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Date: Thu, 28 Aug 2014 12:47:45 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...nel.org, laijs@...fujitsu.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
bobby.prani@...il.com, <""@rjwysocki.net>, tianyu.lan@...el.com
Subject: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and
expedited grace periods
Currently, the expedited grace-period primitives do get_online_cpus().
This greatly simplifies their implementation, but means that calls to
them holding locks that are acquired by CPU-hotplug notifiers (to say
nothing of calls to these primitives from CPU-hotplug notifiers) can
deadlock. But this is starting to become inconvenient:
https://lkml.org/lkml/2014/8/5/754
This commit avoids the deadlock and retains the simplicity by creating
a try_get_online_cpus(), which returns false if the get_online_cpus()
reference count could not immediately be incremented. If a call to
try_get_online_cpus() returns true, the expedited primitives operate
as before. If a call returns false, the expedited primitives fall back
to normal grace-period operations. This falling back of course results
in increased grace-period latency, but only during times when CPU
hotplug operations are actually in flight. The effect should therefore
be negligible during normal operation.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Josh Triplett <josh@...htriplett.org>
Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc: Lan Tianyu <tianyu.lan@...el.com>
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 95978ad7fcdd..b2d9a43012b2 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -213,6 +213,7 @@ extern struct bus_type cpu_subsys;
extern void cpu_hotplug_begin(void);
extern void cpu_hotplug_done(void);
extern void get_online_cpus(void);
+extern bool try_get_online_cpus(void);
extern void put_online_cpus(void);
extern void cpu_hotplug_disable(void);
extern void cpu_hotplug_enable(void);
@@ -230,6 +231,7 @@ int cpu_down(unsigned int cpu);
static inline void cpu_hotplug_begin(void) {}
static inline void cpu_hotplug_done(void) {}
#define get_online_cpus() do { } while (0)
+#define try_get_online_cpus() true
#define put_online_cpus() do { } while (0)
#define cpu_hotplug_disable() do { } while (0)
#define cpu_hotplug_enable() do { } while (0)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index 008388f920d7..4f86465cc317 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -505,6 +505,7 @@ static inline void print_irqtrace_events(struct task_struct *curr)
#define lock_map_acquire(l) lock_acquire_exclusive(l, 0, 0, NULL, _THIS_IP_)
#define lock_map_acquire_read(l) lock_acquire_shared_recursive(l, 0, 0, NULL, _THIS_IP_)
+#define lock_map_acquire_tryread(l) lock_acquire_shared_recursive(l, 0, 1, NULL, _THIS_IP_)
#define lock_map_release(l) lock_release(l, 1, _THIS_IP_)
#ifdef CONFIG_PROVE_LOCKING
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 81e2a388a0f6..356450f09c1f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -79,6 +79,8 @@ static struct {
/* 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)
@@ -91,10 +93,22 @@ void get_online_cpus(void)
mutex_lock(&cpu_hotplug.lock);
cpu_hotplug.refcount++;
mutex_unlock(&cpu_hotplug.lock);
-
}
EXPORT_SYMBOL_GPL(get_online_cpus);
+bool try_get_online_cpus(void)
+{
+ if (cpu_hotplug.active_writer == current)
+ return true;
+ if (!mutex_trylock(&cpu_hotplug.lock))
+ return false;
+ cpuhp_lock_acquire_tryread();
+ cpu_hotplug.refcount++;
+ mutex_unlock(&cpu_hotplug.lock);
+ return true;
+}
+EXPORT_SYMBOL_GPL(try_get_online_cpus);
+
void put_online_cpus(void)
{
if (cpu_hotplug.active_writer == current)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d7a3b13bc94c..04558f0c9d64 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2940,11 +2940,6 @@ static int synchronize_sched_expedited_cpu_stop(void *data)
* restructure your code to batch your updates, and then use a single
* synchronize_sched() instead.
*
- * Note that it is illegal to call this function while holding any lock
- * that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
- * to call this function from a CPU-hotplug notifier. Failing to observe
- * these restriction will result in deadlock.
- *
* This implementation can be thought of as an application of ticket
* locking to RCU, with sync_sched_expedited_started and
* sync_sched_expedited_done taking on the roles of the halves
@@ -2994,7 +2989,12 @@ void synchronize_sched_expedited(void)
*/
snap = atomic_long_inc_return(&rsp->expedited_start);
firstsnap = snap;
- get_online_cpus();
+ if (!try_get_online_cpus()) {
+ /* CPU hotplug operation in flight, fall back to normal GP. */
+ wait_rcu_gp(call_rcu_sched);
+ atomic_long_inc(&rsp->expedited_normal);
+ return;
+ }
WARN_ON_ONCE(cpu_is_offline(raw_smp_processor_id()));
/*
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index fb833811c2f6..821dcf9a3b94 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -793,11 +793,6 @@ sync_rcu_preempt_exp_init(struct rcu_state *rsp, struct rcu_node *rnp)
* In fact, if you are using synchronize_rcu_expedited() in a loop,
* please restructure your code to batch your updates, and then Use a
* single synchronize_rcu() instead.
- *
- * Note that it is illegal to call this function while holding any lock
- * that is acquired by a CPU-hotplug notifier. And yes, it is also illegal
- * to call this function from a CPU-hotplug notifier. Failing to observe
- * these restriction will result in deadlock.
*/
void synchronize_rcu_expedited(void)
{
@@ -819,7 +814,11 @@ void synchronize_rcu_expedited(void)
* being boosted. This simplifies the process of moving tasks
* from leaf to root rcu_node structures.
*/
- get_online_cpus();
+ if (!try_get_online_cpus()) {
+ /* CPU-hotplug operation in flight, fall back to normal GP. */
+ wait_rcu_gp(call_rcu);
+ return;
+ }
/*
* Acquire lock, falling back to synchronize_rcu() if too many
--
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