Adding upstream version 6.1.76.upstream/6.1.76

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 633 insertions, 0 deletions

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
new file mode 100644
index 000000000..c54ea2b6a
--- /dev/null
+++ b/kernel/rcu/rcu_segcblist.c
@@ -0,0 +1,633 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * RCU segmented callback lists, function definitions
+ *
+ * Copyright IBM Corporation, 2017
+ *
+ * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
+ */
+
+#include <linux/cpu.h>
+#include <linux/interrupt.h>
+#include <linux/kernel.h>
+#include <linux/types.h>
+
+#include "rcu_segcblist.h"
+
+/* Initialize simple callback list. */
+void rcu_cblist_init(struct rcu_cblist *rclp)
+{
+	rclp->head = NULL;
+	rclp->tail = &rclp->head;
+	rclp->len = 0;
+}
+
+/*
+ * Enqueue an rcu_head structure onto the specified callback list.
+ */
+void rcu_cblist_enqueue(struct rcu_cblist *rclp, struct rcu_head *rhp)
+{
+	*rclp->tail = rhp;
+	rclp->tail = &rhp->next;
+	WRITE_ONCE(rclp->len, rclp->len + 1);
+}
+
+/*
+ * Flush the second rcu_cblist structure onto the first one, obliterating
+ * any contents of the first.  If rhp is non-NULL, enqueue it as the sole
+ * element of the second rcu_cblist structure, but ensuring that the second
+ * rcu_cblist structure, if initially non-empty, always appears non-empty
+ * throughout the process.  If rdp is NULL, the second rcu_cblist structure
+ * is instead initialized to empty.
+ */
+void rcu_cblist_flush_enqueue(struct rcu_cblist *drclp,
+			      struct rcu_cblist *srclp,
+			      struct rcu_head *rhp)
+{
+	drclp->head = srclp->head;
+	if (drclp->head)
+		drclp->tail = srclp->tail;
+	else
+		drclp->tail = &drclp->head;
+	drclp->len = srclp->len;
+	if (!rhp) {
+		rcu_cblist_init(srclp);
+	} else {
+		rhp->next = NULL;
+		srclp->head = rhp;
+		srclp->tail = &rhp->next;
+		WRITE_ONCE(srclp->len, 1);
+	}
+}
+
+/*
+ * Dequeue the oldest rcu_head structure from the specified callback
+ * list.
+ */
+struct rcu_head *rcu_cblist_dequeue(struct rcu_cblist *rclp)
+{
+	struct rcu_head *rhp;
+
+	rhp = rclp->head;
+	if (!rhp)
+		return NULL;
+	rclp->len--;
+	rclp->head = rhp->next;
+	if (!rclp->head)
+		rclp->tail = &rclp->head;
+	return rhp;
+}
+
+/* Set the length of an rcu_segcblist structure. */
+static void rcu_segcblist_set_len(struct rcu_segcblist *rsclp, long v)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+	atomic_long_set(&rsclp->len, v);
+#else
+	WRITE_ONCE(rsclp->len, v);
+#endif
+}
+
+/* Get the length of a segment of the rcu_segcblist structure. */
+static long rcu_segcblist_get_seglen(struct rcu_segcblist *rsclp, int seg)
+{
+	return READ_ONCE(rsclp->seglen[seg]);
+}
+
+/* Return number of callbacks in segmented callback list by summing seglen. */
+long rcu_segcblist_n_segment_cbs(struct rcu_segcblist *rsclp)
+{
+	long len = 0;
+	int i;
+
+	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
+		len += rcu_segcblist_get_seglen(rsclp, i);
+
+	return len;
+}
+
+/* Set the length of a segment of the rcu_segcblist structure. */
+static void rcu_segcblist_set_seglen(struct rcu_segcblist *rsclp, int seg, long v)
+{
+	WRITE_ONCE(rsclp->seglen[seg], v);
+}
+
+/* Increase the numeric length of a segment by a specified amount. */
+static void rcu_segcblist_add_seglen(struct rcu_segcblist *rsclp, int seg, long v)
+{
+	WRITE_ONCE(rsclp->seglen[seg], rsclp->seglen[seg] + v);
+}
+
+/* Move from's segment length to to's segment. */
+static void rcu_segcblist_move_seglen(struct rcu_segcblist *rsclp, int from, int to)
+{
+	long len;
+
+	if (from == to)
+		return;
+
+	len = rcu_segcblist_get_seglen(rsclp, from);
+	if (!len)
+		return;
+
+	rcu_segcblist_add_seglen(rsclp, to, len);
+	rcu_segcblist_set_seglen(rsclp, from, 0);
+}
+
+/* Increment segment's length. */
+static void rcu_segcblist_inc_seglen(struct rcu_segcblist *rsclp, int seg)
+{
+	rcu_segcblist_add_seglen(rsclp, seg, 1);
+}
+
+/*
+ * Increase the numeric length of an rcu_segcblist structure by the
+ * specified amount, which can be negative.  This can cause the ->len
+ * field to disagree with the actual number of callbacks on the structure.
