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Date: Tue, 1 Jul 2008 10:51:25 +0530
From: Gautham R Shenoy <ego@...ibm.com>
To: Oleg Nesterov <oleg@...sign.ru>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Heiko Carstens <heiko.carstens@...ibm.com>,
Max Krasnyansky <maxk@...lcomm.com>, Paul Jackson <pj@....com>,
Paul Menage <menage@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
Vegard Nossum <vegard.nossum@...il.com>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] workqueues: make get_online_cpus() useable for
work->func()
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@...sign.ru>
That should simplify a lot of subsystems.
Thanks Oleg!
Acked-by: Gautham R Shenoy <ego@...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
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