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>

1 files changed, 12 insertions, 7 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d7a3b13bc94c..133e47223095 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(). */
         }