2 * Copyright (c) 2005 Jeffrey M. Hsu. All rights reserved.
4 * This code is derived from software contributed to The DragonFly Project
5 * by Jeffrey M. Hsu. and Matthew Dillon
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
10 * 1. Redistributions of source code must retain the above copyright
11 * notice, this list of conditions and the following disclaimer.
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.
15 * 3. Neither the name of The DragonFly Project nor the names of its
16 * contributors may be used to endorse or promote products derived
17 * from this software without specific, prior written permission.
19 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
20 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
21 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
22 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
23 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
24 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
25 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
26 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
27 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
28 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
29 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
34 * The implementation is designed to avoid looping when compatible operations
37 * To acquire a spinlock we first increment counta. Then we check if counta
38 * meets our requirements. For an exclusive spinlock it must be 1, of a
39 * shared spinlock it must either be 1 or the SHARED_SPINLOCK bit must be set.
41 * Shared spinlock failure case: Decrement the count, loop until we can
42 * transition from 0 to SHARED_SPINLOCK|1, or until we find SHARED_SPINLOCK
43 * is set and increment the count.
45 * Exclusive spinlock failure case: While maintaining the count, clear the
46 * SHARED_SPINLOCK flag unconditionally. Then use an atomic add to transfer
47 * the count from the low bits to the high bits of counta. Then loop until
48 * all low bits are 0. Once the low bits drop to 0 we can transfer the
49 * count back with an atomic_cmpset_int(), atomically, and return.
51 #include <sys/param.h>
52 #include <sys/systm.h>
53 #include <sys/types.h>
54 #include <sys/kernel.h>
55 #include <sys/sysctl.h>
60 #include <machine/atomic.h>
61 #include <machine/cpu.h>
62 #include <machine/cpufunc.h>
63 #include <machine/specialreg.h>
64 #include <machine/clock.h>
65 #include <sys/indefinite2.h>
66 #include <sys/spinlock.h>
67 #include <sys/spinlock2.h>
70 struct spinlock pmap_spin = SPINLOCK_INITIALIZER(pmap_spin, "pmap_spin");
75 #if !defined(KTR_SPIN_CONTENTION)
76 #define KTR_SPIN_CONTENTION KTR_ALL
78 #define SPIN_STRING "spin=%p type=%c"
79 #define SPIN_ARG_SIZE (sizeof(void *) + sizeof(int))
81 KTR_INFO_MASTER(spin);
83 KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE);
84 KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE);
87 #define logspin(name, spin, type) \
88 KTR_LOG(spin_ ## name, spin, type)
91 static int spin_lock_test_mode;
94 #ifdef DEBUG_LOCKS_LATENCY
96 __read_frequently static long spinlocks_add_latency;
97 SYSCTL_LONG(_debug, OID_AUTO, spinlocks_add_latency, CTLFLAG_RW,
98 &spinlocks_add_latency, 0,
99 "Add spinlock latency");
103 __read_frequently static long spin_backoff_max = 4096;
104 SYSCTL_LONG(_debug, OID_AUTO, spin_backoff_max, CTLFLAG_RW,
105 &spin_backoff_max, 0,
106 "Spinlock exponential backoff limit");
108 /* 1 << n clock cycles, approx */
109 __read_frequently static long spin_window_shift = 8;
110 SYSCTL_LONG(_debug, OID_AUTO, spin_window_shift, CTLFLAG_RW,
111 &spin_window_shift, 0,
112 "Spinlock TSC windowing");
115 * We contested due to another exclusive lock holder. We lose.
117 * We have to unwind the attempt and may acquire the spinlock
118 * anyway while doing so.
121 spin_trylock_contested(struct spinlock *spin)
123 globaldata_t gd = mycpu;
126 * Handle degenerate case, else fail.
128 if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED|0, 1))
130 /*atomic_add_int(&spin->counta, -1);*/
132 crit_exit_raw(gd->gd_curthread);
138 * The spin_lock() inline was unable to acquire the lock and calls this
139 * function with spin->counta already incremented, passing (spin->counta - 1)
140 * to the function (the result of the inline's fetchadd).
