![Tejun Heo](/assets/img/avatar_default.png)
Commit 083b804c4d
("async: use workqueue for worker pool") made it
possible that async jobs are moved from pending to running out-of-order.
While pending async jobs will be queued and dispatched for execution in
the same order, nothing guarantees they'll enter "1) move self to the
running queue" of async_run_entry_fn() in the same order.
Before the conversion, async implemented its own worker pool. An async
worker, upon being woken up, fetches the first item from the pending
list, which kept the executing lists sorted. The conversion to
workqueue was done by adding work_struct to each async_entry and async
just schedules the work item. The queueing and dispatching of such work
items are still in order but now each worker thread is associated with a
specific async_entry and moves that specific async_entry to the
executing list. So, depending on which worker reaches that point
earlier, which is non-deterministic, we may end up moving an async_entry
with larger cookie before one with smaller one.
This broke __lowest_in_progress(). running->domain may not be properly
sorted and is not guaranteed to contain lower cookies than pending list
when not empty. Fix it by ensuring sort-inserting to the running list
and always looking at both pending and running when trying to determine
the lowest cookie.
Over time, the async synchronization implementation became quite messy.
We better restructure it such that each async_entry is linked to two
lists - one global and one per domain - and not move it when execution
starts. There's no reason to distinguish pending and running. They
behave the same for synchronization purposes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
356 lines
11 KiB
C
356 lines
11 KiB
C
/*
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* async.c: Asynchronous function calls for boot performance
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*
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* (C) Copyright 2009 Intel Corporation
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* Author: Arjan van de Ven <arjan@linux.intel.com>
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License
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* as published by the Free Software Foundation; version 2
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* of the License.
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*/
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/*
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Goals and Theory of Operation
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The primary goal of this feature is to reduce the kernel boot time,
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by doing various independent hardware delays and discovery operations
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decoupled and not strictly serialized.
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More specifically, the asynchronous function call concept allows
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certain operations (primarily during system boot) to happen
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asynchronously, out of order, while these operations still
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have their externally visible parts happen sequentially and in-order.
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(not unlike how out-of-order CPUs retire their instructions in order)
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Key to the asynchronous function call implementation is the concept of
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a "sequence cookie" (which, although it has an abstracted type, can be
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thought of as a monotonically incrementing number).
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The async core will assign each scheduled event such a sequence cookie and
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pass this to the called functions.
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The asynchronously called function should before doing a globally visible
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operation, such as registering device numbers, call the
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async_synchronize_cookie() function and pass in its own cookie. The
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async_synchronize_cookie() function will make sure that all asynchronous
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operations that were scheduled prior to the operation corresponding with the
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cookie have completed.
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Subsystem/driver initialization code that scheduled asynchronous probe
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functions, but which shares global resources with other drivers/subsystems
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that do not use the asynchronous call feature, need to do a full
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synchronization with the async_synchronize_full() function, before returning
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from their init function. This is to maintain strict ordering between the
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asynchronous and synchronous parts of the kernel.
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*/
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#include <linux/async.h>
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#include <linux/atomic.h>
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#include <linux/ktime.h>
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#include <linux/export.h>
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#include <linux/wait.h>
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#include <linux/sched.h>
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#include <linux/slab.h>
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#include <linux/workqueue.h>
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static async_cookie_t next_cookie = 1;
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#define MAX_WORK 32768
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static LIST_HEAD(async_pending);
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static ASYNC_DOMAIN(async_running);
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static LIST_HEAD(async_domains);
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static DEFINE_SPINLOCK(async_lock);
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static DEFINE_MUTEX(async_register_mutex);
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struct async_entry {
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struct list_head list;
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struct work_struct work;
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async_cookie_t cookie;
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async_func_ptr *func;
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void *data;
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struct async_domain *running;
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};
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static DECLARE_WAIT_QUEUE_HEAD(async_done);
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static atomic_t entry_count;
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/*
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* MUST be called with the lock held!
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*/
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static async_cookie_t __lowest_in_progress(struct async_domain *running)
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{
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async_cookie_t first_running = next_cookie; /* infinity value */
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async_cookie_t first_pending = next_cookie; /* ditto */
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struct async_entry *entry;
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/*
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* Both running and pending lists are sorted but not disjoint.
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* Take the first cookies from both and return the min.
