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
|
/*-
* SPDX-License-Identifier: BSD-2-Clause-FreeBSD
*
* Copyright (c) 2019, 2020 Jeffrey Roberson <jeff@FreeBSD.org>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice unmodified, this list of conditions, and the following
* disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* $FreeBSD$
*
*/
#ifndef _SYS_SMR_H_
#define _SYS_SMR_H_
#include <sys/_smr.h>
/*
* Safe memory reclamation. See subr_smr.c for a description of the
* algorithm, and smr_types.h for macros to define and access SMR-protected
* data structures.
*
* Readers synchronize with smr_enter()/exit() and writers may either
* free directly to a SMR UMA zone or use smr_synchronize or wait.
*/
/*
* Modular arithmetic for comparing sequence numbers that have
* potentially wrapped. Copied from tcp_seq.h.
*/
#define SMR_SEQ_LT(a, b) ((smr_delta_t)((a)-(b)) < 0)
#define SMR_SEQ_LEQ(a, b) ((smr_delta_t)((a)-(b)) <= 0)
#define SMR_SEQ_GT(a, b) ((smr_delta_t)((a)-(b)) > 0)
#define SMR_SEQ_GEQ(a, b) ((smr_delta_t)((a)-(b)) >= 0)
#define SMR_SEQ_DELTA(a, b) ((smr_delta_t)((a)-(b)))
#define SMR_SEQ_MIN(a, b) (SMR_SEQ_LT((a), (b)) ? (a) : (b))
#define SMR_SEQ_MAX(a, b) (SMR_SEQ_GT((a), (b)) ? (a) : (b))
#define SMR_SEQ_INVALID 0
/* Shared SMR state. */
union s_wr {
struct {
smr_seq_t seq; /* Current write sequence #. */
int ticks; /* tick of last update (LAZY) */
};
uint64_t _pair;
};
struct smr_shared {
const char *s_name; /* Name for debugging/reporting. */
union s_wr s_wr; /* Write sequence */
smr_seq_t s_rd_seq; /* Minimum observed read sequence. */
};
typedef struct smr_shared *smr_shared_t;
/* Per-cpu SMR state. */
struct smr {
smr_seq_t c_seq; /* Current observed sequence. */
smr_shared_t c_shared; /* Shared SMR state. */
int c_deferred; /* Deferred advance counter. */
int c_limit; /* Deferred advance limit. */
int c_flags; /* SMR Configuration */
};
#define SMR_LAZY 0x0001 /* Higher latency write, fast read. */
#define SMR_DEFERRED 0x0002 /* Aggregate updates to wr_seq. */
/*
* Return the current write sequence number. This is not the same as the
* current goal which may be in the future.
*/
static inline smr_seq_t
smr_shared_current(smr_shared_t s)
{
return (atomic_load_int(&s->s_wr.seq));
}
static inline smr_seq_t
smr_current(smr_t smr)
{
return (smr_shared_current(zpcpu_get(smr)->c_shared));
}
/*
* Enter a read section.
*/
static inline void
smr_enter(smr_t smr)
{
critical_enter();
smr = zpcpu_get(smr);
KASSERT((smr->c_flags & SMR_LAZY) == 0,
("smr_enter(%s) lazy smr.", smr->c_shared->s_name));
KASSERT(smr->c_seq == 0,
("smr_enter(%s) does not support recursion.",
smr->c_shared->s_name));
/*
* Store the current observed write sequence number in our
* per-cpu state so that it can be queried via smr_poll().
* Frees that are newer than this stored value will be
* deferred until we call smr_exit().
*
* An acquire barrier is used to synchronize with smr_exit()
* and smr_poll().
*
* It is possible that a long delay between loading the wr_seq
* and storing the c_seq could create a situation where the
* rd_seq advances beyond our stored c_seq. In this situation
* only the observed wr_seq is stale, the fence still orders
* the load. See smr_poll() for details on how this condition
* is detected and handled there.
