On Saturday, April 15, 2017 07:01:26 PM Thomas Gleixner wrote: > Converting the hotplug locking, i.e. get_online_cpus(), to a percpu rwsem > unearthed a circular lock dependency which was hidden from lockdep due to > the lockdep annotation of get_online_cpus() which prevents lockdep from > creating full dependency chains. > > CPU0 CPU1 > ---- ---- > lock((&wfc.work)); > lock(cpu_hotplug_lock.rw_sem); > lock((&wfc.work)); > lock(cpu_hotplug_lock.rw_sem); > > This dependency is established via acpi_processor_start() which calls into > the work queue code. And the work queue code establishes the reverse > dependency. > > This is not a problem of get_online_cpus() recursion, it's a possible > deadlock undetected by lockdep so far. > > The cure is to use cpu_hotplug_disable() instead of get_online_cpus() to > protect the probing from acpi_processor_start(). > > There is a side effect to this: cpu_hotplug_disable() makes a concurrent > cpu hotplug attempt via the sysfs interfaces fail with -EBUSY, but that > probing usually happens during the boot process where no interaction is > possible. Any later invocations are infrequent enough and concurrent > hotplug attempts are so unlikely that the danger of user space visible > regressions is very close to zero. Anyway, thats preferrable over a real > deadlock. > > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: "Rafael J. Wysocki" <rjw@xxxxxxxxxxxxx> > Cc: Len Brown <lenb@xxxxxxxxxx> > Cc: linux-acpi@xxxxxxxxxxxxxxx Acked-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx> > --- > drivers/acpi/processor_driver.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > --- a/drivers/acpi/processor_driver.c > +++ b/drivers/acpi/processor_driver.c > @@ -268,9 +268,9 @@ static int acpi_processor_start(struct d > return -ENODEV; > > /* Protect against concurrent CPU hotplug operations */ > - get_online_cpus(); > + cpu_hotplug_disable(); > ret = __acpi_processor_start(device); > - put_online_cpus(); > + cpu_hotplug_enable(); > return ret; > } > > > -- To unsubscribe from this list: send the line "unsubscribe linux-acpi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