+ * This increase is fully ordered with respect to the callers accesses
+ * both before and after.
+ *
+ * So why on earth is a memory barrier required both before and after
+ * the update to the ->len field???
+ *
+ * The reason is that rcu_barrier() locklessly samples each CPU's ->len
+ * field, and if a given CPU's field is zero, avoids IPIing that CPU.
+ * This can of course race with both queuing and invoking of callbacks.
+ * Failing to correctly handle either of these races could result in
+ * rcu_barrier() failing to IPI a CPU that actually had callbacks queued
+ * which rcu_barrier() was obligated to wait on.  And if rcu_barrier()
+ * failed to wait on such a callback, unloading certain kernel modules
+ * would result in calls to functions whose code was no longer present in
+ * the kernel, for but one example.
+ *
+ * Therefore, ->len transitions from 1->0 and 0->1 have to be carefully
+ * ordered with respect with both list modifications and the rcu_barrier().
+ *
+ * The queuing case is CASE 1 and the invoking case is CASE 2.
+ *
+ * CASE 1: Suppose that CPU 0 has no callbacks queued, but invokes
+ * call_rcu() just as CPU 1 invokes rcu_barrier().  CPU 0's ->len field
+ * will transition from 0->1, which is one of the transitions that must
+ * be handled carefully.  Without the full memory barriers after the ->len
+ * update and at the beginning of rcu_barrier(), the following could happen:
+ *
+ * CPU 0				CPU 1
+ *
+ * call_rcu().
+ *					rcu_barrier() sees ->len as 0.
+ * set ->len = 1.
+ *					rcu_barrier() does nothing.
+ *					module is unloaded.
+ * callback invokes unloaded function!
+ *
+ * With the full barriers, any case where rcu_barrier() sees ->len as 0 will
+ * have unambiguously preceded the return from the racing call_rcu(), which
+ * means that this call_rcu() invocation is OK to not wait on.  After all,
+ * you are supposed to make sure that any problematic call_rcu() invocations
+ * happen before the rcu_barrier().
+ *
+ *
+ * CASE 2: Suppose that CPU 0 is invoking its last callback just as
+ * CPU 1 invokes rcu_barrier().  CPU 0's ->len field will transition from
+ * 1->0, which is one of the transitions that must be handled carefully.
+ * Without the full memory barriers before the ->len update and at the
+ * end of rcu_barrier(), the following could happen:
+ *
+ * CPU 0				CPU 1
+ *
+ * start invoking last callback
+ * set ->len = 0 (reordered)
+ *					rcu_barrier() sees ->len as 0
+ *					rcu_barrier() does nothing.
+ *					module is unloaded
+ * callback executing after unloaded!
+ *
+ * With the full barriers, any case where rcu_barrier() sees ->len as 0
+ * will be fully ordered after the completion of the callback function,
+ * so that the module unloading operation is completely safe.
+ *
+ */
+void rcu_segcblist_add_len(struct rcu_segcblist *rsclp, long v)
+{
+#ifdef CONFIG_RCU_NOCB_CPU
+	smp_mb__before_atomic(); // Read header comment above.
+	atomic_long_add(v, &rsclp->len);
+	smp_mb__after_atomic();  // Read header comment above.
+#else
+	smp_mb(); // Read header comment above.
+	WRITE_ONCE(rsclp->len, rsclp->len + v);
+	smp_mb(); // Read header comment above.
