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Subject[PATCH] generic-ipi: make struct call_function_data lockless
This patch can remove spinlock from struct call_function_data, the 
reasons are below:

1: add a new interface for cpumask named cpumask_test_and_clear_cpu(),
it can atomically test and clear specific cpu, we can use it instead
of cpumask_test_cpu() and cpumask_clear_cpu() and no need data->lock
to protect those in generic_smp_call_function_interrupt().

2: in smp_call_function_many(), after csd_lock() return, the current's
cfd_data is deleted from call_function list, so it not have race
between other cpus, then cfs_data is only used in
smp_call_function_many() that must disable preemption and not from
a hardware interrupthandler or from a bottom half handler to call,
only the correspond cpu can use it, so it not have race in current
cpu, no need cfs_data->lock to protect it.

3: after 1 and 2, cfs_data->lock is only use to protect cfs_data->refs in
generic_smp_call_function_interrupt(), so we can define cfs_data->refs
to atomic_t, and no need cfs_data->lock any more.

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
---
include/linux/cpumask.h | 12 ++++++++++++
kernel/smp.c | 29 ++++++++---------------------
2 files changed, 20 insertions(+), 21 deletions(-)
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index c5ac87c..a370bd5 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -715,6 +715,18 @@ static inline int cpumask_test_and_set_cpu(int cpu, struct cpumask *cpumask)
}

/**
+ * cpumask_test_and_clear_cpu - atomically test and clear a cpu in a cpumask
+ * @cpu: cpu number (< nr_cpu_ids)
+ * @cpumask: the cpumask pointer
+ *
+ * test_and_clear_bit wrapper for cpumasks.
+ */
+static inline int cpumask_test_and_clear_cpu(int cpu, struct cpumask *cpumask)
+{
+ return test_and_clear_bit(cpumask_check(cpu), cpumask_bits(cpumask));
+}
+
+/**
* cpumask_setall - set all cpus (< nr_cpu_ids) in a cpumask
* @dstp: the cpumask pointer
*/
diff --git a/kernel/smp.c b/kernel/smp.c
index ad63d85..7016996 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -29,8 +29,7 @@ enum {

struct call_function_data {
struct call_single_data csd;
- spinlock_t lock;
- unsigned int refs;
+ atomic_t refs;
cpumask_var_t cpumask;
};

@@ -39,9 +38,7 @@ struct call_single_queue {
spinlock_t lock;
};

-static DEFINE_PER_CPU(struct call_function_data, cfd_data) = {
- .lock = __SPIN_LOCK_UNLOCKED(cfd_data.lock),
-};
+static DEFINE_PER_CPU(struct call_function_data, cfd_data);

static int
hotplug_cfd(struct notifier_block *nfb, unsigned long action, void *hcpu)
@@ -191,25 +188,18 @@ void generic_smp_call_function_interrupt(void)
list_for_each_entry_rcu(data, &call_function.queue, csd.list) {
int refs;

- spin_lock(&data->lock);
- if (!cpumask_test_cpu(cpu, data->cpumask)) {
- spin_unlock(&data->lock);
+ if (!cpumask_test_and_clear_cpu(cpu, data->cpumask))
continue;
- }
- cpumask_clear_cpu(cpu, data->cpumask);
- spin_unlock(&data->lock);

data->csd.func(data->csd.info);

- spin_lock(&data->lock);
- WARN_ON(data->refs == 0);
- refs = --data->refs;
+ refs = atomic_sub_return(1, &data->refs);
+ WARN_ON(refs < 0);
if (!refs) {
spin_lock(&call_function.lock);
list_del_rcu(&data->csd.list);
spin_unlock(&call_function.lock);
}
- spin_unlock(&data->lock);

if (refs)
continue;
@@ -391,23 +381,20 @@ void smp_call_function_many(const struct cpumask *mask,
data = &__get_cpu_var(cfd_data);
csd_lock(&data->csd);

- spin_lock_irqsave(&data->lock, flags);
data->csd.func = func;
data->csd.info = info;
cpumask_and(data->cpumask, mask, cpu_online_mask);
cpumask_clear_cpu(this_cpu, data->cpumask);
- data->refs = cpumask_weight(data->cpumask);
+ atomic_set(&data->refs, cpumask_weight(data->cpumask));

- spin_lock(&call_function.lock);
+ spin_lock_irqsave(&call_function.lock, flags);
/*
* Place entry at the _HEAD_ of the list, so that any cpu still
* observing the entry in generic_smp_call_function_interrupt()
* will not miss any other list entries:
*/
list_add_rcu(&data->csd.list, &call_function.queue);
- spin_unlock(&call_function.lock);
-
- spin_unlock_irqrestore(&data->lock, flags);
+ spin_unlock_irqrestore(&call_function.lock, flags);

/*
* Make the list addition visible before sending the ipi.
--
1.6.1.2


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