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>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jens Axboe <jens.axboe@oracle.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
[akpm@linux-foundation.org: use atomic_dec_return()]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 796df12..9b1d458 100644 (file)
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -714,6 +714,18 @@ static inline int cpumask_test_and_set_cpu(int cpu, struct cpumask *cpumask)
        return test_and_set_bit(cpumask_check(cpu), cpumask_bits(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 8e21850..fd47a25 100644 (file)
--- 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)
@@ -196,25 +193,18 @@ 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_dec_return(&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;
@@ -419,23 +409,20 @@ 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.