+#endif
+}
+
+/*
+ * Increase the numeric length of an rcu_segcblist structure by one.
+ * This can cause the ->len field to disagree with the actual number of
+ * callbacks on the structure.  This increase is fully ordered with respect
+ * to the callers accesses both before and after.
+ */
+void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp)
+{
+	rcu_segcblist_add_len(rsclp, 1);
+}
+
+/*
+ * Initialize an rcu_segcblist structure.
+ */
+void rcu_segcblist_init(struct rcu_segcblist *rsclp)
+{
+	int i;
+
+	BUILD_BUG_ON(RCU_NEXT_TAIL + 1 != ARRAY_SIZE(rsclp->gp_seq));
+	BUILD_BUG_ON(ARRAY_SIZE(rsclp->tails) != ARRAY_SIZE(rsclp->gp_seq));
+	rsclp->head = NULL;
+	for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
+		rsclp->tails[i] = &rsclp->head;
+		rcu_segcblist_set_seglen(rsclp, i, 0);
+	}
+	rcu_segcblist_set_len(rsclp, 0);
+	rcu_segcblist_set_flags(rsclp, SEGCBLIST_ENABLED);
+}
+
+/*
+ * Disable the specified rcu_segcblist structure, so that callbacks can
+ * no longer be posted to it.  This structure must be empty.
+ */
+void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
+{
+	WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
+	WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
+	rcu_segcblist_clear_flags(rsclp, SEGCBLIST_ENABLED);
+}
+
+/*
+ * Mark the specified rcu_segcblist structure as offloaded (or not)
+ */
+void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload)
+{
+	if (offload)
+		rcu_segcblist_set_flags(rsclp, SEGCBLIST_LOCKING | SEGCBLIST_OFFLOADED);
+	else
+		rcu_segcblist_clear_flags(rsclp, SEGCBLIST_OFFLOADED);
+}
+
+/*
+ * Does the specified rcu_segcblist structure contain callbacks that
+ * are ready to be invoked?
+ */
+bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp)
+{
+	return rcu_segcblist_is_enabled(rsclp) &&
+	       &rsclp->head != READ_ONCE(rsclp->tails[RCU_DONE_TAIL]);
+}
+
+/*
+ * Does the specified rcu_segcblist structure contain callbacks that
+ * are still pending, that is, not yet ready to be invoked?
+ */
+bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp)
+{
+	return rcu_segcblist_is_enabled(rsclp) &&
+	       !rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL);
+}
+
+/*
+ * Return a pointer to the first callback in the specified rcu_segcblist
+ * structure.  This is useful for diagnostics.
+ */
+struct rcu_head *rcu_segcblist_first_cb(struct rcu_segcblist *rsclp)
+{
+	if (rcu_segcblist_is_enabled(rsclp))
+		return rsclp->head;
+	return NULL;
+}
+
+/*
+ * Return a pointer to the first pending callback in the specified
+ * rcu_segcblist structure.  This is useful just after posting a given
+ * callback -- if that callback is the first pending callback, then
+ * you cannot rely on someone else having already started up the required
+ * grace period.
+ */
+struct rcu_head *rcu_segcblist_first_pend_cb(struct rcu_segcblist *rsclp)
+{
+	if (rcu_segcblist_is_enabled(rsclp))
+		return *rsclp->tails[RCU_DONE_TAIL];
+	return NULL;
+}
+
+/*
+ * Return false if there are no CBs awaiting grace periods, otherwise,
+ * return true and store the nearest waited-upon grace period into *lp.
+ */
+bool rcu_segcblist_nextgp(struct rcu_segcblist *rsclp, unsigned long *lp)
+{
+	if (!rcu_segcblist_pend_cbs(rsclp))
+		return false;
+	*lp = rsclp->gp_seq[RCU_WAIT_TAIL];
+	return true;
+}
+
+/*
+ * Enqueue the specified callback onto the specified rcu_segcblist
+ * structure, updating accounting as needed.  Note that the ->len
+ * field may be accessed locklessly, hence the WRITE_ONCE().
+ * The ->len field is used by rcu_barrier() and friends to determine
+ * if it must post a callback on this structure, and it is OK
+ * for rcu_barrier() to sometimes post callbacks needlessly, but
+ * absolutely not OK for it to ever miss posting a callback.
