Re: [PATCH 1/2] cpuhotplug: use rw_semaphore for cpu_hotplug

Paul E. McKenney
2009-05-30T01:53:49Z

On Fri, May 29, 2009 at 04:29:30PM +0800, Lai Jiangshan wrote:
>
> Current get_online_cpus()/put_online_cpus() re-implement
> a rw_semaphore, so it is converted to a real rw_semaphore in this fix.
> It simplifies codes, and is good for read.
>
> And misc fix:
> 1) Add comments for cpu_hotplug.active_writer.
> 2) The theoretical disadvantage described in cpu_hotplug_begin()'s
> comments is no longer existed when we use rw_semaphore,
> so this part of comments was removed.
>
> [Impact: improve get_online_cpus()/put_online_cpus() ]

Actually, it turns out that for my purposes it is only necessary to check:

cpu_hotplug.active_writer != NULL

The only time that it is unsafe to invoke get_online_cpus() is when
in a notifier, and in that case the value of cpu_hotplug.active_writer
is stable. There could be false positives, but these are harmless, as
the fallback is simply synchronize_sched().

Even this is only needed should the deadlock scenario you pointed out
arise in practice.

As Oleg noted, there are some "interesting" constraints on
get_online_cpus(). Adding Gautham Shenoy to CC for his views.

Thanx, Paul

> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> ---
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 395b697..62198ec 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -14,6 +14,7 @@
> #include <linux/kthread.h>
> #include <linux/stop_machine.h>
> #include <linux/mutex.h>
> +#include <linux/rwsem.h>
>
> #ifdef CONFIG_SMP
> /* Serializes the updates to cpu_online_mask, cpu_present_mask */
> @@ -27,20 +28,21 @@ static __cpuinitdata RAW_NOTIFIER_HEAD(cpu_chain);
> static int cpu_hotplug_disabled;
>
> 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.
> + * active_writer makes get_online_cpus()/put_online_cpus() are allowd
> + * to be nested in cpu_hotplug_begin()/cpu_hotplug_done().
> + *
> + * Thus, get_online_cpus()/put_online_cpus() can be called in
> + * CPU notifiers.
> */
> - int refcount;
> + struct task_struct *active_writer;
> + struct rw_semaphore rwlock;
> } cpu_hotplug;
>
> void __init cpu_hotplug_init(void)
> {
> cpu_hotplug.active_writer = NULL;
> - mutex_init(&cpu_hotplug.lock);
> - cpu_hotplug.refcount = 0;
> + init_rwsem(&cpu_hotplug.rwlock);
> }
>
> #ifdef CONFIG_HOTPLUG_CPU
> @@ -50,9 +52,7 @@ 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);
> + down_read(&cpu_hotplug.rwlock);
>
> }
> EXPORT_SYMBOL_GPL(get_online_cpus);
> @@ -61,10 +61,7 @@ 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);
> + up_read(&cpu_hotplug.rwlock);
>
> }
> EXPORT_SYMBOL_GPL(put_online_cpus);
> @@ -86,45 +83,25 @@ 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 reader.
> *
> * Note that during a cpu-hotplug operation, the new readers, if any,
> - * will be blocked by the cpu_hotplug.lock
> + * will be blocked by the cpu_hotplug.rwlock
> *
> * 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)
> {
> + down_write(&cpu_hotplug.rwlock);
> cpu_hotplug.active_writer = current;
> -
> - for (;;) {
> - mutex_lock(&cpu_hotplug.lock);
> - if (likely(!cpu_hotplug.refcount))
> - break;
> - __set_current_state(TASK_UNINTERRUPTIBLE);
> - mutex_unlock(&cpu_hotplug.lock);
> - schedule();
> - }
> }
>
> static void cpu_hotplug_done(void)
> {
> cpu_hotplug.active_writer = NULL;
> - mutex_unlock(&cpu_hotplug.lock);
> + up_write(&cpu_hotplug.rwlock);
> }
> /* Need to know about CPUs going up/down? */
> int __ref register_cpu_notifier(struct notifier_block *nb)
>
>