On Mon, Sep 23, 2013 at 07:32:03PM +0200, Oleg Nesterov wrote:
> > static void cpuhp_wait_refcount(void)
> > {
> > for (;;) {
> > unsigned int rc1, rc2;
> >
> > rc1 = cpuhp_refcount();
> > set_current_state(TASK_UNINTERRUPTIBLE); /* MB */
> > rc2 = cpuhp_refcount();
> >
> > if (rc1 == rc2 && !rc1)
>
> But this only makes the race above "theoretical ** 2". Both
> cpuhp_refcount()'s can be equally fooled.
>
> Looks like, cpuhp_refcount() should take all per-cpu cpuhp_lock's
> before it reads __cpuhp_refcount.
Ah, so SRCU has a solution for this using a sequence count.
So now we drop from a no memory barriers fast path, into a memory
barrier 'slow' path into blocking.
Only once we block do we hit global state..
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -16,6 +16,7 @@
#include <linux/node.h>
#include <linux/compiler.h>
#include <linux/cpumask.h>
+#include <linux/percpu.h>
struct device;
@@ -173,10 +174,50 @@ extern struct bus_type cpu_subsys;
#ifdef CONFIG_HOTPLUG_CPU
/* Stop CPUs going up and down. */
+extern void cpu_hotplug_init_task(struct task_struct *p);
+
extern void cpu_hotplug_begin(void);
extern void cpu_hotplug_done(void);
-extern void get_online_cpus(void);
-extern void put_online_cpus(void);
+
+extern int __cpuhp_writer;
+DECLARE_PER_CPU(unsigned int, __cpuhp_refcount);
+
+extern void __get_online_cpus(void);
+
+static inline void get_online_cpus(void)
+{
+ might_sleep();
+
+ /* Support reader-in-reader recursion */
+ if (current->cpuhp_ref++) {
+ barrier();
+ return;
+ }
+
+ preempt_disable();
+ if (likely(!__cpuhp_writer))
+ __this_cpu_inc(__cpuhp_refcount);
+ else
+ __get_online_cpus();
+ preempt_enable();
+}
+
+extern void __put_online_cpus(void);
+
+static inline void put_online_cpus(void)
+{
+ barrier();
+ if (--current->cpuhp_ref)
+ return;
+
+ preempt_disable();
+ if (likely(!__cpuhp_writer))
+ __this_cpu_dec(__cpuhp_refcount);
+ else
+ __put_online_cpus();
+ preempt_enable();
+}
+
extern void cpu_hotplug_disable(void);
extern void cpu_hotplug_enable(void);
#define hotcpu_notifier(fn, pri) cpu_notifier(fn, pri)
@@ -200,6 +241,8 @@ static inline void cpu_hotplug_driver_un
#else /* CONFIG_HOTPLUG_CPU */
+static inline void cpu_hotplug_init_task(struct task_struct *p) {}
+
static inline void cpu_hotplug_begin(void) {}
static inline void cpu_hotplug_done(void) {}
#define get_online_cpus() do { } while (0)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1454,6 +1454,9 @@ struct task_struct {
unsigned int sequential_io;
unsigned int sequential_io_avg;
#endif
+#ifdef CONFIG_HOTPLUG_CPU
+ int cpuhp_ref;
+#endif
};
/* Future-safe accessor for struct task_struct's cpus_allowed. */
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -49,88 +49,148 @@ static int cpu_hotplug_disabled;
#ifdef CONFIG_HOTPLUG_CPU
-static struct {
- struct task_struct *active_writer;
- struct mutex lock; /* Synchronizes accesses to refcount, */
+int __cpuhp_writer;
+EXPORT_SYMBOL_GPL(__cpuhp_writer);
+
+DEFINE_PER_CPU(unsigned int, __cpuhp_refcount);
+EXPORT_PER_CPU_SYMBOL_GPL(__cpuhp_refcount);
+
+static DEFINE_PER_CPU(unsigned int, cpuhp_seq);
+static atomic_t cpuhp_waitcount;
+static DECLARE_WAIT_QUEUE_HEAD(cpuhp_readers);
+static DECLARE_WAIT_QUEUE_HEAD(cpuhp_writer);
+
+void cpu_hotplug_init_task(struct task_struct *p)
+{
+ p->cpuhp_ref = 0;
+}
+
+void __get_online_cpus(void)
+{
+ if (__cpuhp_writer == 1) {
+ /* See __srcu_read_lock() */
+ __this_cpu_inc(__cpuhp_refcount);
+ smp_mb();
+ __this_cpu_inc(cpuhp_seq);
+ return;
+ }
+
+ atomic_inc(&cpuhp_waitcount);
+
/*
- * Also blocks the new readers during
- * an ongoing cpu hotplug operation.
+ * We either call schedule() in the wait, or we'll fall through
+ * and reschedule on the preempt_enable() in get_online_cpus().
*/
- int refcount;
-} cpu_hotplug = {
- .active_writer = NULL,
- .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
- .refcount = 0,
-};
+ preempt_enable_no_resched();
+ wait_event(cpuhp_readers, !__cpuhp_writer);
+ preempt_disable();
-void get_online_cpus(void)
+ /*
+ * XXX list_empty_careful(&cpuhp_readers.task_list) ?