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*/
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if (!list_empty(&running->domain)) {
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entry = list_first_entry(&running->domain, typeof(*entry), list);
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first_running = entry->cookie;
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}
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list_for_each_entry(entry, &async_pending, list) {
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if (entry->running == running) {
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first_pending = entry->cookie;
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break;
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}
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}
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return min(first_running, first_pending);
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}
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static async_cookie_t lowest_in_progress(struct async_domain *running)
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{
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unsigned long flags;
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async_cookie_t ret;
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spin_lock_irqsave(&async_lock, flags);
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ret = __lowest_in_progress(running);
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spin_unlock_irqrestore(&async_lock, flags);
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return ret;
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}
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/*
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* pick the first pending entry and run it
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*/
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static void async_run_entry_fn(struct work_struct *work)
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{
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struct async_entry *entry =
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container_of(work, struct async_entry, work);
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struct async_entry *pos;
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unsigned long flags;
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ktime_t uninitialized_var(calltime), delta, rettime;
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struct async_domain *running = entry->running;
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/* 1) move self to the running queue, make sure it stays sorted */
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spin_lock_irqsave(&async_lock, flags);
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list_for_each_entry_reverse(pos, &running->domain, list)
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if (entry->cookie < pos->cookie)
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break;
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list_move_tail(&entry->list, &pos->list);
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spin_unlock_irqrestore(&async_lock, flags);
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/* 2) run (and print duration) */
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if (initcall_debug && system_state == SYSTEM_BOOTING) {
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printk(KERN_DEBUG "calling %lli_%pF @ %i\n",
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(long long)entry->cookie,
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entry->func, task_pid_nr(current));
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calltime = ktime_get();
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}
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entry->func(entry->data, entry->cookie);
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if (initcall_debug && system_state == SYSTEM_BOOTING) {
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rettime = ktime_get();
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delta = ktime_sub(rettime, calltime);
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printk(KERN_DEBUG "initcall %lli_%pF returned 0 after %lld usecs\n",
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(long long)entry->cookie,
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entry->func,
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(long long)ktime_to_ns(delta) >> 10);
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}
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/* 3) remove self from the running queue */
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spin_lock_irqsave(&async_lock, flags);
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list_del(&entry->list);
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if (running->registered && --running->count == 0)
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list_del_init(&running->node);
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/* 4) free the entry */
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kfree(entry);
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atomic_dec(&entry_count);
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spin_unlock_irqrestore(&async_lock, flags);
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/* 5) wake up any waiters */
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wake_up(&async_done);
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}
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static async_cookie_t __async_schedule(async_func_ptr *ptr, void *data, struct async_domain *running)
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{
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struct async_entry *entry;
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unsigned long flags;
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async_cookie_t newcookie;
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/* allow irq-off callers */
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entry = kzalloc(sizeof(struct async_entry), GFP_ATOMIC);
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/*
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* If we're out of memory or if there's too much work
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* pending already, we execute synchronously.
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*/
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if (!entry || atomic_read(&entry_count) > MAX_WORK) {
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kfree(entry);
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spin_lock_irqsave(&async_lock, flags);
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newcookie = next_cookie++;
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spin_unlock_irqrestore(&async_lock, flags);
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/* low on memory.. run synchronously */
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ptr(data, newcookie);
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return newcookie;
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}
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INIT_WORK(&entry->work, async_run_entry_fn);
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entry->func = ptr;
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entry->data = data;
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entry->running = running;
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spin_lock_irqsave(&async_lock, flags);
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newcookie = entry->cookie = next_cookie++;
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list_add_tail(&entry->list, &async_pending);
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if (running->registered && running->count++ == 0)
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list_add_tail(&running->node, &async_domains);
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atomic_inc(&entry_count);
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spin_unlock_irqrestore(&async_lock, flags);
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/* mark that this task has queued an async job, used by module init */
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current->flags |= PF_USED_ASYNC;
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/* schedule for execution */
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queue_work(system_unbound_wq, &entry->work);
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return newcookie;
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}
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/**
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* async_schedule - schedule a function for asynchronous execution
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* @ptr: function to execute asynchronously
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* @data: data pointer to pass to the function
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*
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* Returns an async_cookie_t that may be used for checkpointing later.
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* Note: This function may be called from atomic or non-atomic contexts.
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*/
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async_cookie_t async_schedule(async_func_ptr *ptr, void *data)
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{
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return __async_schedule(ptr, data, &async_running);
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}
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EXPORT_SYMBOL_GPL(async_schedule);
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/**
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* async_schedule_domain - schedule a function for asynchronous execution within a certain domain
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* @ptr: function to execute asynchronously
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* @data: data pointer to pass to the function
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* @running: running list for the domain
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*
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* Returns an async_cookie_t that may be used for checkpointing later.
