lib: add light-weight queuing mechanism.

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.

This is particularly useful when work items are queued from BH or IRQ
context, and when they are handled one at a time by dedicated threads.

Avoiding any locking when enqueueing means there is no need to disable
BH or interrupts, which is generally best avoided (particularly when
there are any RT tasks on the machine).

This solution is superior to using "list_head" links because we need
half as many pointers in the data structures, and because list_head
lists would need locking to add items to the queue.

This solution is superior to a bespoke solution as all locking and
container_of casting is integrated, so the interface is simple.

Despite the similar name, this solution meets a distinctly different
need to kfifo.  kfifo provides a fixed sized circular buffer to which
data can be added at one end and removed at the other, and does not
provide any locking.  lwq does not have any size limit and works with
data structures (objects?) rather than data (bytes).

A unit test for basic functionality, which runs at boot time, is included.

Signed-off-by: NeilBrown <neilb@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: David Gow <davidgow@google.com>
Cc: linux-kernel@vger.kernel.org
Message-Id: <20230911111333.4d1a872330e924a00acb905b@linux-foundation.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

1 files changed, 158 insertions, 0 deletions

diff --git a/lib/lwq.c b/lib/lwq.c
new file mode 100644
index 000000000000..57d080a4d53d
--- /dev/null
+++ b/lib/lwq.c
@@ -0,0 +1,158 @@
+// 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.
+ *
+ * This is particularly suitable when work items are queued in BH or
+ * IRQ context, and where work items are handled one at a time by
+ * dedicated threads.
+ */
+#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;
+}
+EXPORT_SYMBOL_GPL(__lwq_dequeue);
+
+/**
+ * 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;
+}
+EXPORT_SYMBOL_GPL(lwq_dequeue_all);
+
+#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);
+		if (!t)
+			break;
+		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);
+		if (t)
+			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*/