[PATCH rcu 16/20] srcu: Check for readers at module-exit time

["Paul E. McKenney" <paulmck@kernel.org>]

If a given statically allocated in-module srcu_struct structure was ever
used for updates, srcu_module_going() will invoke cleanup_srcu_struct()
at module-exit time.  This will check for the error case of SRCU readers
persisting past module-exit time.  On the other hand, if this srcu_struct
structure never went through a grace period, srcu_module_going() only
invokes free_percpu(), which would result in strange failures if SRCU
readers persisted past module-exit time.

This commit therefore adds a srcu_readers_active() check to
srcu_module_going(), splatting if readers have persisted and refraining
from invoking free_percpu() in that case.  Better to leak memory than
to suffer silent memory corruption!

[ paulmck: Apply Zhang, Qiang1 feedback on memory leak. ]

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/srcutree.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 169a6513b739..f9dd6ed5503e 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -1911,7 +1911,8 @@ static void srcu_module_going(struct module *mod)
 		if (!rcu_seq_state(smp_load_acquire(&ssp->srcu_sup->srcu_gp_seq_needed)) &&
 		    !WARN_ON_ONCE(!ssp->srcu_sup->sda_is_static))
 			cleanup_srcu_struct(ssp);
-		free_percpu(ssp->sda);
+		if (!WARN_ON(srcu_readers_active(ssp)))
+			free_percpu(ssp->sda);
 	}
 }
 
-- 
2.40.0.rc2