1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * Copyright 2023 Red Hat
4 */
5
6 #include "funnel-queue.h"
7
8 #include "cpu.h"
9 #include "memory-alloc.h"
10 #include "permassert.h"
11
vdo_make_funnel_queue(struct funnel_queue ** queue_ptr)12 int vdo_make_funnel_queue(struct funnel_queue **queue_ptr)
13 {
14 int result;
15 struct funnel_queue *queue;
16
17 result = vdo_allocate(1, struct funnel_queue, "funnel queue", &queue);
18 if (result != VDO_SUCCESS)
19 return result;
20
21 /*
22 * Initialize the stub entry and put it in the queue, establishing the invariant that
23 * queue->newest and queue->oldest are never null.
24 */
25 queue->stub.next = NULL;
26 queue->newest = &queue->stub;
27 queue->oldest = &queue->stub;
28
29 *queue_ptr = queue;
30 return VDO_SUCCESS;
31 }
32
vdo_free_funnel_queue(struct funnel_queue * queue)33 void vdo_free_funnel_queue(struct funnel_queue *queue)
34 {
35 vdo_free(queue);
36 }
37
get_oldest(struct funnel_queue * queue)38 static struct funnel_queue_entry *get_oldest(struct funnel_queue *queue)
39 {
40 /*
41 * Barrier requirements: We need a read barrier between reading a "next" field pointer
42 * value and reading anything it points to. There's an accompanying barrier in
43 * vdo_funnel_queue_put() between its caller setting up the entry and making it visible.
44 */
45 struct funnel_queue_entry *oldest = queue->oldest;
46 struct funnel_queue_entry *next = READ_ONCE(oldest->next);
47
48 if (oldest == &queue->stub) {
49 /*
50 * When the oldest entry is the stub and it has no successor, the queue is
51 * logically empty.
52 */
53 if (next == NULL)
54 return NULL;
55 /*
56 * The stub entry has a successor, so the stub can be dequeued and ignored without
57 * breaking the queue invariants.
58 */
59 oldest = next;
60 queue->oldest = oldest;
61 next = READ_ONCE(oldest->next);
62 }
63
64 /*
65 * We have a non-stub candidate to dequeue. If it lacks a successor, we'll need to put the
66 * stub entry back on the queue first.
67 */
68 if (next == NULL) {
69 struct funnel_queue_entry *newest = READ_ONCE(queue->newest);
70
71 if (oldest != newest) {
72 /*
73 * Another thread has already swung queue->newest atomically, but not yet
74 * assigned previous->next. The queue is really still empty.
75 */
76 return NULL;
77 }
78
79 /*
80 * Put the stub entry back on the queue, ensuring a successor will eventually be
81 * seen.
82 */
83 vdo_funnel_queue_put(queue, &queue->stub);
84
85 /* Check again for a successor. */
86 next = READ_ONCE(oldest->next);
87 if (next == NULL) {
88 /*
89 * We lost a race with a producer who swapped queue->newest before we did,
90 * but who hasn't yet updated previous->next. Try again later.
91 */
92 return NULL;
93 }
94 }
95
96 return oldest;
97 }
98
99 /*
100 * Poll a queue, removing the oldest entry if the queue is not empty. This function must only be
101 * called from a single consumer thread.
102 */
vdo_funnel_queue_poll(struct funnel_queue * queue)103 struct funnel_queue_entry *vdo_funnel_queue_poll(struct funnel_queue *queue)
104 {
105 struct funnel_queue_entry *oldest = get_oldest(queue);
106
107 if (oldest == NULL)
108 return oldest;
109
110 /*
111 * Dequeue the oldest entry and return it. Only one consumer thread may call this function,
112 * so no locking, atomic operations, or fences are needed; queue->oldest is owned by the
113 * consumer and oldest->next is never used by a producer thread after it is swung from NULL
114 * to non-NULL.
115 */
116 queue->oldest = READ_ONCE(oldest->next);
117 /*
118 * Make sure the caller sees the proper stored data for this entry. Since we've already
119 * fetched the entry pointer we stored in "queue->oldest", this also ensures that on entry
120 * to the next call we'll properly see the dependent data.
121 */
122 smp_rmb();
123 /*
124 * If "oldest" is a very light-weight work item, we'll be looking for the next one very
125 * soon, so prefetch it now.
126 */
127 uds_prefetch_address(queue->oldest, true);
128 WRITE_ONCE(oldest->next, NULL);
129 return oldest;
130 }
131
132 /*
133 * Check whether the funnel queue is empty or not. If the queue is in a transition state with one
134 * or more entries being added such that the list view is incomplete, this function will report the
135 * queue as empty.
136 */
vdo_is_funnel_queue_empty(struct funnel_queue * queue)137 bool vdo_is_funnel_queue_empty(struct funnel_queue *queue)
138 {
139 return get_oldest(queue) == NULL;
140 }
141
142 /*
143 * Check whether the funnel queue is idle or not. If the queue has entries available to be
144 * retrieved, it is not idle. If the queue is in a transition state with one or more entries being
145 * added such that the list view is incomplete, it may not be possible to retrieve an entry with
146 * the vdo_funnel_queue_poll() function, but the queue will not be considered idle.
147 */
vdo_is_funnel_queue_idle(struct funnel_queue * queue)148 bool vdo_is_funnel_queue_idle(struct funnel_queue *queue)
149 {
150 /*
151 * Oldest is not the stub, so there's another entry, though if next is NULL we can't
152 * retrieve it yet.
153 */
154 if (queue->oldest != &queue->stub)
155 return false;
156
157 /*
158 * Oldest is the stub, but newest has been updated by _put(); either there's another,
159 * retrievable entry in the list, or the list is officially empty but in the intermediate
160 * state of having an entry added.
161 *
162 * Whether anything is retrievable depends on whether stub.next has been updated and become
163 * visible to us, but for idleness we don't care. And due to memory ordering in _put(), the
164 * update to newest would be visible to us at the same time or sooner.
165 */
166 if (READ_ONCE(queue->newest) != &queue->stub)
167 return false;
168
169 return true;
170 }
171