lwq is a FIFO single-linked queue that only requires a spinlock
for dequeueing, which happens in process context. Enqueueing is atomic
with no spinlock and can happen in any context.
Include a unit test for basic functionality - runs at boot time. Does
not use kunit framework.
Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
include/linux/lwq.h | 120 +++++++++++++++++++++++++++++++++++
lib/Kconfig | 5 ++
lib/Makefile | 2 +-
lib/lwq.c | 149 ++++++++++++++++++++++++++++++++++++++++++++
4 files changed, 275 insertions(+), 1 deletion(-)
create mode 100644 include/linux/lwq.h
create mode 100644 lib/lwq.c
diff --git a/include/linux/lwq.h b/include/linux/lwq.h
new file mode 100644
index 000000000000..52b9c81b493a
--- /dev/null
+++ b/include/linux/lwq.h
@@ -0,0 +1,120 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#ifndef LWQ_H
+#define LWQ_H
+/*
+ * light-weight single-linked queue built from llist
+ *
+ * Entries can be enqueued from any context with no locking.
+ * Entries can be dequeued from process context with integrated locking.
+ */
+#include <linux/container_of.h>
+#include <linux/spinlock.h>
+#include <linux/llist.h>
+
+struct lwq_node {
+ struct llist_node node;
+};
+
+struct lwq {
+ spinlock_t lock;
+ struct llist_node *ready; /* entries to be dequeued */
+ struct llist_head new; /* entries being enqueued */
+};
+
+/**
+ * lwq_init - initialise a lwq
+ * @q: the lwq object
+ */
+static inline void lwq_init(struct lwq *q)
+{
+ spin_lock_init(&q->lock);
+ q->ready = NULL;
+ init_llist_head(&q->new);
+}
+
+/**
+ * lwq_empty - test if lwq contains any entry
+ * @q: the lwq object
+ *
+ * This empty test contains an acquire barrier so that if a wakeup
+ * is sent when lwq_dequeue returns true, it is safe to go to sleep after
+ * a test on lwq_empty().
+ */
+static inline bool lwq_empty(struct lwq *q)
+{
+ /* acquire ensures ordering wrt lwq_enqueue() */
+ return smp_load_acquire(&q->ready) == NULL && llist_empty(&q->new);
+}
+
+struct llist_node *__lwq_dequeue(struct lwq *q);
+/**
+ * lwq_dequeue - dequeue first (oldest) entry from lwq
+ * @q: the queue to dequeue from
+ * @type: the type of object to return
+ * @member: them member in returned object which is an lwq_node.
+ *
+ * Remove a single object from the lwq and return it. This will take
+ * a spinlock and so must always be called in the same context, typcially
+ * process contet.
+ */
+#define lwq_dequeue(q, type, member) \
+ ({ struct llist_node *_n = __lwq_dequeue(q); \
+ _n ? container_of(_n, type, member.node) : NULL; })
+
+struct llist_node *lwq_dequeue_all(struct lwq *q);
+
+/**
+ * lwq_for_each_safe - iterate over detached queue allowing deletion
+ * @_n: iterator variable
+ * @_t1: temporary struct llist_node **
+ * @_t2: temporary struct llist_node *
+ * @_l: address of llist_node pointer from lwq_dequeue_all()
+ * @_member: member in _n where lwq_node is found.
+ *
+ * Iterate over members in a dequeued list. If the iterator variable
+ * is set to NULL, the iterator removes that entry from the queue.
+ */
+#define lwq_for_each_safe(_n, _t1, _t2, _l, _member) \
+ for (_t1 = (_l); \
+ *(_t1) ? (_n = container_of(*(_t1), typeof(*(_n)), _member.node),\
+ _t2 = ((*_t1)->next), \
+ true) \
+ : false; \
+ (_n) ? (_t1 = &(_n)->_member.node.next, 0) \
+ : ((*(_t1) = (_t2)), 0))
+
+/**
+ * lwq_enqueue - add a new item to the end of the queue
+ * @n - the lwq_node embedded in the item to be added
+ * @q - the lwq to append to.
+ *
+ * No locking is needed to append to the queue so this can
+ * be called from any context.
+ * Return %true is the list may have previously been empty.
+ */
+static inline bool lwq_enqueue(struct lwq_node *n, struct lwq *q)
+{
+ /* acquire enqures ordering wrt lwq_dequeue */
+ return llist_add(&n->node, &q->new) &&
+ smp_load_acquire(&q->ready) == NULL;
+}
+
+/**
+ * lwq_enqueue_batch - add a list of new items to the end of the queue
+ * @n - the lwq_node embedded in the first item to be added
+ * @q - the lwq to append to.
+ *
+ * No locking is needed to append to the queue so this can
+ * be called from any context.
+ * Return %true is the list may have previously been empty.
+ */
+static inline bool lwq_enqueue_batch(struct llist_node *n, struct lwq *q)
+{
+ struct llist_node *e = n;
+
+ /* acquire enqures ordering wrt lwq_dequeue */
+ return llist_add_batch(llist_reverse_order(n), e, &q->new) &&
+ smp_load_acquire(&q->ready) == NULL;
+}
+#endif /* LWQ_H */
diff --git a/lib/Kconfig b/lib/Kconfig
index 5c2da561c516..6620bdba4f94 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -763,3 +763,8 @@ config ASN1_ENCODER
config POLYNOMIAL
tristate
+
+config LWQ_TEST
+ bool "RPC: enable boot-time test for lwq queuing"
+ help
+ Enable boot-time test of lwq functionality.
diff --git a/lib/Makefile b/lib/Makefile
index 1ffae65bb7ee..4b67c2d6af62 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -45,7 +45,7 @@ obj-y += lockref.o
obj-y += bcd.o sort.o parser.o debug_locks.o random32.o \
bust_spinlocks.o kasprintf.o bitmap.o scatterlist.o \
list_sort.o uuid.o iov_iter.o clz_ctz.o \
- bsearch.o find_bit.o llist.o memweight.o kfifo.o \
+ bsearch.o find_bit.o llist.o lwq.o memweight.o kfifo.o \
percpu-refcount.o rhashtable.o base64.o \
once.o refcount.o rcuref.o usercopy.o errseq.o bucket_locks.o \
generic-radix-tree.o
diff --git a/lib/lwq.c b/lib/lwq.c
new file mode 100644
index 000000000000..d6be6dda3867
--- /dev/null
+++ b/lib/lwq.c
@@ -0,0 +1,149 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Light weight single-linked queue.
