[PATCH 01/19] srcutiny: Make a Tiny SRCU grace period imply an RCU grace period

[Paul E. McKenney]

In non-preemptible kernels, a Tiny SRCU grace period implies an RCU
grace period because any context switch suffices.  But in preemptible
kernels, it is possible for a Tiny SRCU grace period to elapse without
a corresponding RCU grace period.

Which was OK until RCU Tasks Trace was re-implemented in terms
of SRCU-fast, which in Tiny SRCU is implemented as SRCU, which is
already fast.  And RCU Tasks Trace grace periods are required to imply
RCU grace periods.  This commit therefore adds a synchronize_rcu(),
but only in preemptible kernels.

Because preemptible Tiny SRCU is not in mainline, this added call to
synchronize_rcu() will not slow anything down:  The comparison would
instead be with TREE SRCU.  But if this added call ever becomes a problem,
the Tiny SRCU srcu_struct structure could track whether or not this is for
SRCU-fast, and to add the synchronize_rcu() only in the SRCU-fast case.
However, at the moment, this is seen as unnecessary complexity.

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

diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 5de9a6905838..32b37d63d58a 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -159,6 +159,8 @@ void srcu_drive_gp(struct work_struct *wp)
 	WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1);
 	WRITE_ONCE(ssp->srcu_gp_waiting, true);  /* srcu_read_unlock() wakes! */
 	preempt_enable();
+	if (IS_ENABLED(CONFIG_PREEMPTION))
+		synchronize_rcu(); // Needed for RCU Tasks Trace to imply RCU grace period
 	do {
 		// Deadlock issues prevent __srcu_read_unlock() from
 		// doing an unconditional wakeup, so polling is required.
-- 
2.40.1