srcu: Provide internal interface to start a Tree SRCU grace period

mainline inclusion
from mainline-5.10.62
commit fdf66e5a7fc87d3213c21eef4472b71d54fdf736
bugzilla: 182217 https://gitee.com/openeuler/kernel/issues/I4EFOS

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fdf66e5a7fc87d3213c21eef4472b71d54fdf736

--------------------------------

commit 29d2bb94 upstream.

There is a need for a polling interface for SRCU grace periods.
This polling needs to initiate an SRCU grace period without having
to queue (and manage) a callback.  This commit therefore splits the
Tree SRCU __call_srcu() function into callback-initialization and
queuing/start-grace-period portions, with the latter in a new function
named srcu_gp_start_if_needed().  This function may be passed a NULL
callback pointer, in which case it will refrain from queuing anything.

Why have the new function mess with queuing?  Locking considerations,
of course!

Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/Reported-by: NKent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: NNeeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: NPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Acked-by: NWeilong Chen <chenweilong@huawei.com>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

kernel/rcu/srcutree.c

@@ -810,6 +810,42 @@ 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)
+{
+	unsigned long flags;
+	int idx;
+	bool needexp = false;
+	bool needgp = false;
+	unsigned long s;
+	struct srcu_data *sdp;
+
+	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);
+	rcu_segcblist_advance(&sdp->srcu_cblist,
+			      rcu_seq_current(&ssp->srcu_gp_seq));
+	s = rcu_seq_snap(&ssp->srcu_gp_seq);
+	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
+	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
+		sdp->srcu_gp_seq_needed = s;
+		needgp = true;
+	}
+	if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
+		sdp->srcu_gp_seq_needed_exp = s;
+		needexp = true;
+	}
+	spin_unlock_irqrestore_rcu_node(sdp, flags);
+	if (needgp)
+		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
+	else if (needexp)
+		srcu_funnel_exp_start(ssp, sdp->mynode, s);
+	srcu_read_unlock(ssp, idx);
+}
+
 /*
  * Enqueue an SRCU callback on the srcu_data structure associated with
  * the current CPU and the specified srcu_struct structure, initiating
@@ -841,13 +877,6 @@ static void srcu_leak_callback(struct rcu_head *rhp)
 static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 			rcu_callback_t func, bool do_norm)
 {
-	unsigned long flags;
-	int idx;
-	bool needexp = false;
-	bool needgp = false;
-	unsigned long s;
-	struct srcu_data *sdp;
-
 	check_init_srcu_struct(ssp);
 	if (debug_rcu_head_queue(rhp)) {
 		/* Probable double call_srcu(), so leak the callback. */
@@ -856,28 +885,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 		return;
 	}
 	rhp->func = func;
-	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);
-	rcu_segcblist_advance(&sdp->srcu_cblist,
-			      rcu_seq_current(&ssp->srcu_gp_seq));
-	s = rcu_seq_snap(&ssp->srcu_gp_seq);
-	(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
-	if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
-		sdp->srcu_gp_seq_needed = s;
-		needgp = true;
-	}
-	if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
-		sdp->srcu_gp_seq_needed_exp = s;
-		needexp = true;
-	}
-	spin_unlock_irqrestore_rcu_node(sdp, flags);
-	if (needgp)
-		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
-	else if (needexp)
-		srcu_funnel_exp_start(ssp, sdp->mynode, s);
-	srcu_read_unlock(ssp, idx);
+	srcu_gp_start_if_needed(ssp, rhp, do_norm);
 }
 
 /**