+ */
+void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
+			   struct rcu_head *rhp)
+{
+	rcu_segcblist_inc_len(rsclp);
+	rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
+	rhp->next = NULL;
+	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
+	WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], &rhp->next);
+}
+
+/*
+ * Entrain the specified callback onto the specified rcu_segcblist at
+ * the end of the last non-empty segment.  If the entire rcu_segcblist
+ * is empty, make no change, but return false.
+ *
+ * This is intended for use by rcu_barrier()-like primitives, -not-
+ * for normal grace-period use.  IMPORTANT:  The callback you enqueue
+ * will wait for all prior callbacks, NOT necessarily for a grace
+ * period.  You have been warned.
+ */
+bool rcu_segcblist_entrain(struct rcu_segcblist *rsclp,
+			   struct rcu_head *rhp)
+{
+	int i;
+
+	if (rcu_segcblist_n_cbs(rsclp) == 0)
+		return false;
+	rcu_segcblist_inc_len(rsclp);
+	smp_mb(); /* Ensure counts are updated before callback is entrained. */
+	rhp->next = NULL;
+	for (i = RCU_NEXT_TAIL; i > RCU_DONE_TAIL; i--)
+		if (rsclp->tails[i] != rsclp->tails[i - 1])
+			break;
+	rcu_segcblist_inc_seglen(rsclp, i);
+	WRITE_ONCE(*rsclp->tails[i], rhp);
+	for (; i <= RCU_NEXT_TAIL; i++)
+		WRITE_ONCE(rsclp->tails[i], &rhp->next);
+	return true;
+}
+
+/*
+ * Extract only those callbacks ready to be invoked from the specified
+ * rcu_segcblist structure and place them in the specified rcu_cblist
+ * structure.
+ */
+void rcu_segcblist_extract_done_cbs(struct rcu_segcblist *rsclp,
+				    struct rcu_cblist *rclp)
+{
+	int i;
+
+	if (!rcu_segcblist_ready_cbs(rsclp))
+		return; /* Nothing to do. */
+	rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_DONE_TAIL);
+	*rclp->tail = rsclp->head;
+	WRITE_ONCE(rsclp->head, *rsclp->tails[RCU_DONE_TAIL]);
+	WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
+	rclp->tail = rsclp->tails[RCU_DONE_TAIL];
+	for (i = RCU_CBLIST_NSEGS - 1; i >= RCU_DONE_TAIL; i--)
+		if (rsclp->tails[i] == rsclp->tails[RCU_DONE_TAIL])
+			WRITE_ONCE(rsclp->tails[i], &rsclp->head);
+	rcu_segcblist_set_seglen(rsclp, RCU_DONE_TAIL, 0);
+}
+
+/*
+ * Extract only those callbacks still pending (not yet ready to be
+ * invoked) from the specified rcu_segcblist structure and place them in
+ * the specified rcu_cblist structure.  Note that this loses information
+ * about any callbacks that might have been partway done waiting for
+ * their grace period.  Too bad!  They will have to start over.
+ */
+void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp,
+				    struct rcu_cblist *rclp)
+{
+	int i;
+
+	if (!rcu_segcblist_pend_cbs(rsclp))
+		return; /* Nothing to do. */
+	rclp->len = 0;
+	*rclp->tail = *rsclp->tails[RCU_DONE_TAIL];
+	rclp->tail = rsclp->tails[RCU_NEXT_TAIL];
+	WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL);
+	for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++) {
+		rclp->len += rcu_segcblist_get_seglen(rsclp, i);
+		WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]);
+		rcu_segcblist_set_seglen(rsclp, i, 0);
+	}
+}
+
+/*
+ * Insert counts from the specified rcu_cblist structure in the
+ * specified rcu_segcblist structure.
+ */
+void rcu_segcblist_insert_count(struct rcu_segcblist *rsclp,
+				struct rcu_cblist *rclp)
+{
+	rcu_segcblist_add_len(rsclp, rclp->len);
+}
+
+/*
+ * Move callbacks from the specified rcu_cblist to the beginning of the
+ * done-callbacks segment of the specified rcu_segcblist.
