On Mon, Feb 15, 2016 at 03:17:55PM +0100, Peter Zijlstra wrote: > On Mon, Feb 15, 2016 at 02:36:43PM +0200, Joonas Lahtinen wrote: > > Instead of implementing a custom locked reference counting, use lockref. > > > > Current implementation leads to a deadlock splat on Intel SKL platforms > > when lockdep debugging is enabled. > > > > This is due to few of CPUfreq drivers (including Intel P-state) having this; > > policy->rwsem is locked during driver initialization and the functions > > called > > during init that actually apply CPU limits use get_online_cpus (because they > > have other calling paths too), which will briefly lock cpu_hotplug.lock to > > increase cpu_hotplug.refcount. > > > > On later calling path, when doing a suspend, when cpu_hotplug_begin() is > > called > > in disable_nonboot_cpus(), callbacks to CPUfreq functions get called after, > > which will lock policy->rwsem and cpu_hotplug.lock is already held by > > cpu_hotplug_begin() and we do have a potential deadlock scenario reported by > > our CI system (though it is a very unlikely one). See the Bugzilla link for > > more > > details. > > I've been meaning to change the thing into a percpu-rwsem, I just > haven't had time to look into the lockdep splat that generated.
The below has plenty lockdep issues because percpu-rwsem is reader-writer fair (like the regular rwsem), so it does throw up a fair number of very icky issues. If at all possible, I'd really rather fix those and have a 'saner' hotplug lock, rather than muddle on with open-coded horror lock we have now. --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -225,6 +225,8 @@ extern struct bus_type cpu_subsys; #ifdef CONFIG_HOTPLUG_CPU /* Stop CPUs going up and down. */ +extern void cpu_hotplug_init_task(struct task_struct *p); + extern void cpu_hotplug_begin(void); extern void cpu_hotplug_done(void); extern void get_online_cpus(void); @@ -242,6 +244,8 @@ int cpu_down(unsigned int cpu); #else /* CONFIG_HOTPLUG_CPU */ +static inline void cpu_hotplug_init_task(struct task_struct *p) {} + static inline void cpu_hotplug_begin(void) {} static inline void cpu_hotplug_done(void) {} #define get_online_cpus() do { } while (0) --- a/include/linux/percpu-rwsem.h +++ b/include/linux/percpu-rwsem.h @@ -16,6 +16,15 @@ struct percpu_rw_semaphore { wait_queue_head_t write_waitq; }; +#define DEFINE_STATIC_PERCPU_RWSEM(name) \ +static DEFINE_PER_CPU(unsigned int, __percpu_rwsem_frc_##name); \ +static struct percpu_rw_semaphore name = { \ + .rss = __RCU_SYNC_INITIALIZER(name.rss, RCU_SCHED_SYNC), \ + .fast_read_ctr = &__percpu_rwsem_frc_##name, \ + .rw_sem = __RWSEM_INITIALIZER(name.rw_sem), \ + .write_waitq = __WAIT_QUEUE_HEAD_INITIALIZER(name.write_waitq), \ +} + extern void percpu_down_read(struct percpu_rw_semaphore *); extern int percpu_down_read_trylock(struct percpu_rw_semaphore *); extern void percpu_up_read(struct percpu_rw_semaphore *); @@ -33,9 +42,11 @@ extern void percpu_free_rwsem(struct per __percpu_init_rwsem(brw, #brw, &rwsem_key); \ }) - #define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem) +#define percpu_rwsem_assert_held(sem) \ + lockdep_assert_held(&(sem)->rw_sem) + static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem, bool read, unsigned long ip) { --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1403,6 +1403,9 @@ struct task_struct { struct task_struct *last_wakee; int wake_cpu; +#ifdef CONFIG_HOTPLUG_CPU + int cpuhp_ref; +#endif #endif int on_rq; --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -23,6 +23,7 @@ #include <linux/tick.h> #include <linux/irq.h> #include <trace/events/power.h> +#include <linux/percpu-rwsem.h> #include "smpboot.h" @@ -51,121 +52,52 @@ EXPORT_SYMBOL(cpu_notifier_register_done static RAW_NOTIFIER_HEAD(cpu_chain); -/* If set, cpu_up and cpu_down will return -EBUSY and do nothing. +/* + * If set, cpu_up