How about the pseudo-code below?
workqueue_mutex is only used to protect "struct list_head workqueues",
all workqueue operations can run in parallel with cpuhotplug callback path.
take_over_work(), migrate_sequence, CPU_LOCK_ACQUIRE/RELEASE go away.
I'd like to make a couple of cleanups (and fix schedule_on_each_cpu) before
sending the patch, but if somebody doesn't like this intrusive change, he
can nack it right now.
Oleg.
struct cpu_workqueue_srtuct {
...
int should_stop;
...
};
// also used by flush_work/flush_workqueue
static cpumask_t cpu_populated_map __read_mostly;
/*
* NOTE: the caller must not touch *cwq if this func returns true
*/
static inline int cwq_should_stop(struct cpu_workqueue_struct *cwq)
{
int should_stop = cwq->should_stop;
if (unlikely(should_stop)) {
spin_lock_irq(&cwq->lock);
should_stop = cwq->should_stop && list_empty(&cwq->worklist);
if (should_stop)
cwq->thread = NULL;
spin_unlock_irq(&cwq->lock);
}
return should_stop;
}
static int worker_thread(void *cwq)
{
while (!cwq_should_stop(cwq)) {
...
run_workqueue();
...
}
}
static int create_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
{
struct task_struct *p;
spin_lock_irq(&cwq->lock);
cwq->should_stop = 0;
p = cwq->thread;
spin_unlock_irq(&cwq->lock);
if (!p) {
struct workqueue_struct *wq = cwq->wq;
const char *fmt = is_single_threaded(wq) ? "%s" : "%s/%d";
p = kthread_create(worker_thread, cwq, fmt, wq->name, cpu);
/*
* Nobody can add the work_struct to this cwq,
* if (caller is __create_workqueue)
* nobody should see this wq
* else // caller is CPU_UP_PREPARE
* cpu is not on cpu_online_map
* so we can abort safely.
*/
if (IS_ERR(p))
return PTR_ERR(p);
if (!is_single_threaded(wq))
kthread_bind(p, cpu);
/*
* Cancels affinity if the caller is CPU_UP_PREPARE.
* Needs a cleanup, but OK.
*/
wake_up_process(p);
cwq->thread = p;
}
return 0;
}
struct workqueue_struct *__create_workqueue(const char *name,
int singlethread, int freezeable)
{
struct workqueue_struct *wq;
struct cpu_workqueue_struct *cwq;
int err = 0, cpu;
wq = kzalloc(sizeof(*wq), GFP_KERNEL);
if (!wq)
return NULL;
wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct);
if (!wq->cpu_wq) {
kfree(wq);
return NULL;
}
wq->name = name;
wq->freezeable = freezeable;
if (singlethread) {
INIT_LIST_HEAD(&wq->list);
cwq = init_cpu_workqueue(wq, singlethread_cpu);
err = create_workqueue_thread(cwq, singlethread_cpu);
} else {
mutex_lock(&workqueue_mutex);
list_add(&wq->list, &workqueues);
for_each_possible_cpu(cpu) {
cwq = init_cpu_workqueue(wq, cpu);
if (err || !cpu_isset(cpu, cpu_populated_map))
continue;
err = create_workqueue_thread(cwq, cpu);
}
mutex_unlock(&workqueue_mutex);
}
if (err) {
destroy_workqueue(wq);
wq = NULL;
}
return wq;
}
static void cleanup_workqueue_thread(struct workqueue_struct *wq, int cpu)
{
struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
struct wq_barrier barr;
int alive = 0;
spin_lock_irq(&cwq->lock);
if (cwq->thread != NULL) {
insert_wq_barrier(cwq, &barr, 1);
cwq->should_stop = 1;
alive = 1;
}
spin_unlock_irq(&cwq->lock);
if (alive) {
wait_for_completion(&barr.done);
while (unlikely(cwq->thread != NULL))
cpu_relax();
/*
* Wait until cwq->thread unlocks cwq->lock,
* it won't touch *cwq after that.
*/
smp_rmb();
spin_unlock_wait(&cwq->lock);
}
}
void destroy_workqueue(struct workqueue_struct *wq)
{
if (is_single_threaded(wq))
cleanup_workqueue_thread(wq, singlethread_cpu);
else {
int cpu;
mutex_lock(&workqueue_mutex);
list_del(&wq->list);
mutex_unlock(&workqueue_mutex);
for_each_cpu_mask(cpu, cpu_populated_map)
cleanup_workqueue_thread(wq, cpu);
}
free_percpu(wq->cpu_wq);
kfree(wq);
}
static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
unsigned long action,
void *hcpu)
{
struct workqueue_struct *wq;
struct cpu_workqueue_struct *cwq;
unsigned int cpu = (unsigned long)hcpu;
int ret = NOTIFY_OK;
mutex_lock(&workqueue_mutex);
if (action == CPU_UP_PREPARE)
cpu_set(cpu, cpu_populated_map);
list_for_each_entry(wq, &workqueues, list) {
cwq = per_cpu_ptr(wq->cpu_wq, cpu);
switch (action) {
case CPU_UP_PREPARE:
if (create_workqueue_thread(cwq, cpu))
ret = NOTIFY_BAD;
break;
case CPU_ONLINE:
set_cpus_allowed(cwq->thread, cpumask_of_cpu(cpu));
break;
case CPU_UP_CANCELED:
case CPU_DEAD:
cwq->should_stop = 1;
wake_up(&cwq->more_work);
break;
}
if (ret != NOTIFY_OK) {
printk(KERN_ERR "workqueue for %i failed\n", cpu);
break;
}
}
mutex_unlock(&workqueue_mutex);
return ret;
}
void init_workqueues(void)
{
...
cpu_populated_map = cpu_online_map;
...
}
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