• Thomas Gleixner's avatar
    PCI: Use cpu_hotplug_disable() instead of get_online_cpus() · 1ddd45f8
    Thomas Gleixner authored
    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. There are several variants of this. And
    example is:
    
    Chain exists of:
    
    cpu_hotplug_lock.rw_sem --> drm_global_mutex --> &item->mutex
    
    CPU0                    CPU1
    ----                    ----
    lock(&item->mutex);
                            lock(drm_global_mutex);
                            lock(&item->mutex);
    lock(cpu_hotplug_lock.rw_sem);
    
    because there are dependencies through workqueues. The call chain is:
    
    	get_online_cpus
    	apply_workqueue_attrs
    	__alloc_workqueue_key
    	ttm_mem_global_init
    	ast_ttm_mem_global_init
    	drm_global_item_ref
    	ast_mm_init
    	ast_driver_load
    	drm_dev_register
    	drm_get_pci_dev
    	ast_pci_probe
    	local_pci_probe
    	work_for_cpu_fn
    	process_one_work
    	worker_thread
    
    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 PCI probing.
    
    There is a side effect to this: cpu_hotplug_disable() makes a concurrent
    cpu hotplug attempt via the sysfs interfaces fail with -EBUSY, but PCI
    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: default avatarThomas Gleixner <tglx@linutronix.de>
    Acked-by: default avatarIngo Molnar <mingo@kernel.org>
    Acked-by: default avatarBjorn Helgaas <bhelgaas@google.com>
    Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: linux-pci@vger.kernel.org
    Cc: Sebastian Siewior <bigeasy@linutronix.de>
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Link: http://lkml.kernel.org/r/20170524081548.691198590@linutronix.de
    1ddd45f8
pci-driver.c 36 KB