summaryrefslogtreecommitdiff
path: root/kernel/rcu/rcu_segcblist.c
blob: 9bd5f6023c21f940905961d9e02504c79a9fe7c4 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
// 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/types.h>
#include <linux/kernel.h>
#include <linux/interrupt.h>
#include <linux/rcupdate.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;
	rclp->len_lazy = 0;
}

/*
 * Dequeue the oldest rcu_head structure from the specified callback
 * list.  This function assumes that the callback is non-lazy, but
 * the caller can later invoke rcu_cblist_dequeued_lazy() if it
 * finds otherwise (and if it cares about laziness).  This allows
 * different users to have different ways of determining laziness.
 */
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;
}

/*
 * 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;
	rsclp->len = 0;
	rsclp->len_lazy = 0;
}

/*
 * 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));
	WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
	rsclp->tails[RCU_NEXT_TAIL] = NULL;
}

/*
 * 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 != 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;
}

/*
 * 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, bool lazy)
{
	WRITE_ONCE(rsclp->len, rsclp->len + 1); /* ->len sampled locklessly. */
	if (lazy)
		rsclp->len_lazy++;
	smp_mb(); /* Ensure counts are updated before callback is enqueued. */
	rhp->next = NULL;
	*rsclp->tails[RCU_NEXT_TAIL] = rhp;
	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, bool lazy)
{
	int i;

	if (rcu_segcblist_n_cbs(rsclp) == 0)
		return false;
	WRITE_ONCE(rsclp->len, rsclp->len + 1);
	if (lazy)
		rsclp->len_lazy++;
	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;
	*rsclp->tails[i] = rhp;
	for (; i <= RCU_NEXT_TAIL; i++)
		rsclp->tails[i] = &rhp->next;
	return true;
}

/*
 * Extract only the counts from the specified rcu_segcblist structure,
 * and place them in the specified rcu_cblist structure.  This function
 * supports both callback orphaning and invocation, hence the separation
 * of counts and callbacks.  (Callbacks ready for invocation must be
 * orphaned and adopted separately from pending callbacks, but counts
 * apply to all callbacks.  Locking must be used to make sure that
 * both orphaned-callbacks lists are consistent.)
 */
void rcu_segcblist_extract_count(struct rcu_segcblist *rsclp,
					       struct rcu_cblist *rclp)
{
	rclp->len_lazy += rsclp->len_lazy;
	rclp->len += rsclp->len;
	rsclp->len_lazy = 0;
	WRITE_ONCE(rsclp->len, 0); /* ->len sampled locklessly. */
}

/*
 * 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->tail = rsclp->head;
	rsclp->head = *rsclp->tails[RCU_DONE_TAIL];
	*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])
			rsclp->tails[i] = &rsclp->head;
}

/*
 * 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->tail = *rsclp->tails[RCU_DONE_TAIL];
	rclp->tail = rsclp->tails[RCU_NEXT_TAIL];
	*rsclp->tails[RCU_DONE_TAIL] = NULL;
	for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++)
		rsclp->tails[i] = rsclp->tails[RCU_DONE_TAIL];
}

/*
 * 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)
{
	rsclp->len_lazy += rclp->len_lazy;
	/* ->len sampled locklessly. */
	WRITE_ONCE(rsclp->len, rsclp->len + rclp->len);
	rclp->len_lazy = 0;
	rclp->len = 0;
}

/*
 * 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. */
	*rclp->tail = rsclp->head;
	rsclp->head = rclp->head;
	for (i = RCU_DONE_TAIL; i < RCU_CBLIST_NSEGS; i++)
		if (&rsclp->head == rsclp->tails[i])
			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. */
	*rsclp->tails[RCU_NEXT_TAIL] = rclp->head;
	rsclp->tails[RCU_NEXT_TAIL] = rclp->tail;
	rclp->head = NULL;
	rclp->tail = &rclp->head;
}

/*
 * 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;
		rsclp->tails[RCU_DONE_TAIL] = rsclp->tails[i];
	}

	/* 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++)
		rsclp->tails[j] = rsclp->tails[RCU_DONE_TAIL];

	/*
	 * Callbacks moved, so clean up the misordered ->tails[] pointers
	 * that now point into the middle of the list of ready-to-invoke
	 * callbacks.  The overall effect is to copy down the later pointers
	 * into the gap that was created by the now-ready 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. */
		rsclp->tails[j] = rsclp->tails[i];
		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;

	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.
	 */
	if (++i >= RCU_NEXT_TAIL)
		return false;

	/*
	 * 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++) {
		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;

	rcu_cblist_init(&donecbs);
	rcu_cblist_init(&pendcbs);
	rcu_segcblist_extract_count(src_rsclp, &donecbs);
	rcu_segcblist_extract_done_cbs(src_rsclp, &donecbs);
	rcu_segcblist_extract_pend_cbs(src_rsclp, &pendcbs);
	rcu_segcblist_insert_count(dst_rsclp, &donecbs);
	rcu_segcblist_insert_done_cbs(dst_rsclp, &donecbs);
	rcu_segcblist_insert_pend_cbs(dst_rsclp, &pendcbs);
	rcu_segcblist_init(src_rsclp);
}