and cpu_down will return -EBUSY and do nothing. * Should always be manipulated under cpu_add_remove_lock */ static int cpu_hotplug_disabled; #ifdef CONFIG_HOTPLUG_CPU -static struct { - struct task_struct *active_writer; - /* wait queue to wake up the active_writer */ - wait_queue_head_t wq; - /* verifies that no writer will get active while readers are active */ - struct mutex lock; - /* - * Also blocks the new readers during - * an ongoing cpu hotplug operation. - */ - atomic_t refcount; - -#ifdef CONFIG_DEBUG_LOCK_ALLOC - struct lockdep_map dep_map; -#endif -} cpu_hotplug = { - .active_writer = NULL, - .wq = __WAIT_QUEUE_HEAD_INITIALIZER(cpu_hotplug.wq), - .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock), -#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_tryread() \ - lock_map_acquire_tryread(&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) +DEFINE_STATIC_PERCPU_RWSEM(hotplug); +void cpu_hotplug_init_task(struct task_struct *p) +{ + if (WARN_ON_ONCE(p->cpuhp_ref)) + p->cpuhp_ref = 0; +} void get_online_cpus(void) { might_sleep(); - if (cpu_hotplug.active_writer == current) + + if (current->cpuhp_ref++) /* read recursion */ return; - cpuhp_lock_acquire_read(); - mutex_lock(&cpu_hotplug.lock); - atomic_inc(&cpu_hotplug.refcount); - mutex_unlock(&cpu_hotplug.lock); + + percpu_down_read(&hotplug); } EXPORT_SYMBOL_GPL(get_online_cpus); void put_online_cpus(void) { - int refcount; - - if (cpu_hotplug.active_writer == current) + if (--current->cpuhp_ref) return; - refcount = atomic_dec_return(&cpu_hotplug.refcount); - if (WARN_ON(refcount < 0)) /* try to fix things up */ - atomic_inc(&cpu_hotplug.refcount); - - if (refcount <= 0 && waitqueue_active(&cpu_hotplug.wq)) - wake_up(&cpu_hotplug.wq); - - cpuhp_lock_release(); - + percpu_up_read(&hotplug); } EXPORT_SYMBOL_GPL(put_online_cpus); -/* - * This ensures that the hotplug operation can begin only when the - * refcount goes to zero. - * - * Note that during a cpu-hotplug operation, the new readers, if any, - * will be blocked by the cpu_hotplug.lock - * - * Since cpu_hotplug_begin() is always called after invoking - * cpu_maps_update_begin(), we can be sure that only one writer is active. - * - * Note that theoretically, there is a possibility of a livelock: - * - Refcount goes to zero, last reader wakes up the sleeping - * writer. - * - Last reader unlocks the cpu_hotplug.lock. - * - A new reader arrives at this moment, bumps up the refcount. - * - The writer acquires the cpu_hotplug.lock finds the refcount - * non zero and goes to sleep again. - * - * However, this is very difficult to achieve in practice since - * get_online_cpus() not an api which is called all that often. - * - */ void cpu_hotplug_begin(void) { - DEFINE_WAIT(wait); - - cpu_hotplug.active_writer = current; - cpuhp_lock_acquire(); - - for (;;) { - mutex_lock(&cpu_hotplug.lock); - prepare_to_wait(&cpu_hotplug.wq, &wait, TASK_UNINTERRUPTIBLE); - if (likely(!atomic_read(&cpu_hotplug.refcount))) - break; - mutex_unlock(&cpu_hotplug.lock); - schedule(); - } - finish_wait(&cpu_hotplug.wq, &wait); + percpu_down_write(&hotplug); + current->cpuhp_ref++; /* allow read-in-write recursion */ } void cpu_hotplug_done(void) { - cpu_hotplug.active_writer = NULL; - mutex_unlock(&cpu_hotplug.lock); - cpuhp_lock_release(); + current->cpuhp_ref--; + percpu_up_write(&hotplug); } /* --- a/kernel/fork.c +++ b/kernel/fork.c @@ -1414,6 +1414,8 @@ static struct task_struct *copy_process( p->sequential_io_avg = 0; #endif + cpu_hotplug_init_task(p); + /* Perform scheduler related setup. Assign this task to a CPU. */ retval = sched_fork(clone_flags, p); if (retval) --- a/lib/Kconfig +++ b/lib/Kconfig @@ -53,6 +53,11 @@ config GENERIC_IO config STMP_DEVICE bool +config PERCPU_RWSEM_HOTPLUG + def_bool y + depends on HOTPLUG_CPU + select PERCPU_RWSEM + config ARCH_USE_CMPXCHG_LOCKREF bool