Messages in this thread Patch in this message | | | Date | Tue, 09 Jun 2009 20:07:09 +0800 | From | Lai Jiangshan <> | Subject | [PATCH -mm] cpuhotplug: introduce try_get_online_cpus() take 3 |
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It's for -mm tree.
It also works for mainline if you apply this at first: http://lkml.org/lkml/2009/2/17/58
Subject: [PATCH -mm] cpuhotplug: introduce try_get_online_cpus() take 3
get_online_cpus() is a typically coarsely granular lock. It's a source of ABBA deadlock.
Thanks to the CPU notifiers, Some subsystem's global lock will be required after cpu_hotplug.lock. Subsystem's global lock is coarsely granular lock too, thus a lot's of lock in kernel should be required after cpu_hotplug.lock(if we need cpu_hotplug.lock held too)
Otherwise it may come to a ABBA deadlock like this:
thread 1 | thread 2 _cpu_down() | Lock a-kernel-lock. cpu_hotplug_begin() | down_write(&cpu_hotplug.lock) | __raw_notifier_call_chain(CPU_DOWN_PREPARE) | get_online_cpus() ------------------------------------------------------------------------ Lock a-kernel-lock.(wait thread2) | down_read(&cpu_hotplug.lock) (wait thread 1)
But CPU online/offline are happened very rarely, get_online_cpus() returns success quickly in all probability. So it's an asinine behavior that get_online_cpus() is not allowed to be required after we had held "a-kernel-lock".
To dispel the ABBA deadlock, this patch introduces try_get_online_cpus(). It returns fail very rarely. It gives the caller a chance to select an alternative way to finish works, instead of sleeping or deadlock.
Changed from V1 Lockless for get_online_cpus()'s fast path
Changed from V2 Fix patch as Oleg Nesterov valuable suggestions.
Suggested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> --- diff --git a/include/linux/cpu.h b/include/linux/cpu.h index 4d668e0..eeb9ca5 100644 --- a/include/linux/cpu.h +++ b/include/linux/cpu.h @@ -99,6 +99,7 @@ extern struct sysdev_class cpu_sysdev_class; extern void get_online_cpus(void); extern void put_online_cpus(void); +extern int try_get_online_cpus(void); #define hotcpu_notifier(fn, pri) { \ static struct notifier_block fn##_nb __cpuinitdata = \ { .notifier_call = fn, .priority = pri }; \ @@ -112,6 +113,7 @@ int cpu_down(unsigned int cpu); #define get_online_cpus() do { } while (0) #define put_online_cpus() do { } while (0) +static inline int try_get_online_cpus(void) { return 1; } #define hotcpu_notifier(fn, pri) do { (void)(fn); } while (0) /* These aren't inline functions due to a GCC bug. */ #define register_hotcpu_notifier(nb) ({ (void)(nb); 0; }) diff --git a/kernel/cpu.c b/kernel/cpu.c index 609fae1..afecc95 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -14,6 +14,8 @@ #include <linux/kthread.h> #include <linux/stop_machine.h> #include <linux/mutex.h> +#include <asm/atomic.h> +#include <linux/wait.h> #ifdef CONFIG_SMP /* Serializes the updates to cpu_online_mask, cpu_present_mask */ @@ -26,16 +28,23 @@ static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain); */ static int cpu_hotplug_disabled; +/* + * @cpu_hotplug is a special read-write semaphore with these semantics: + * 1) It is read-preference and allows reader-in-reader recursion. + * 2) It allows writer to downgrade to a reader when required. + * (allows reader-in-writer recursion.) + * 3) It allows only one thread to require the write-side lock at most. + * (cpu_add_remove_lock ensures it.) + */ static struct { struct task_struct *active_writer; - struct mutex lock; /* Synchronizes accesses to refcount, */ - /* - * Also blocks the new readers during - * an ongoing cpu hotplug operation. - */ - int refcount; + wait_queue_head_t sleeping_readers; + /* refcount = 0 means the writer owns the lock. */ + atomic_t refcount; } cpu_hotplug = { - .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock), + NULL, + __WAIT_QUEUE_HEAD_INITIALIZER(cpu_hotplug.sleeping_readers), + ATOMIC_INIT(1), }; #ifdef CONFIG_HOTPLUG_CPU @@ -45,10 +54,9 @@ void get_online_cpus(void) might_sleep(); if (cpu_hotplug.active_writer == current) return; - mutex_lock(&cpu_hotplug.lock); - cpu_hotplug.refcount++; - mutex_unlock(&cpu_hotplug.lock); + wait_event(cpu_hotplug.sleeping_readers, + likely(atomic_inc_not_zero(&cpu_hotplug.refcount))); } EXPORT_SYMBOL_GPL(get_online_cpus); @@ -56,14 +64,27 @@ void put_online_cpus(void) { if (cpu_hotplug.active_writer == current) return; - mutex_lock(&cpu_hotplug.lock); - if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer)) - wake_up_process(cpu_hotplug.active_writer); - mutex_unlock(&cpu_hotplug.lock); + if (unlikely(atomic_dec_and_test(&cpu_hotplug.refcount))) { + /* atomic_dec_and_test() implies smp_mb__after_atomic_dec() */ + BUG_ON(!cpu_hotplug.active_writer); + wake_up_process(cpu_hotplug.active_writer); + } } EXPORT_SYMBOL_GPL(put_online_cpus); +int try_get_online_cpus(void) +{ + if (cpu_hotplug.active_writer == current) + return 1; + + if (likely(atomic_inc_not_zero(&cpu_hotplug.refcount))) + return 1; + + return 0; +} +EXPORT_SYMBOL_GPL(try_get_online_cpus); + #endif /* CONFIG_HOTPLUG_CPU */ /* @@ -81,46 +102,42 @@ void cpu_maps_update_done(void) } /* - * This ensures that the hotplug operation can begin only when the - * refcount goes to zero. + * This ensures that the hotplug operation can begin only when + * there is no ongoing reader. * * Note that during a cpu-hotplug operation, the new readers, if any, - * will be blocked by the cpu_hotplug.lock + * will be blocked and queued at cpu_hotplug.sleeping_readers. * * 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. - * */ static void cpu_hotplug_begin(void) { cpu_hotplug.active_writer = current; + /* atomic_dec_and_test() implies smp_mb__before_atomic_dec() */ + if (atomic_dec_and_test(&cpu_hotplug.refcount)) + return; + for (;;) { - mutex_lock(&cpu_hotplug.lock); - if (likely(!cpu_hotplug.refcount)) + set_current_state(TASK_UNINTERRUPTIBLE); + if (!atomic_read(&cpu_hotplug.refcount)) break; - __set_current_state(TASK_UNINTERRUPTIBLE); - mutex_unlock(&cpu_hotplug.lock); schedule(); } + + __set_current_state(TASK_RUNNING); } static void cpu_hotplug_done(void) { + atomic_inc(&cpu_hotplug.refcount); + wake_up_all(&cpu_hotplug.sleeping_readers); + cpu_hotplug.active_writer = NULL; - mutex_unlock(&cpu_hotplug.lock); } + /* Need to know about CPUs going up/down? */ int __ref register_cpu_notifier(struct notifier_block *nb) {
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