142 * Note that we implement both exclusive and shared spinlocks, so we cannot
143 * use atomic_swap_int(). Instead, we try to use atomic_fetchadd_int()
144 * to put most of the burden on the cpu. Atomic_cmpset_int() (cmpxchg)
145 * can cause a lot of unnecessary looping in situations where it is just
146 * trying to increment the count.
148 * Similarly, we leave the SHARED flag intact and incur slightly more
149 * overhead when switching from shared to exclusive. This allows us to
150 * use atomic_fetchadd_int() for both spinlock types in the critical
153 * The exponential (n^1.5) backoff algorithm is designed to both reduce
154 * cache bus contention between cpu cores and sockets, and to allow some
155 * bursting of exclusive locks in heavily contended situations to improve
158 * The exclusive lock priority mechanism prevents even heavily contended
159 * exclusive locks from being starved by shared locks
162 _spin_lock_contested(struct spinlock *spin, const char *ident, int value)
164 indefinite_info_t info;
170 * WARNING! Caller has already incremented the lock. We must
171 * increment the count value (from the inline's fetch-add)
174 * Handle the degenerate case where the spinlock is flagged SHARED
175 * with only our reference. We can convert it to EXCLUSIVE.
177 if (value == (SPINLOCK_SHARED | 1) - 1) {
178 if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED | 1, 1))
181 /* ++value; value not used after this */
182 info.type = 0; /* avoid improper gcc warning */
183 info.ident = NULL; /* avoid improper gcc warning */
187 * Transfer our exclusive request to the high bits and clear the
188 * SPINLOCK_SHARED bit if it was set. This makes the spinlock
189 * appear exclusive, preventing any NEW shared or exclusive
190 * spinlocks from being obtained while we wait for existing
191 * shared or exclusive holders to unlock.
193 * Don't tread on earlier exclusive waiters by stealing the lock
194 * away early if the low bits happen to now be 1.
196 * The shared unlock understands that this may occur.
198 ovalue = atomic_fetchadd_int(&spin->counta, SPINLOCK_EXCLWAIT - 1);
199 ovalue += SPINLOCK_EXCLWAIT - 1;
200 if (ovalue & SPINLOCK_SHARED) {
201 atomic_clear_int(&spin->counta, SPINLOCK_SHARED);
202 ovalue &= ~SPINLOCK_SHARED;
206 expbackoff = (expbackoff + 1) * 3 / 2;
207 if (expbackoff == 6) /* 1, 3, 6, 10, ... */
208 indefinite_init(&info, ident, 0, 'S');
209 if ((rdtsc() >> spin_window_shift) % ncpus != mycpuid) {
210 for (loop = expbackoff; loop; --loop)
216 * If the low bits are zero, try to acquire the exclusive lock
217 * by transfering our high bit reservation to the low bits.
219 * NOTE: Avoid unconditional atomic op by testing ovalue,
220 * otherwise we get cache bus armageddon.
222 * NOTE: We must also ensure that the SHARED bit is cleared.
223 * It is possible for it to wind up being set on a
224 * shared lock override of the EXCLWAIT bits.
226 ovalue = spin->counta;
228 if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0) {
231 nvalue= ((ovalue - SPINLOCK_EXCLWAIT) | 1) &
233 if (atomic_fcmpset_int(&spin->counta, &ovalue, nvalue))
237 if (expbackoff > 6 + spin_backoff_max)
238 expbackoff = 6 + spin_backoff_max;
239 if (expbackoff >= 6) {
240 if (indefinite_check(&info))
245 indefinite_done(&info);
249 * The spin_lock_shared() inline was unable to acquire the lock and calls
250 * this function with spin->counta already incremented.
252 * This is not in the critical path unless there is contention between
253 * shared and exclusive holders.
