2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
4 * Copyright (c) 2019, 2020 Jeffrey Roberson <jeff@FreeBSD.org>
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
9 * 1. Redistributions of source code must retain the above copyright
10 * notice unmodified, this list of conditions, and the following
12 * 2. Redistributions in binary form must reproduce the above copyright
13 * notice, this list of conditions and the following disclaimer in the
14 * documentation and/or other materials provided with the distribution.
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
17 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
18 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
19 * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
20 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
21 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
22 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
23 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
24 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
25 * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
37 * Safe memory reclamation. See subr_smr.c for a description of the
40 * Readers synchronize with smr_enter()/exit() and writers may either
41 * free directly to a SMR UMA zone or use smr_synchronize or wait.
45 * Modular arithmetic for comparing sequence numbers that have
46 * potentially wrapped. Copied from tcp_seq.h.
48 #define SMR_SEQ_LT(a, b) ((int32_t)((a)-(b)) < 0)
49 #define SMR_SEQ_LEQ(a, b) ((int32_t)((a)-(b)) <= 0)
50 #define SMR_SEQ_GT(a, b) ((int32_t)((a)-(b)) > 0)
51 #define SMR_SEQ_GEQ(a, b) ((int32_t)((a)-(b)) >= 0)
53 #define SMR_SEQ_INVALID 0
55 /* Shared SMR state. */
57 const char *s_name; /* Name for debugging/reporting. */
58 smr_seq_t s_wr_seq; /* Current write sequence #. */
59 smr_seq_t s_rd_seq; /* Minimum observed read sequence. */
61 typedef struct smr_shared *smr_shared_t;
63 /* Per-cpu SMR state. */
65 smr_seq_t c_seq; /* Current observed sequence. */
66 smr_shared_t c_shared; /* Shared SMR state. */
70 * Return the current write sequence number.
72 static inline smr_seq_t
73 smr_shared_current(smr_shared_t s)
76 return (atomic_load_int(&s->s_wr_seq));
79 static inline smr_seq_t
80 smr_current(smr_t smr)
83 return (smr_shared_current(zpcpu_get(smr)->c_shared));
87 * Enter a read section.
95 KASSERT(smr->c_seq == 0,
96 ("smr_enter(%s) does not support recursion.",
97 smr->c_shared->s_name));
100 * Store the current observed write sequence number in our
101 * per-cpu state so that it can be queried via smr_poll().
102 * Frees that are newer than this stored value will be
103 * deferred until we call smr_exit().
105 * An acquire barrier is used to synchronize with smr_exit()
108 * It is possible that a long delay between loading the wr_seq
109 * and storing the c_seq could create a situation where the
110 * rd_seq advances beyond our stored c_seq. In this situation
111 * only the observed wr_seq is stale, the fence still orders
112 * the load. See smr_poll() for details on how this condition
113 * is detected and handled there.
115 /* This is an add because we do not have atomic_store_acq_int */
116 atomic_add_acq_int(&smr->c_seq, smr_shared_current(smr->c_shared));
120 * Exit a read section.
126 smr = zpcpu_get(smr);
127 CRITICAL_ASSERT(curthread);
128 KASSERT(smr->c_seq != SMR_SEQ_INVALID,
129 ("smr_exit(%s) not in a smr section.", smr->c_shared->s_name));
132 * Clear the recorded sequence number. This allows poll() to
133 * detect CPUs not in read sections.
135 * Use release semantics to retire any stores before the sequence
138 atomic_store_rel_int(&smr->c_seq, SMR_SEQ_INVALID);
143 * Advances the write sequence number. Returns the sequence number
144 * required to ensure that all modifications are visible to readers.
146 smr_seq_t smr_advance(smr_t smr);
149 * Returns true if a goal sequence has been reached. If
150 * wait is true this will busy loop until success.
152 bool smr_poll(smr_t smr, smr_seq_t goal, bool wait);
154 /* Create a new SMR context. */
155 smr_t smr_create(const char *name);
156 void smr_destroy(smr_t smr);
159 * Blocking wait for all readers to observe 'goal'.
162 smr_wait(smr_t smr, smr_seq_t goal)
165 return (smr_poll(smr, goal, true));
169 * Synchronize advances the write sequence and returns when all
170 * readers have observed it.
172 * If your application can cache a sequence number returned from
173 * smr_advance() and poll or wait at a later time there will
174 * be less chance of busy looping while waiting for readers.
177 smr_synchronize(smr_t smr)
180 smr_wait(smr, smr_advance(smr));
183 /* Only at startup. */
186 #endif /* _SYS_SMR_H_ */