+ */
+void rcu_segcblist_insert_done_cbs(struct rcu_segcblist *rsclp,
+				   struct rcu_cblist *rclp)
+{
+	int i;
+
+	if (!rclp->head)
+		return; /* No callbacks to move. */
+	rcu_segcblist_add_seglen(rsclp, RCU_DONE_TAIL, rclp->len);
+	*rclp->tail = rsclp->head;
+	WRITE_ONCE(rsclp->head, rclp->head);
+	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
+		if (&rsclp->head == rsclp->tails[i])
+			WRITE_ONCE(rsclp->tails[i], rclp->tail);
+		else
+			break;
+	rclp->head = NULL;
+	rclp->tail = &rclp->head;
+}
+
+/*
+ * Move callbacks from the specified rcu_cblist to the end of the
+ * new-callbacks segment of the specified rcu_segcblist.
+ */
+void rcu_segcblist_insert_pend_cbs(struct rcu_segcblist *rsclp,
+				   struct rcu_cblist *rclp)
+{
+	if (!rclp->head)
+		return; /* Nothing to do. */
+
+	rcu_segcblist_add_seglen(rsclp, RCU_NEXT_TAIL, rclp->len);
+	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rclp->head);
+	WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], rclp->tail);
+}
+
+/*
+ * Advance the callbacks in the specified rcu_segcblist structure based
+ * on the current value passed in for the grace-period counter.
+ */
+void rcu_segcblist_advance(struct rcu_segcblist *rsclp, unsigned long seq)
+{
+	int i, j;
+
+	WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
+	if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
+		return;
+
+	/*
+	 * Find all callbacks whose ->gp_seq numbers indicate that they
+	 * are ready to invoke, and put them into the RCU_DONE_TAIL segment.
+	 */
+	for (i = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++) {
+		if (ULONG_CMP_LT(seq, rsclp->gp_seq[i]))
+			break;
+		WRITE_ONCE(rsclp->tails[RCU_DONE_TAIL], rsclp->tails[i]);
+		rcu_segcblist_move_seglen(rsclp, i, RCU_DONE_TAIL);
+	}
+
+	/* If no callbacks moved, nothing more need be done. */
+	if (i == RCU_WAIT_TAIL)
+		return;
+
+	/* Clean up tail pointers that might have been misordered above. */
+	for (j = RCU_WAIT_TAIL; j < i; j++)
+		WRITE_ONCE(rsclp->tails[j], rsclp->tails[RCU_DONE_TAIL]);
+
+	/*
+	 * Callbacks moved, so there might be an empty RCU_WAIT_TAIL
+	 * and a non-empty RCU_NEXT_READY_TAIL.  If so, copy the
+	 * RCU_NEXT_READY_TAIL segment to fill the RCU_WAIT_TAIL gap
+	 * created by the now-ready-to-invoke segments.
+	 */
+	for (j = RCU_WAIT_TAIL; i < RCU_NEXT_TAIL; i++, j++) {
+		if (rsclp->tails[j] == rsclp->tails[RCU_NEXT_TAIL])
+			break;  /* No more callbacks. */
+		WRITE_ONCE(rsclp->tails[j], rsclp->tails[i]);
+		rcu_segcblist_move_seglen(rsclp, i, j);
+		rsclp->gp_seq[j] = rsclp->gp_seq[i];
+	}
+}
+
+/*
+ * "Accelerate" callbacks based on more-accurate grace-period information.
+ * The reason for this is that RCU does not synchronize the beginnings and
+ * ends of grace periods, and that callbacks are posted locally.  This in
+ * turn means that the callbacks must be labelled conservatively early
+ * on, as getting exact information would degrade both performance and
+ * scalability.  When more accurate grace-period information becomes
+ * available, previously posted callbacks can be "accelerated", marking
+ * them to complete at the end of the earlier grace period.
+ *
+ * This function operates on an rcu_segcblist structure, and also the
+ * grace-period sequence number seq at which new callbacks would become
+ * ready to invoke.  Returns true if there are callbacks that won't be
+ * ready to invoke until seq, false otherwise.
