[PATCH RFC tip/core/rcu 5/5] srcu: Provide polling interfaces for Tree SRCU grace periods

[paulmck]

From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>

There is a need for a polling interface for SRCU grace
periods, so this commit supplies get_state_synchronize_srcu(),
start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
purpose.  The first can be used if future grace periods are inevitable
(perhaps due to a later call_srcu() invocation), the second if future
grace periods might not otherwise happen, and the third to check if a
grace period has elapsed since the corresponding call to either of the
first two.

As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
the return value from either get_state_synchronize_srcu() or
start_poll_synchronize_srcu() must be passed in to a later call to
poll_state_synchronize_srcu().

Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@xxxxxxxxxxxxxx/
Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
[ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/srcutree.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 60 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index d930ece..015d80e 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -810,7 +810,8 @@ static void srcu_leak_callback(struct rcu_head *rhp)
 /*
  * Start an SRCU grace period, and also queue the callback if non-NULL.
  */
-static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rhp, bool do_norm)
+static unsigned long srcu_gp_start_if_needed(struct srcu_struct *ssp,
+					     struct rcu_head *rhp, bool do_norm)
 {
 	unsigned long flags;
 	int idx;
@@ -822,7 +823,8 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
 	idx = srcu_read_lock(ssp);
 	sdp = raw_cpu_ptr(ssp->sda);
 	spin_lock_irqsave_rcu_node(sdp, flags);
-	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
+	if (rhp)
+		rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
 	rcu_segcblist_advance(&sdp->srcu_cblist,
 			      rcu_seq_current(&ssp->srcu_gp_seq));
 	s = rcu_seq_snap(&ssp->srcu_gp_seq);
@@ -841,6 +843,7 @@ static void srcu_gp_start_if_needed(struct srcu_struct *ssp, struct rcu_head *rh
 	else if (needexp)
 		srcu_funnel_exp_start(ssp, sdp->mynode, s);
 	srcu_read_unlock(ssp, idx);
+	return s;
 }
 
 /*
@@ -882,7 +885,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 		return;
 	}
 	rhp->func = func;
-	srcu_gp_start_if_needed(ssp, rhp, do_norm);
+	(void)srcu_gp_start_if_needed(ssp, rhp, do_norm);
 }
 
 /**
@@ -1011,6 +1014,60 @@ void synchronize_srcu(struct srcu_struct *ssp)
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);
 
+/**
+ * get_state_synchronize_srcu - Provide an end-of-grace-period cookie
+ * @ssp: srcu_struct to provide cookie for.
+ *
+ * This function returns a cookie that can be passed to
+ * poll_state_synchronize_srcu(), which will return true if a full grace
+ * period has elapsed in the meantime.  It is the caller's responsibility
+ * to make sure that grace period happens, for example, by invoking
+ * call_srcu() after return from get_state_synchronize_srcu().
+ */
+unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
+{
+	// Any prior manipulation of SRCU-protected data must happen
+        // before the load from ->srcu_gp_seq.
+	smp_mb();
+	return rcu_seq_snap(&ssp->srcu_gp_seq);
+}
+EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
+
+/**
+ * start_poll_synchronize_srcu - Provide cookie and start grace period
+ * @ssp: srcu_struct to provide cookie for.
+ *
+ * This function returns a cookie that can be passed to
+ * poll_state_synchronize_srcu(), which will return true if a full grace
+ * period has elapsed in the meantime.  Unlike get_state_synchronize_srcu(),
+ * this function also ensures that any needed SRCU grace period will be
+ * started.  This convenience does come at a cost in terms of CPU overhead.
+ */
+unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
+{
+	return srcu_gp_start_if_needed(ssp, NULL, true);
+}
+EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
+
+/**
+ * poll_state_synchronize_srcu - Has cookie's grace period ended?
+ * @ssp: srcu_struct to provide cookie for.
+ * @cookie: Return value from get_state_synchronize_srcu() or start_poll_synchronize_srcu().
+ *
+ * This function takes the cookie that was returned from either
+ * get_state_synchronize_srcu() or start_poll_synchronize_srcu(), and
+ * returns @true if an SRCU grace period elapsed since the time that the
+ * cookie was created.
+ */
+bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
+{
+	if (!rcu_seq_done(&ssp->srcu_gp_seq, cookie))
+		return false;
+	smp_mb(); // ^^^
+	return true;
+}
+EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
+
 /*
  * Callback function for srcu_barrier() use.
  */
-- 
2.9.5