On 2015/9/4 22:16, Rafael J. Wysocki wrote: > 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? Hi Rafael, Adding lock_device_hotplug() to smp_init() doesn't solve the issue. So it seems to be an false alarm of lockdep, and I don't know how to get rid of such an lockdep false alarm:( Thanks! Gerry -- 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
