srcutree: Explicitly note DEFINE_SRCU() needs for srcu_barrier()

In the core kernel, an srcu_struct structure created by DEFINE_SRCU()
or friends lives as long as the kernel does, so there are no particular
requirements surrounding the end of that structure's life.  In contrast,
when DEFINE_SRCU() and friends are used within a module, the corresponding
srcu_struct structures' lifetimes end when that module exits.  This in
turn means that if such a structure was passed to call_srcu(), then
srcu_barrier() must be invoked after the last call_srcu() invocation
but before the module exits.

This commit therefore adds a comment stating this.

Reported-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
index ad9d9658b0a2..93b76e543894 100644
--- a/include/linux/srcutree.h
+++ b/include/linux/srcutree.h
@@ -214,6 +214,12 @@ struct srcu_struct {
  * instead of smp_mb(), and given that the first (for example)
  * srcu_read_lock_fast() might race with the first synchronize_srcu(),
  * this different must be specified at initialization time.
+ *
+ * If you use any of the DEFINE_SRCU() functions within a module, the
+ * module-entry code will invoke init_srcu_struct() and the module-exit
+ * code will invoke cleanup_srcu_struct().  This means that if your module
+ * passes the resulting srcu_struct structure to call_srcu(), you will
+ * need to also pass this structure to srcu_barrier() prior to module exit.
  */
 #ifdef MODULE
 # define __DEFINE_SRCU(name, fast, is_static)							\