From: Heiko Carstens

Convert stop_machine to a workqueue based approach. Instead of using kernel
threads for stop_machine we now use a an rt workqueue to synchronize all
cpus.
This has the advantage that all needed per cpu threads are already created
when stop_machine gets called. And therefore a call to stop_machine won't
fail anymore. This is needed for s390 which needs a mechanism to synchronize
all cpus without allocating any memory.
As Rusty pointed out free_module() needs a non-failing stop_machine interface
as well.

As a side effect the stop_machine code gets simplified.

Signed-off-by: Heiko Carstens
---
kernel/stop_machine.c | 109 ++++++++++++++++++--------------------------------
1 file changed, 40 insertions(+), 69 deletions(-)

Index: linux-2.6/kernel/stop_machine.c
================================================== =================
--- linux-2.6.orig/kernel/stop_machine.c
+++ linux-2.6/kernel/stop_machine.c
@@ -37,9 +37,13 @@ struct stop_machine_data {
/* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
static unsigned int num_threads;
static atomic_t thread_ack;
-static struct completion finished;
static DEFINE_MUTEX(lock);

+static struct workqueue_struct *stop_machine_wq;
+static struct stop_machine_data active, idle;
+static const cpumask_t *active_cpus;
+static void *stop_machine_work;
+
static void set_state(enum stopmachine_state newstate)
{
/* Reset ack counter. */
@@ -51,21 +55,25 @@ static void set_state(enum stopmachine_s
/* Last one to ack a state moves to the next state. */
static void ack_state(void)
{
- if (atomic_dec_and_test(&thread_ack)) {
- /* If we're the last one to ack the EXIT, we're finished. */
- if (state == STOPMACHINE_EXIT)
- complete(&finished);
- else
- set_state(state + 1);
- }
+ if (atomic_dec_and_test(&thread_ack))
+ set_state(state + 1);
}

-/* This is the actual thread which stops the CPU. It exits by itself rather
- * than waiting for kthread_stop(), because it's easier for hotplug CPU. */
-static int stop_cpu(struct stop_machine_data *smdata)
+/* This is the actual function which stops the CPU. It runs
+ * in the context of a dedicated stopmachine workqueue. */
+static void stop_cpu(struct work_struct *unused)
{
enum stopmachine_state curstate = STOPMACHINE_NONE;
+ struct stop_machine_data *smdata = &idle;
+ int cpu = smp_processor_id();

+ if (!active_cpus) {
+ if (cpu == first_cpu(cpu_online_map))
+ smdata = &active;
+ } else {
+ if (cpu_isset(cpu, *active_cpus))
+ smdata = &active;
+ }
/* Simple state machine */
do {
/* Chill out and ensure we re-read stopmachine_state. */
@@ -90,7 +98,6 @@ static int stop_cpu(struct stop_machine_
} while (curstate != STOPMACHINE_EXIT);

local_irq_enable();
- do_exit(0);
}

/* Callback for CPUs which aren't supposed to do anything. */
@@ -101,78 +108,34 @@ static int chill(void *unused)

int __stop_machine(int (*fn)(void *), void *data, const cpumask_t *cpus)
{
- int i, err;
- struct stop_machine_data active, idle;
- struct task_struct **threads;
+ struct work_struct *sm_work;
+ int i;

+ /* Set up initial state. */
+ mutex_lock(&lock);
+ num_threads = num_online_cpus();
+ active_cpus = cpus;
active.fn = fn;
active.data = data;
active.fnret = 0;
idle.fn = chill;
idle.data = NULL;

- /* This could be too big for stack on large machines. */
- threads = kcalloc(NR_CPUS, sizeof(threads[0]), GFP_KERNEL);
- if (!threads)
- return -ENOMEM;
-
- /* Set up initial state. */
- mutex_lock(&lock);
- init_completion(&finished);
- num_threads = num_online_cpus();
set_state(STOPMACHINE_PREPARE);

- for_each_online_cpu(i) {
- struct stop_machine_data *smdata = &idle;
- struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
-
- if (!cpus) {
- if (i == first_cpu(cpu_online_map))
- smdata = &active;
- } else {
- if (cpu_isset(i, *cpus))
- smdata = &active;
- }
-
- threads[i] = kthread_create((void *)stop_cpu, smdata, "kstop%u",
- i);
- if (IS_ERR(threads[i])) {
- err = PTR_ERR(threads[i]);
- threads[i] = NULL;
- goto kill_threads;
- }
-
- /* Place it onto correct cpu. */
- kthread_bind(threads[i], i);
-
- /* Make it highest prio. */
- if (sched_setscheduler_nocheck(threads[i], SCHED_FIFO, &param))
- BUG();
- }
-
- /* We've created all the threads. Wake them all: hold this CPU so one
+ /* Schedule the stop_cpu work on all cpus: hold this CPU so one
* doesn't hit this CPU until we're ready. */
get_cpu();
- for_each_online_cpu(i)
- wake_up_process(threads[i]);
-
+ for_each_online_cpu(i) {
+ sm_work = percpu_ptr(stop_machine_work, i);
+ INIT_WORK(sm_work, stop_cpu);
+ queue_work_on(i, stop_machine_wq, sm_work);
+ }
/* This will release the thread on our CPU. */
put_cpu();
- wait_for_completion(&finished);
+ flush_workqueue(stop_machine_wq);
mutex_unlock(&lock);
-
- kfree(threads);
-
return active.fnret;
-
-kill_threads:
- for_each_online_cpu(i)
- if (threads[i])
- kthread_stop(threads[i]);
- mutex_unlock(&lock);
-
- kfree(threads);
- return err;
}

int stop_machine(int (*fn)(void *), void *data, const cpumask_t *cpus)
@@ -187,3 +150,11 @@ int stop_machine(int (*fn)(void *), void
return ret;
}
EXPORT_SYMBOL_GPL(stop_machine);
+
+static int __init stop_machine_init(void)
+{
+ stop_machine_wq = create_rt_workqueue("kstop");
+ stop_machine_work = alloc_percpu(struct work_struct);
+ return 0;
+}
+early_initcall(stop_machine_init);

--
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