srcu: Create concurrency-safe helper for initiating size transition

Once there are contention-initiated size transitions, it will be
possible for rcutorture to initiate a transition at the same time
as a contention-initiated transition.  This commit therefore creates
a concurrency-safe helper function named srcu_transition_to_big() to
safely initiate size transitions.

Co-developed-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 64993a172cfff547f1e646bba848720690f1ca1c..c9460374d4371f1fef77b2fda2e14c0068d716b3 100644 (file)
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -272,6 +272,25 @@ EXPORT_SYMBOL_GPL(init_srcu_struct);
 
 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
 
+/*
+ * Initiate an idempotent transition to SRCU_SIZE_BIG.
+ */
+static void srcu_transition_to_big(struct srcu_struct *ssp)
+{
+       unsigned long flags;
+
+       /* Double-checked locking on ->srcu_size-state. */
+       if (smp_load_acquire(&ssp->srcu_size_state) != SRCU_SIZE_SMALL)
+               return;
+       spin_lock_irqsave_rcu_node(ssp, flags);
+       if (smp_load_acquire(&ssp->srcu_size_state) != SRCU_SIZE_SMALL) {
+               spin_unlock_irqrestore_rcu_node(ssp, flags);
+               return;
+       }
+       smp_store_release(&ssp->srcu_size_state, SRCU_SIZE_ALLOC);
+       spin_unlock_irqrestore_rcu_node(ssp, flags);
+}
+
 /*
  * First-use initialization of statically allocated srcu_struct
  * structure.  Wiring up the combining tree is more than can be
@@ -1523,8 +1542,8 @@ void srcu_torture_stats_print(struct srcu_struct *ssp, char *tt, char *tf)
                }
                pr_cont(" T(%ld,%ld)\n", s0, s1);
        }
-       if (READ_ONCE(ssp->srcu_size_state) == SRCU_SIZE_SMALL && convert_to_big == 2)
-               WRITE_ONCE(ssp->srcu_size_state, SRCU_SIZE_ALLOC);
+       if (convert_to_big == 2)
+               srcu_transition_to_big(ssp);
 }
 EXPORT_SYMBOL_GPL(srcu_torture_stats_print);