*/
/* This is an add because we do not have atomic_store_acq_int */
atomic_add_acq_int(&smr->c_seq, smr_shared_current(smr->c_shared));
}
/*
* Exit a read section.
*/
static inline void
smr_exit(smr_t smr)
{
smr = zpcpu_get(smr);
CRITICAL_ASSERT(curthread);
KASSERT((smr->c_flags & SMR_LAZY) == 0,
("smr_exit(%s) lazy smr.", smr->c_shared->s_name));
KASSERT(smr->c_seq != SMR_SEQ_INVALID,
("smr_exit(%s) not in a smr section.", smr->c_shared->s_name));
/*
* Clear the recorded sequence number. This allows poll() to
* detect CPUs not in read sections.
*
* Use release semantics to retire any stores before the sequence
* number is cleared.
*/
atomic_store_rel_int(&smr->c_seq, SMR_SEQ_INVALID);
critical_exit();
}
/*
* Enter a lazy smr section. This is used for read-mostly state that
* can tolerate a high free latency.
*/
static inline void
smr_lazy_enter(smr_t smr)
{
critical_enter();
smr = zpcpu_get(smr);
KASSERT((smr->c_flags & SMR_LAZY) != 0,
("smr_lazy_enter(%s) non-lazy smr.", smr->c_shared->s_name));
KASSERT(smr->c_seq == 0,
("smr_lazy_enter(%s) does not support recursion.",
smr->c_shared->s_name));
/*
* This needs no serialization. If an interrupt occurs before we
* assign sr_seq to c_seq any speculative loads will be discarded.
* If we assign a stale wr_seq value due to interrupt we use the
* same algorithm that renders smr_enter() safe.
*/
atomic_store_int(&smr->c_seq, smr_shared_current(smr->c_shared));
}
/*
* Exit a lazy smr section. This is used for read-mostly state that
* can tolerate a high free latency.
*/
static inline void
smr_lazy_exit(smr_t smr)
{
smr = zpcpu_get(smr);
CRITICAL_ASSERT(curthread);
KASSERT((smr->c_flags & SMR_LAZY) != 0,
("smr_lazy_enter(%s) non-lazy smr.", smr->c_shared->s_name));
KASSERT(smr->c_seq != SMR_SEQ_INVALID,
("smr_lazy_exit(%s) not in a smr section.", smr->c_shared->s_name));
/*
* All loads/stores must be retired before the sequence becomes
* visible. The fence compiles away on amd64. Another
* alternative would be to omit the fence but store the exit
* time and wait 1 tick longer.
*/
atomic_thread_fence_rel();
atomic_store_int(&smr->c_seq, SMR_SEQ_INVALID);
critical_exit();
}
/*
* Advances the write sequence number. Returns the sequence number
* required to ensure that all modifications are visible to readers.
*/
smr_seq_t smr_advance(smr_t smr);
/*
* Returns true if a goal sequence has been reached. If
* wait is true this will busy loop until success.
*/
bool smr_poll(smr_t smr, smr_seq_t goal, bool wait);
/* Create a new SMR context. */
smr_t smr_create(const char *name, int limit, int flags);
/* Destroy the context. */
void smr_destroy(smr_t smr);
/*
* Blocking wait for all readers to observe 'goal'.
*/
static inline bool
smr_wait(smr_t smr, smr_seq_t goal)
{
return (smr_poll(smr, goal, true));
}
/*
* Synchronize advances the write sequence and returns when all
* readers have observed it.
*
* If your application can cache a sequence number returned from
* smr_advance() and poll or wait at a later time there will
* be less chance of busy looping while waiting for readers.
*/
static inline void
smr_synchronize(smr_t smr)
{
smr_wait(smr, smr_advance(smr));
}
/* Only at startup. */
void smr_init(void);
#endif /* _SYS_SMR_H_ */
|
