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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-08-25 20:25:06 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2014-09-18 16:22:27 -0700
commitdd56af42bd829c6e770ed69812bd65a04eaeb1e4 (patch)
tree6d510ac693c3dfb05dcc383cb60acd3e1e8cab6c /kernel/rcu
parentec4518aad8329364af373f4bf7f4eff25a01a339 (diff)
downloadop-kernel-dev-dd56af42bd829c6e770ed69812bd65a04eaeb1e4.zip
op-kernel-dev-dd56af42bd829c6e770ed69812bd65a04eaeb1e4.tar.gz
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, as can be seen here: https://lkml.org/lkml/2014/8/5/754. The problem in this case is that some developers need to acquire a mutex from a CPU-hotplug notifier, but also need to hold it across a synchronize_rcu_expedited(). As noted above, this currently results in deadlock. 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@linux.vnet.ibm.com> Cc: Josh Triplett <josh@joshtriplett.org> Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net> Tested-by: Lan Tianyu <tianyu.lan@intel.com>
Diffstat (limited to 'kernel/rcu')
-rw-r--r--kernel/rcu/tree.c19
-rw-r--r--kernel/rcu/tree_plugin.h11
2 files changed, 17 insertions, 13 deletions
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d7a3b13..133e472 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()));
/*
@@ -3041,7 +3041,12 @@ void synchronize_sched_expedited(void)
* and they started after our first try, so their grace
* period works for us.
*/
- get_online_cpus();
+ if (!try_get_online_cpus()) {
+ /* CPU hotplug operation in flight, use normal GP. */
+ wait_rcu_gp(call_rcu_sched);
+ atomic_long_inc(&rsp->expedited_normal);
+ return;
+ }
snap = atomic_long_read(&rsp->expedited_start);
smp_mb(); /* ensure read is before try_stop_cpus(). */
}
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e2c5910..387dd45 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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