+ */
+bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
+{
+	int i, j;
+
+	WARN_ON_ONCE(!rcu_segcblist_is_enabled(rsclp));
+	if (rcu_segcblist_restempty(rsclp, RCU_DONE_TAIL))
+		return false;
+
+	/*
+	 * Find the segment preceding the oldest segment of callbacks
+	 * whose ->gp_seq[] completion is at or after that passed in via
+	 * "seq", skipping any empty segments.  This oldest segment, along
+	 * with any later segments, can be merged in with any newly arrived
+	 * callbacks in the RCU_NEXT_TAIL segment, and assigned "seq"
+	 * as their ->gp_seq[] grace-period completion sequence number.
+	 */
+	for (i = RCU_NEXT_READY_TAIL; i > RCU_DONE_TAIL; i--)
+		if (rsclp->tails[i] != rsclp->tails[i - 1] &&
+		    ULONG_CMP_LT(rsclp->gp_seq[i], seq))
+			break;
+
+	/*
+	 * If all the segments contain callbacks that correspond to
+	 * earlier grace-period sequence numbers than "seq", leave.
+	 * Assuming that the rcu_segcblist structure has enough
+	 * segments in its arrays, this can only happen if some of
+	 * the non-done segments contain callbacks that really are
+	 * ready to invoke.  This situation will get straightened
+	 * out by the next call to rcu_segcblist_advance().
+	 *
+	 * Also advance to the oldest segment of callbacks whose
+	 * ->gp_seq[] completion is at or after that passed in via "seq",
+	 * skipping any empty segments.
+	 *
+	 * Note that segment "i" (and any lower-numbered segments
+	 * containing older callbacks) will be unaffected, and their
+	 * grace-period numbers remain unchanged.  For example, if i ==
+	 * WAIT_TAIL, then neither WAIT_TAIL nor DONE_TAIL will be touched.
+	 * Instead, the CBs in NEXT_TAIL will be merged with those in
+	 * NEXT_READY_TAIL and the grace-period number of NEXT_READY_TAIL
+	 * would be updated.  NEXT_TAIL would then be empty.
+	 */
+	if (rcu_segcblist_restempty(rsclp, i) || ++i >= RCU_NEXT_TAIL)
+		return false;
+
+	/* Accounting: everything below i is about to get merged into i. */
+	for (j = i + 1; j <= RCU_NEXT_TAIL; j++)
+		rcu_segcblist_move_seglen(rsclp, j, i);
+
+	/*
+	 * Merge all later callbacks, including newly arrived callbacks,
+	 * into the segment located by the for-loop above.  Assign "seq"
+	 * as the ->gp_seq[] value in order to correctly handle the case
+	 * where there were no pending callbacks in the rcu_segcblist
+	 * structure other than in the RCU_NEXT_TAIL segment.
+	 */
+	for (; i < RCU_NEXT_TAIL; i++) {
+		WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_NEXT_TAIL]);
+		rsclp->gp_seq[i] = seq;
+	}
+	return true;
+}
+
+/*
+ * Merge the source rcu_segcblist structure into the destination
+ * rcu_segcblist structure, then initialize the source.  Any pending
+ * callbacks from the source get to start over.  It is best to
+ * advance and accelerate both the destination and the source
+ * before merging.
+ */
+void rcu_segcblist_merge(struct rcu_segcblist *dst_rsclp,
+			 struct rcu_segcblist *src_rsclp)
+{
+	struct rcu_cblist donecbs;
+	struct rcu_cblist pendcbs;
+
+	lockdep_assert_cpus_held();
+
+	rcu_cblist_init(&donecbs);
+	rcu_cblist_init(&pendcbs);
+
+	rcu_segcblist_extract_done_cbs(src_rsclp, &donecbs);
+	rcu_segcblist_extract_pend_cbs(src_rsclp, &pendcbs);
+
+	/*
+	 * No need smp_mb() before setting length to 0, because CPU hotplug
+	 * lock excludes rcu_barrier.
+	 */
+	rcu_segcblist_set_len(src_rsclp, 0);
+
+	rcu_segcblist_insert_count(dst_rsclp, &donecbs);
+	rcu_segcblist_insert_count(dst_rsclp, &pendcbs);
+	rcu_segcblist_insert_done_cbs(dst_rsclp, &donecbs);
+	rcu_segcblist_insert_pend_cbs(dst_rsclp, &pendcbs);
+
+	rcu_segcblist_init(src_rsclp);
+}