+ *
+ * Entries are enqueued to the head of an llist, with no blocking.
+ * This can happen in any context.
+ *
+ * Entries are dequeued using a spinlock to protect against
+ * multiple access. The llist is staged in reverse order, and refreshed
+ * from the llist when it exhausts.
+ */
+#include <linux/rcupdate.h>
+#include <linux/lwq.h>
+
+struct llist_node *__lwq_dequeue(struct lwq *q)
+{
+ struct llist_node *this;
+
+ if (lwq_empty(q))
+ return NULL;
+ spin_lock(&q->lock);
+ this = q->ready;
+ if (!this && !llist_empty(&q->new)) {
+ /* ensure queue doesn't appear transiently lwq_empty */
+ smp_store_release(&q->ready, (void *)1);
+ this = llist_reverse_order(llist_del_all(&q->new));
+ if (!this)
+ q->ready = NULL;
+ }
+ if (this)
+ q->ready = llist_next(this);
+ spin_unlock(&q->lock);
+ return this;
+}
+
+/**
+ * lwq_dequeue_all - dequeue all currently enqueued objects
+ * @q: the queue to dequeue from
+ *
+ * Remove and return a linked list of llist_nodes of all the objects that were
+ * in the queue. The first on the list will be the object that was least
+ * recently enqueued.
+ */
+struct llist_node *lwq_dequeue_all(struct lwq *q)
+{
+ struct llist_node *r, *t, **ep;
+
+ if (lwq_empty(q))
+ return NULL;
+
+ spin_lock(&q->lock);
+ r = q->ready;
+ q->ready = NULL;
+ t = llist_del_all(&q->new);
+ spin_unlock(&q->lock);
+ ep = &r;
+ while (*ep)
+ ep = &(*ep)->next;
+ *ep = llist_reverse_order(t);
+ return r;
+}
+
+#if IS_ENABLED(CONFIG_LWQ_TEST)
+
+#include <linux/module.h>
+#include <linux/slab.h>
+#include <linux/wait_bit.h>
+#include <linux/kthread.h>
+#include <linux/delay.h>
+struct tnode {
+ struct lwq_node n;
+ int i;
+ int c;
+};
+
+static int lwq_exercise(void *qv)
+{
+ struct lwq *q = qv;
+ int cnt;
+ struct tnode *t;
+
+ for (cnt = 0; cnt < 10000; cnt++) {
+ wait_var_event(q, (t = lwq_dequeue(q, struct tnode, n)) != NULL);
+ t->c++;
+ if (lwq_enqueue(&t->n, q))
+ wake_up_var(q);
+ }
+ while (!kthread_should_stop())
+ schedule_timeout_idle(1);
+ return 0;
+}
+
+static int lwq_test(void)
+{
+ int i;
+ struct lwq q;
+ struct llist_node *l, **t1, *t2;
+ struct tnode *t;
+ struct task_struct *threads[8];
+
+ printk(KERN_INFO "testing lwq....\n");
+ lwq_init(&q);
+ printk(KERN_INFO " lwq: run some threads\n");
+ for (i = 0; i < ARRAY_SIZE(threads); i++)
+ threads[i] = kthread_run(lwq_exercise, &q, "lwq-test-%d", i);
+ for (i = 0; i < 100; i++) {
+ t = kmalloc(sizeof(*t), GFP_KERNEL);
+ t->i = i;
+ t->c = 0;
+ if (lwq_enqueue(&t->n, &q))
+ wake_up_var(&q);
+ };
+ /* wait for threads to exit */
+ for (i = 0; i < ARRAY_SIZE(threads); i++)
+ if (!IS_ERR_OR_NULL(threads[i]))
+ kthread_stop(threads[i]);
+ printk(KERN_INFO " lwq: dequeue first 50:");
+ for (i = 0; i < 50 ; i++) {
+ if (i && (i % 10) == 0) {
+ printk(KERN_CONT "\n");
+ printk(KERN_INFO " lwq: ... ");
+ }
+ t = lwq_dequeue(&q, struct tnode, n);
+ printk(KERN_CONT " %d(%d)", t->i, t->c);
+ kfree(t);
+ }
+ printk(KERN_CONT "\n");
+ l = lwq_dequeue_all(&q);
+ printk(KERN_INFO " lwq: delete the multiples of 3 (test lwq_for_each_safe())\n");
+ lwq_for_each_safe(t, t1, t2, &l, n) {
+ if ((t->i % 3) == 0) {
+ t->i = -1;
+ kfree(t);
+ t = NULL;
+ }
+ }
+ if (l)
+ lwq_enqueue_batch(l, &q);
+ printk(KERN_INFO " lwq: dequeue remaining:");
+ while ((t = lwq_dequeue(&q, struct tnode, n)) != NULL) {
+ printk(KERN_CONT " %d", t->i);
+ kfree(t);
+ }
+ printk(KERN_CONT "\n");
+ return 0;
+}
+
+module_init(lwq_test);
+#endif /* CONFIG_LWQ_TEST*/
--
2.41.0
