On Mon, Feb 10, 2014 at 02:45:55PM +0530, Srivatsa S. Bhat wrote: > Hi Gautham, > > On 02/08/2014 12:41 AM, Gautham R Shenoy wrote: > > On Thu, Feb 06, 2014 at 07:41:03PM +0100, Oleg Nesterov wrote: > >> On 02/06, Srivatsa S. Bhat wrote: > >>> > >>> The following method of CPU hotplug callback registration is not safe > >>> due to the possibility of an ABBA deadlock involving the > >>> cpu_add_remove_lock > >>> and the cpu_hotplug.lock. > >> > >> Off-topic, but perhaps it also makes sense to add the lockdep annotations > >> later, to catch other similar problems. Currently get_online_cpus() > >> acquires > >> nothing from lockdep pov. > > > > Well, both get/put_online_cpus() as well as cpu_hotplug_begin/end() > > take the cpu_hotplug.lock mutex. So ideally the lockdep annotations of > > mutex_lock/unlock() should have worked. > > The reason lockdep doesn't catch the lock-inversion (ABBA) deadlock between > cpu_hotplug.lock (from get_online_cpus) and cpu_add_remove_lock (from > cpu_maps_update_begin) is because, in the following path, the > cpu_add_remove_lock is acquired after *releasing* the cpu_hotplug.lock mutex. >
Right. I get it now! > get_online_cpus(); // acquire mutex; update counter; release mutex > > register_cpu_notifier(); // acquire cpu_add_remove_lock ... > > put_online_cpus(); > > > If it hasn't, then the > > following lockdep annotations to cpu-hotplug locking should do the > > trick. > > > > This patch looks good to me. I have a couple of suggestions though.. > Thanks. I have incorporated the suggestions. Could you check if the following looks good ? --- Add lockdep annotations for get/put_online_cpus() and cpu_hotplug_begin()/cpu_hotplug_end(). Cc: Oleg Nesterov <[email protected]> Cc: Srivatsa Bhat <[email protected]> Signed-off-by: Gautham R. Shenoy <[email protected]> --- kernel/cpu.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/kernel/cpu.c b/kernel/cpu.c index deff2e6..33caf5e 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -19,6 +19,7 @@ #include <linux/mutex.h> #include <linux/gfp.h> #include <linux/suspend.h> +#include <linux/lockdep.h> #include "smpboot.h" @@ -57,17 +58,30 @@ static struct { * an ongoing cpu hotplug operation. */ int refcount; + +#ifdef CONFIG_DEBUG_LOCK_ALLOC + struct lockdep_map dep_map; +#endif } cpu_hotplug = { .active_writer = NULL, .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock), .refcount = 0, +#ifdef CONFIG_DEBUG_LOCK_ALLOC + .dep_map = {.name = "cpu_hotplug.lock" }, +#endif }; +/* Lockdep annotations for get/put_online_cpus() and cpu_hotplug_begin/end() */ +#define cpuhp_lock_acquire_read() lock_map_acquire_read(&cpu_hotplug.dep_map) +#define cpuhp_lock_acquire() lock_map_acquire(&cpu_hotplug.dep_map) +#define cpuhp_lock_release() lock_map_release(&cpu_hotplug.dep_map) + void get_online_cpus(void) { might_sleep(); if (cpu_hotplug.active_writer == current) return; + cpuhp_lock_acquire_read(); mutex_lock(&cpu_hotplug.lock); cpu_hotplug.refcount++; mutex_unlock(&cpu_hotplug.lock); @@ -87,6 +101,7 @@ void put_online_cpus(void) if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) wake_up_process(cpu_hotplug.active_writer); mutex_unlock(&cpu_hotplug.lock); + cpuhp_lock_release(); } EXPORT_SYMBOL_GPL(put_online_cpus); @@ -117,6 +132,7 @@ void cpu_hotplug_begin(void) { cpu_hotplug.active_writer = current; + cpuhp_lock_acquire(); for (;;) { mutex_lock(&cpu_hotplug.lock); if (likely(!cpu_hotplug.refcount)) @@ -131,6 +147,7 @@ void cpu_hotplug_done(void) { cpu_hotplug.active_writer = NULL; mutex_unlock(&cpu_hotplug.lock); + cpuhp_lock_release(); } /* -- 1.8.3.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