+ */
+ if (atomic_dec_and_test(&cpuhp_waitcount))
+ wake_up_all(&cpuhp_writer);
+}
+EXPORT_SYMBOL_GPL(__get_online_cpus);
+
+void __put_online_cpus(void)
{
- might_sleep();
- if (cpu_hotplug.active_writer == current)
- return;
- mutex_lock(&cpu_hotplug.lock);
- cpu_hotplug.refcount++;
- mutex_unlock(&cpu_hotplug.lock);
+ /* See __srcu_read_unlock() */
+ smp_mb();
+ this_cpu_dec(__cpuhp_refcount);
+ /* Prod writer to recheck readers_active */
+ wake_up_all(&cpuhp_writer);
}
-EXPORT_SYMBOL_GPL(get_online_cpus);
+EXPORT_SYMBOL_GPL(__put_online_cpus);
-void put_online_cpus(void)
+static unsigned int cpuhp_seq(void)
{
- if (cpu_hotplug.active_writer == current)
- return;
- mutex_lock(&cpu_hotplug.lock);
+ unsigned int seq = 0;
+ int cpu;
- if (WARN_ON(!cpu_hotplug.refcount))
- cpu_hotplug.refcount++; /* try to fix things up */
+ for_each_possible_cpu(cpu)
+ seq += per_cpu(cpuhp_seq, cpu);
- if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
- wake_up_process(cpu_hotplug.active_writer);
- mutex_unlock(&cpu_hotplug.lock);
+ return seq;
+}
+
+static unsigned int cpuhp_refcount(void)
+{
+ unsigned int refcount = 0;
+ int cpu;
+ for_each_possible_cpu(cpu)
+ refcount += per_cpu(__cpuhp_refcount, cpu);
+
+ return refcount;
+}
+
+/*
+ * See srcu_readers_active_idx_check()
+ */
+static bool cpuhp_readers_active_check(void)
+{
+ unsigned int seq = cpuhp_seq();
+
+ smp_mb();
+
+ if (cpuhp_refcount() != 0)
+ return false;
+
+ smp_mb();
+
+ return cpuhp_seq() == seq;
}
-EXPORT_SYMBOL_GPL(put_online_cpus);
/*
* This ensures that the hotplug operation can begin only when the
* refcount goes to zero.
*
- * Note that during a cpu-hotplug operation, the new readers, if any,
- * will be blocked by the cpu_hotplug.lock
- *
* Since cpu_hotplug_begin() is always called after invoking
* cpu_maps_update_begin(), we can be sure that only one writer is active.
- *
- * Note that theoretically, there is a possibility of a livelock:
- * - Refcount goes to zero, last reader wakes up the sleeping
- * writer.
- * - Last reader unlocks the cpu_hotplug.lock.
- * - A new reader arrives at this moment, bumps up the refcount.
- * - The writer acquires the cpu_hotplug.lock finds the refcount
- * non zero and goes to sleep again.
- *
- * However, this is very difficult to achieve in practice since
- * get_online_cpus() not an api which is called all that often.
- *
*/
void cpu_hotplug_begin(void)
{
- cpu_hotplug.active_writer = current;
+ unsigned int count = 0;
+ int cpu;
- for (;;) {
- mutex_lock(&cpu_hotplug.lock);
- if (likely(!cpu_hotplug.refcount))
- break;
- __set_current_state(TASK_UNINTERRUPTIBLE);
- mutex_unlock(&cpu_hotplug.lock);
- schedule();
- }
+ lockdep_assert_held(&cpu_add_remove_lock);
+
+ /* allow reader-in-writer recursion */
+ current->cpuhp_ref++;
+
+ /* make readers take the slow path */
+ __cpuhp_writer = 1;
+
+ /* See percpu_down_write() */
+ synchronize_sched();
+
+ /* make readers block */
+ __cpuhp_writer = 2;
+
+ /* Wait for all readers to go away */
+ wait_event(cpuhp_writer, cpuhp_readers_active_check());
}
void cpu_hotplug_done(void)
{
- cpu_hotplug.active_writer = NULL;
- mutex_unlock(&cpu_hotplug.lock);
+ /* Signal the writer is done, no fast path yet */
+ __cpuhp_writer = 1;
+ wake_up_all(&cpuhp_readers);
+
+ /* See percpu_up_write() */
+ synchronize_sched();
+
+ /* Let em rip */
+ __cpuhp_writer = 0
+ current->cpuhp_ref--;
+
+ /*
+ * Wait for any pending readers to be running. This ensures readers
+ * after writer and avoids writers starving readers.
+ */
+ wait_event(cpuhp_writer, !atomic_read(&cpuhp_waitcount));
}
/*
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1736,6 +1736,8 @@ static void __sched_fork(unsigned long c
INIT_LIST_HEAD(&p->numa_entry);
p->numa_group = NULL;
#endif /* CONFIG_NUMA_BALANCING */
+
+ cpu_hotplug_init_task(p);
}
#ifdef CONFIG_NUMA_BALANCING
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
