On Thu, Sep 3, 2015 at 6:19 PM, Tejun Heo <tj@xxxxxxxxxx> wrote: >>> Hello, Rafael. >>> >>> On Thu, Sep 03, 2015 at 02:58:16AM +0200, Rafael J. Wysocki wrote: >>>> So acpi_device_hotplug() calls lock_device_hotplug() which simply >>>> acquires device_hotplug_lock. It is held throughout the entire >>>> hot-add/hot-remove code path. >>>> >>>> Witing anything to /sys/devices/system/cpu/cpux/online goes through >>>> online_store() in drivers/base/core.c and that does >>>> lock_device_hotplug_sysfs() which then attempts to acquire >>>> device_hotplug_lock using mutex_trylock(). And it only calls >>>> either device_online() or device_offline() if it ends up with the >>>> lock held. >>>> >>>> Quite frankly, I don't see how these particular two code paths can >>>> deadlock in any way. >>>> >>>> So either a third code path is involved which is not executed >>>> under device_hotplug_lock, or lockdep needs to be told to actually >>>> take device_hotplug_lock into account in this case IMO. >>> >>> Hmm... all sysfs rw functions are protected from removal. ie. by >>> default, removal of a sysfs file drains in-flight rw operations, so >>> the hot plug path grabs a lock and then tries to remove a file and >>> writing to the online file makes the file's write method to try to >>> grab the same lock. It deadlocks if the hotunplug path already has >>> the lock and trying to drain the online file for removal. >> >> My point is that you cannot get into that situation. If hotplug >> already holds device_hotplug_lock, the write to "online" will end up >> doing restart_syscall(). >> >> If the "online" code path is holding the lock, hotplug cannot acquire >> it and cannot proceed. >> >> Am I missing anything? > Hi Rafael, > I think your are right. The lock_device_hotplug_sysfs() has > already provided a solution for such a deadlock scenario. And there's > another related code path at boot as: > smp_init() > ->cpu_up() > ->cpu_hotplug_begin() > So it seems to be a false alarm. Any way to teach lockdep > about this to get rid of the false alarm? Well, maybe we could call lock_device_hotplug() from that code path too? Thanks, Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-hotplug" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
