Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

On Thu, Feb 06, 2014 at 07:41:03PM +0100, Oleg Nesterov wrote:
> On 02/06, Srivatsa S. Bhat wrote:
> >
> > The following method of CPU hotplug callback registration is not safe
> > due to the possibility of an ABBA deadlock involving the cpu_add_remove_lock
> > and the cpu_hotplug.lock.
> 
> Off-topic, but perhaps it also makes sense to add the lockdep annotations
> later, to catch other similar problems. Currently get_online_cpus() acquires
> nothing from lockdep pov.

Well, both get/put_online_cpus() as well as cpu_hotplug_begin/end()
take the cpu_hotplug.lock mutex. So ideally the lockdep annotations of
mutex_lock/unlock() should have worked. If it hasn't, then the
following lockdep annotations to cpu-hotplug locking should do the
trick.

Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
---
 kernel/cpu.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index deff2e6..3d2dd1c 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -19,6 +19,7 @@
 #include <linux/mutex.h>
 #include <linux/gfp.h>
 #include <linux/suspend.h>
+#include <linux/lockdep.h>
 
 #include "smpboot.h"
 
@@ -57,21 +58,34 @@ static struct {
 	 * an ongoing cpu hotplug operation.
 	 */
 	int refcount;
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lockdep_map dep_map;
+#endif
 } cpu_hotplug = {
 	.active_writer = NULL,
 	.lock = __MUTEX_INITIALIZER(cpu_hotplug.lock),
 	.refcount = 0,
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	.dep_map = {.name = "cpu_hotplug.lock" },
+#endif
 };
 
+#define cphp_lock_acquire_read(l, s, t, i) lock_acquire_shared_recursive(l, s, t, NULL, i)
+#define cphp_lock_acquire(l, s, t, i)      lock_acquire_exclusive(l, s, t, NULL, i)
+#define cphp_lock_release(l, n, i)      lock_release(l, n, i)
+
 void get_online_cpus(void)
 {
 	might_sleep();
 	if (cpu_hotplug.active_writer == current)
 		return;
+	cphp_lock_acquire_read(&cpu_hotplug.dep_map, 0, 0, _RET_IP_);
 	mutex_lock(&cpu_hotplug.lock);
 	cpu_hotplug.refcount++;
 	mutex_unlock(&cpu_hotplug.lock);
 
+
 }
 EXPORT_SYMBOL_GPL(get_online_cpus);
 
@@ -79,6 +93,7 @@ void put_online_cpus(void)
 {
 	if (cpu_hotplug.active_writer == current)
 		return;
+
 	mutex_lock(&cpu_hotplug.lock);
 
 	if (WARN_ON(!cpu_hotplug.refcount))
@@ -87,6 +102,7 @@ void put_online_cpus(void)
 	if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
 		wake_up_process(cpu_hotplug.active_writer);
 	mutex_unlock(&cpu_hotplug.lock);
+	cphp_lock_release(&cpu_hotplug.dep_map, 1, _RET_IP_);
 
 }
 EXPORT_SYMBOL_GPL(put_online_cpus);
@@ -117,6 +133,7 @@ void cpu_hotplug_begin(void)
 {
 	cpu_hotplug.active_writer = current;
 
+	cphp_lock_acquire(&cpu_hotplug.dep_map, 0, 0, _RET_IP_);
 	for (;;) {
 		mutex_lock(&cpu_hotplug.lock);
 		if (likely(!cpu_hotplug.refcount))
@@ -131,6 +148,7 @@ void cpu_hotplug_done(void)
 {
 	cpu_hotplug.active_writer = NULL;
 	mutex_unlock(&cpu_hotplug.lock);
+	cphp_lock_release(&cpu_hotplug.dep_map, 1, _RET_IP_);
 }
 
 /*
-- 
1.8.3.1