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* @running may be used in the async_synchronize_*_domain() functions
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* to wait within a certain synchronization domain rather than globally.
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* A synchronization domain is specified via the running queue @running to use.
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* Note: This function may be called from atomic or non-atomic contexts.
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*/
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async_cookie_t async_schedule_domain(async_func_ptr *ptr, void *data,
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struct async_domain *running)
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{
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return __async_schedule(ptr, data, running);
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}
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EXPORT_SYMBOL_GPL(async_schedule_domain);
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/**
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* async_synchronize_full - synchronize all asynchronous function calls
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*
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* This function waits until all asynchronous function calls have been done.
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*/
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void async_synchronize_full(void)
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{
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mutex_lock(&async_register_mutex);
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do {
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struct async_domain *domain = NULL;
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spin_lock_irq(&async_lock);
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if (!list_empty(&async_domains))
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domain = list_first_entry(&async_domains, typeof(*domain), node);
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spin_unlock_irq(&async_lock);
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async_synchronize_cookie_domain(next_cookie, domain);
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} while (!list_empty(&async_domains));
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mutex_unlock(&async_register_mutex);
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}
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EXPORT_SYMBOL_GPL(async_synchronize_full);
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/**
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* async_unregister_domain - ensure no more anonymous waiters on this domain
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* @domain: idle domain to flush out of any async_synchronize_full instances
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*
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* async_synchronize_{cookie|full}_domain() are not flushed since callers
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* of these routines should know the lifetime of @domain
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*
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* Prefer ASYNC_DOMAIN_EXCLUSIVE() declarations over flushing
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*/
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void async_unregister_domain(struct async_domain *domain)
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{
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mutex_lock(&async_register_mutex);
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spin_lock_irq(&async_lock);
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WARN_ON(!domain->registered || !list_empty(&domain->node) ||
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!list_empty(&domain->domain));
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domain->registered = 0;
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spin_unlock_irq(&async_lock);
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mutex_unlock(&async_register_mutex);
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}
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EXPORT_SYMBOL_GPL(async_unregister_domain);
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/**
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* async_synchronize_full_domain - synchronize all asynchronous function within a certain domain
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* @domain: running list to synchronize on
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*
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* This function waits until all asynchronous function calls for the
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* synchronization domain specified by the running list @domain have been done.
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*/
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void async_synchronize_full_domain(struct async_domain *domain)
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{
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async_synchronize_cookie_domain(next_cookie, domain);
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}
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EXPORT_SYMBOL_GPL(async_synchronize_full_domain);
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/**
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* async_synchronize_cookie_domain - synchronize asynchronous function calls within a certain domain with cookie checkpointing
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* @cookie: async_cookie_t to use as checkpoint
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* @running: running list to synchronize on
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*
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* This function waits until all asynchronous function calls for the
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* synchronization domain specified by running list @running submitted
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* prior to @cookie have been done.
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*/
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void async_synchronize_cookie_domain(async_cookie_t cookie, struct async_domain *running)
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{
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ktime_t uninitialized_var(starttime), delta, endtime;
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if (!running)
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return;
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if (initcall_debug && system_state == SYSTEM_BOOTING) {
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printk(KERN_DEBUG "async_waiting @ %i\n", task_pid_nr(current));
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starttime = ktime_get();
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}
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wait_event(async_done, lowest_in_progress(running) >= cookie);
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if (initcall_debug && system_state == SYSTEM_BOOTING) {
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endtime = ktime_get();
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delta = ktime_sub(endtime, starttime);
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printk(KERN_DEBUG "async_continuing @ %i after %lli usec\n",
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task_pid_nr(current),
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(long long)ktime_to_ns(delta) >> 10);
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}
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}
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EXPORT_SYMBOL_GPL(async_synchronize_cookie_domain);
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/**
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* async_synchronize_cookie - synchronize asynchronous function calls with cookie checkpointing
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* @cookie: async_cookie_t to use as checkpoint
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*
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* This function waits until all asynchronous function calls prior to @cookie
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* have been done.
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*/
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void async_synchronize_cookie(async_cookie_t cookie)
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{
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async_synchronize_cookie_domain(cookie, &async_running);
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}
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EXPORT_SYMBOL_GPL(async_synchronize_cookie);
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