The cpu hotplug lock is a rwsem with read-in-write and read-in-read
recursion. Implement it as such.
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
include/linux/cpu.h | 6 +
include/linux/percpu-rwsem.h | 10 ++-
include/linux/sched.h | 4 +
init/main.c | 1
kernel/cpu.c | 133 +++++++++++++------------------------------
kernel/fork.c | 2
lib/Kconfig | 5 +
7 files changed, 66 insertions(+), 95 deletions(-)
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -224,6 +224,9 @@ extern struct bus_type cpu_subsys;
#ifdef CONFIG_HOTPLUG_CPU
/* Stop CPUs going up and down. */
+extern void cpu_hotplug_init(void);
+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);
@@ -242,6 +245,9 @@ int cpu_down(unsigned int cpu);
#else /* CONFIG_HOTPLUG_CPU */
+static inline void cpu_hotplug_init(void) {}
+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/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -20,12 +20,10 @@ extern void __percpu_down_read(struct pe
extern bool __percpu_down_read_trylock(struct percpu_rw_semaphore *);
extern void __percpu_up_read(struct percpu_rw_semaphore *);
-static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
+static inline void _percpu_down_read(struct percpu_rw_semaphore *sem)
{
might_sleep();
- rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
-
preempt_disable();
/*
* We are in an RCU-sched read-side critical section, so the writer
@@ -46,6 +44,12 @@ static inline void percpu_down_read(stru
*/
}
+static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
+{
+ rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
+ _percpu_down_read(sem);
+}
+
static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
bool ret = true;
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1389,6 +1389,10 @@ struct task_struct {
unsigned int btrace_seq;
#endif
+#ifdef CONFIG_HOTPLUG_CPU
+ int cpuhp_ref;
+#endif
+
unsigned int policy;
int nr_cpus_allowed;
cpumask_t cpus_allowed;
--- a/init/main.c
+++ b/init/main.c
@@ -588,6 +588,7 @@ asmlinkage __visible void __init start_k
sched_clock_postinit();
perf_event_init();
profile_init();
+ cpu_hotplug_init();
call_function_init();
WARN(!irqs_disabled(), "Interrupts were enabled early\n");
early_boot_irqs_disabled = false;
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -22,6 +22,7 @@
#include <linux/lockdep.h>
#include <linux/tick.h>
#include <trace/events/power.h>
+#include <linux/percpu-rwsem.h>
#include "smpboot.h"
@@ -50,7 +51,8 @@ EXPORT_SYMBOL(cpu_notifier_register_done
static RAW_NOTIFIER_HEAD(cpu_chain);
-/* If set, cpu_up and cpu_down will return -EBUSY and do nothing.
+/*
+ * If set, cpu_up and cpu_down will return -EBUSY and do nothing.
* Should always be manipulated under cpu_add_remove_lock
*/
static int cpu_hotplug_disabled;
@@ -58,126 +60,72 @@ static int cpu_hotplug_disabled;
#ifdef CONFIG_HOTPLUG_CPU
static struct {
- struct task_struct *active_writer;
- /* wait queue to wake up the active_writer */
- wait_queue_head_t wq;
- /* verifies that no writer will get active while readers are active */
- struct mutex lock;
- /*
- * Also blocks the new readers during
- * an ongoing cpu hotplug operation.