255 * Exclusive locks have priority over shared locks. However, this can
256 * cause shared locks to be starved when large numbers of threads are
257 * competing for exclusive locks so the shared lock code uses TSC-windowing
258 * to selectively ignore the exclusive priority mechanism. This has the
259 * effect of allowing a limited number of shared locks to compete against
260 * exclusive waiters at any given moment.
262 * Note that shared locks do not implement exponential backoff. Instead,
263 * the shared lock simply polls the lock value. One cpu_pause() is built
264 * into indefinite_check().
267 _spin_lock_shared_contested(struct spinlock *spin, const char *ident)
269 indefinite_info_t info;
273 * Undo the inline's increment.
275 ovalue = atomic_fetchadd_int(&spin->counta, -1) - 1;
277 indefinite_init(&info, ident, 0, 's');
280 #ifdef DEBUG_LOCKS_LATENCY
282 for (j = spinlocks_add_latency; j > 0; --j)
288 * Loop until we can acquire the shared spinlock. Note that
289 * the low bits can be zero while the high EXCLWAIT bits are
290 * non-zero. In this situation exclusive requesters have
291 * priority (otherwise shared users on multiple cpus can hog
294 * NOTE: Reading spin->counta prior to the swap is extremely
295 * important on multi-chip/many-core boxes. On 48-core
296 * this one change improves fully concurrent all-cores
297 * compiles by 100% or better.
299 * I can't emphasize enough how important the pre-read
300 * is in preventing hw cache bus armageddon on
301 * multi-chip systems. And on single-chip/multi-core
302 * systems it just doesn't hurt.
307 * Ignore the EXCLWAIT bits if we are inside our window.
309 if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0 &&
310 (rdtsc() >> spin_window_shift) % ncpus == mycpuid) {
311 if (atomic_fcmpset_int(&spin->counta, &ovalue,
312 ovalue | SPINLOCK_SHARED | 1)) {
319 * Check ovalue tightly (no exponential backoff for shared
320 * locks, that would result in horrible performance. Instead,
321 * shared locks depend on the exclusive priority mechanism
322 * to avoid starving exclusive locks).
325 if (atomic_fcmpset_int(&spin->counta, &ovalue,
326 SPINLOCK_SHARED | 1)) {
333 * If SHARED is already set, go for the increment, improving
334 * the exclusive to multiple-readers transition.
336 if (ovalue & SPINLOCK_SHARED) {
337 ovalue = atomic_fetchadd_int(&spin->counta, 1);
338 /* ovalue += 1; NOT NEEDED */
339 if (ovalue & SPINLOCK_SHARED)
341 ovalue = atomic_fetchadd_int(&spin->counta, -1);
345 if (indefinite_check(&info))
348 * ovalue was wrong anyway, just reload
350 ovalue = spin->counta;
352 indefinite_done(&info);
356 * If INVARIANTS is enabled various spinlock timing tests can be run
357 * by setting debug.spin_lock_test:
359 * 1 Test the indefinite wait code
360 * 2 Time the best-case exclusive lock overhead (spin_test_count)
361 * 3 Time the best-case shared lock overhead (spin_test_count)
366 static int spin_test_count = 10000000;
367 SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0,
368 "Number of iterations to use for spinlock wait code test");
371 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS)
373 struct spinlock spin;
378 if ((error = priv_check(curthread, PRIV_ROOT)) != 0)
380 if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0)
384 * Indefinite wait test
387 spin_init(&spin, "sysctllock");
388 spin_lock(&spin); /* force an indefinite wait */
389 spin_lock_test_mode = 1;
391 spin_unlock(&spin); /* Clean up the spinlock count */
393 spin_lock_test_mode = 0;
397 * Time best-case exclusive spinlocks
400 globaldata_t gd = mycpu;
402 spin_init(&spin, "sysctllocktest");
403 for (i = spin_test_count; i > 0; --i) {
404 _spin_lock_quick(gd, &spin, "test");
405 spin_unlock_quick(gd, &spin);
412 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT,
413 0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code");
415 #endif /* INVARIANTS */