Re: [PATCH 1/2] workqueues: make get_online_cpus() useable for work->func()

[Gautham R Shenoy <ego@in.ibm.com>]

On Sun, Jun 29, 2008 at 08:51:31PM +0400, Oleg Nesterov wrote:
> workqueue_cpu_callback(CPU_DEAD) flushes cwq->thread under
> cpu_maps_update_begin(). This means that the multithreaded workqueues can't
> use get_online_cpus() due to the possible deadlock, very bad and very old
> problem.
> 
> Introduce the new state, CPU_POST_DEAD, which is called after
> cpu_hotplug_done() but before cpu_maps_update_done().
> 
> Change workqueue_cpu_callback() to use CPU_POST_DEAD instead of CPU_DEAD.
> This means that create/destroy functions can't rely on get_online_cpus()
> any longer and should take cpu_add_remove_lock instead.
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
That should simplify a lot of subsystems.
Thanks Oleg!

Acked-by: Gautham R Shenoy <ego@in.ibm.com>

> 
>  include/linux/notifier.h |    2 ++
>  kernel/cpu.c             |    5 +++++
>  kernel/workqueue.c       |   18 +++++++++---------
>  3 files changed, 16 insertions(+), 9 deletions(-)
> 
> --- 26-rc2/include/linux/notifier.h~WQ_4_GET_ONLINE_CPUS	2008-05-18 15:44:16.000000000 +0400
> +++ 26-rc2/include/linux/notifier.h	2008-05-18 15:44:16.000000000 +0400
> @@ -213,6 +213,8 @@ static inline int notifier_to_errno(int 
>  #define CPU_DEAD		0x0007 /* CPU (unsigned)v dead */
>  #define CPU_DYING		0x0008 /* CPU (unsigned)v not running any task,
>  				        * not handling interrupts, soon dead */
> +#define CPU_POST_DEAD		0x0009 /* CPU (unsigned)v dead, cpu_hotplug
> +					* lock is dropped */
> 
>  /* Used for CPU hotplug events occuring while tasks are frozen due to a suspend
>   * operation in progress
> --- 26-rc2/kernel/cpu.c~WQ_4_GET_ONLINE_CPUS	2008-05-18 15:44:18.000000000 +0400
> +++ 26-rc2/kernel/cpu.c	2008-06-29 20:03:19.000000000 +0400
> @@ -261,6 +261,11 @@ out_allowed:
>  	set_cpus_allowed_ptr(current, &old_allowed);
>  out_release:
>  	cpu_hotplug_done();
> +	if (!err) {
> +		if (raw_notifier_call_chain(&cpu_chain, CPU_POST_DEAD | mod,
> +					    hcpu) == NOTIFY_BAD)
> +			BUG();
> +	}
>  	return err;
>  }
> 
> --- 26-rc2/kernel/workqueue.c~WQ_4_GET_ONLINE_CPUS	2008-06-29 18:34:06.000000000 +0400
> +++ 26-rc2/kernel/workqueue.c	2008-06-29 19:36:27.000000000 +0400
> @@ -791,7 +791,7 @@ struct workqueue_struct *__create_workqu
>  		err = create_workqueue_thread(cwq, singlethread_cpu);
>  		start_workqueue_thread(cwq, -1);
>  	} else {
> -		get_online_cpus();
> +		cpu_maps_update_begin();
>  		spin_lock(&workqueue_lock);
>  		list_add(&wq->list, &workqueues);
>  		spin_unlock(&workqueue_lock);
> @@ -803,7 +803,7 @@ struct workqueue_struct *__create_workqu
>  			err = create_workqueue_thread(cwq, cpu);
>  			start_workqueue_thread(cwq, cpu);
>  		}
> -		put_online_cpus();
> +		cpu_maps_update_done();
>  	}
> 
>  	if (err) {
> @@ -817,8 +817,8 @@ EXPORT_SYMBOL_GPL(__create_workqueue_key
>  static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq)
>  {
>  	/*
> -	 * Our caller is either destroy_workqueue() or CPU_DEAD,
> -	 * get_online_cpus() protects cwq->thread.
> +	 * Our caller is either destroy_workqueue() or CPU_POST_DEAD,
> +	 * cpu_add_remove_lock protects cwq->thread.
>  	 */
>  	if (cwq->thread == NULL)
>  		return;
> @@ -828,7 +828,7 @@ static void cleanup_workqueue_thread(str
> 
>  	flush_cpu_workqueue(cwq);
>  	/*
> -	 * If the caller is CPU_DEAD and cwq->worklist was not empty,
> +	 * If the caller is CPU_POST_DEAD and cwq->worklist was not empty,
>  	 * a concurrent flush_workqueue() can insert a barrier after us.
>  	 * However, in that case run_workqueue() won't return and check
>  	 * kthread_should_stop() until it flushes all work_struct's.
> @@ -852,14 +852,14 @@ void destroy_workqueue(struct workqueue_
>  	const cpumask_t *cpu_map = wq_cpu_map(wq);
>  	int cpu;
> 
> -	get_online_cpus();
> +	cpu_maps_update_begin();
>  	spin_lock(&workqueue_lock);
>  	list_del(&wq->list);
>  	spin_unlock(&workqueue_lock);
> 
>  	for_each_cpu_mask(cpu, *cpu_map)
>  		cleanup_workqueue_thread(per_cpu_ptr(wq->cpu_wq, cpu));
> -	put_online_cpus();
> +	cpu_maps_update_done();
> 
>  	free_percpu(wq->cpu_wq);
>  	kfree(wq);
> @@ -898,7 +898,7 @@ static int __devinit workqueue_cpu_callb
> 
>  		case CPU_UP_CANCELED:
>  			start_workqueue_thread(cwq, -1);
> -		case CPU_DEAD:
> +		case CPU_POST_DEAD:
>  			cleanup_workqueue_thread(cwq);
>  			break;
>  		}
> @@ -906,7 +906,7 @@ static int __devinit workqueue_cpu_callb
> 
>  	switch (action) {
>  	case CPU_UP_CANCELED:
> -	case CPU_DEAD:
> +	case CPU_POST_DEAD:
>  		cpu_clear(cpu, cpu_populated_map);
>  	}
> 
> 

-- 
Thanks and Regards
gautham