- */
- atomic_t refcount;
-
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
- struct lockdep_map dep_map;
-#endif
-} cpu_hotplug = {
- .active_writer = NULL,
- .wq = __WAIT_QUEUE_HEAD_INITIALIZER(cpu_hotplug.wq),
- .lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
-#ifdef CONFIG_DEBUG_LOCK_ALLOC
- .dep_map = {.name = "cpu_hotplug.lock" },
-#endif
-};
-
-/* Lockdep annotations for get/put_online_cpus() and cpu_hotplug_begin/end() */
-#define cpuhp_lock_acquire_read() lock_map_acquire_read(&cpu_hotplug.dep_map)
-#define cpuhp_lock_acquire_tryread() \
- lock_map_acquire_tryread(&cpu_hotplug.dep_map)
-#define cpuhp_lock_acquire() lock_map_acquire(&cpu_hotplug.dep_map)
-#define cpuhp_lock_release() lock_map_release(&cpu_hotplug.dep_map)
+ struct percpu_rw_semaphore rwsem;
+ struct task_struct *writer;
+} cpu_hotplug = { .writer = &init_task, };
+
+void cpu_hotplug_init(void)
+{
+ percpu_init_rwsem(&cpu_hotplug.rwsem);
+ cpu_hotplug.writer = NULL;
+}
+void cpu_hotplug_init_task(struct task_struct *p)
+{
+ p->cpuhp_ref = 0;
+}
void get_online_cpus(void)
{
might_sleep();
- if (cpu_hotplug.active_writer == current)
+
+ /* read in write recursion */
+ if (cpu_hotplug.writer == current)
+ return;
+
+ /* read in read recursion */
+ if (current->cpuhp_ref++)
return;
- cpuhp_lock_acquire_read();
- mutex_lock(&cpu_hotplug.lock);
- atomic_inc(&cpu_hotplug.refcount);
- mutex_unlock(&cpu_hotplug.lock);
+
+ lock_map_acquire_read(&cpu_hotplug.rwsem.rw_sem.dep_map);
+ _percpu_down_read(&cpu_hotplug.rwsem);
}
EXPORT_SYMBOL_GPL(get_online_cpus);
bool try_get_online_cpus(void)
{
- if (cpu_hotplug.active_writer == current)
+ if (cpu_hotplug.writer == current)
return true;
- if (!mutex_trylock(&cpu_hotplug.lock))
- return false;
- cpuhp_lock_acquire_tryread();
- atomic_inc(&cpu_hotplug.refcount);
- mutex_unlock(&cpu_hotplug.lock);
- return true;
+
+ if (current->cpuhp_ref++)
+ return true;
+
+ return percpu_down_read_trylock(&cpu_hotplug.rwsem);
}
EXPORT_SYMBOL_GPL(try_get_online_cpus);
void put_online_cpus(void)
{
- int refcount;
-
- if (cpu_hotplug.active_writer == current)
+ if (cpu_hotplug.writer == current)
return;
- refcount = atomic_dec_return(&cpu_hotplug.refcount);
- if (WARN_ON(refcount < 0)) /* try to fix things up */
- atomic_inc(&cpu_hotplug.refcount);
-
- if (refcount <= 0 && waitqueue_active(&cpu_hotplug.wq))
- wake_up(&cpu_hotplug.wq);
-
- cpuhp_lock_release();
+ if (--current->cpuhp_ref)
+ return;
+ percpu_up_read(&cpu_hotplug.rwsem);
}
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)
{
- DEFINE_WAIT(wait);
-
- cpu_hotplug.active_writer = current;
- cpuhp_lock_acquire();
-
- for (;;) {
- mutex_lock(&cpu_hotplug.lock);
- prepare_to_wait(&cpu_hotplug.wq, &wait, TASK_UNINTERRUPTIBLE);
- if (likely(!atomic_read(&cpu_hotplug.refcount)))
- break;
- mutex_unlock(&cpu_hotplug.lock);
- schedule();
- }
- finish_wait(&cpu_hotplug.wq, &wait);
+ percpu_down_write(&cpu_hotplug.rwsem);
+ cpu_hotplug.writer = current;
}
void cpu_hotplug_done(void)
{
- cpu_hotplug.active_writer = NULL;
- mutex_unlock(&cpu_hotplug.lock);
- cpuhp_lock_release();
+ cpu_hotplug.writer = NULL;
+ percpu_up_write(&cpu_hotplug.rwsem);
}
/*
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1410,6 +1410,8 @@ static struct task_struct *copy_process(
p->sequential_io_avg = 0;
#endif
+ cpu_hotplug_init_task(p);
+
/* Perform scheduler related setup. Assign this task to a CPU. */
retval = sched_fork(clone_flags, p);
if (retval)
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -56,6 +56,11 @@ config STMP_DEVICE
config PERCPU_RWSEM
bool
+config PERCPU_RWSEM_HOTPLUG
+ def_bool y
+ depends on HOTPLUG_CPU
+ select PERCPU_RWSEM
+
config ARCH_USE_CMPXCHG_LOCKREF
bool
--
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