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 #ifdef _KERNEL_VIRTUAL
74 struct spinlock pmap_spin = SPINLOCK_INITIALIZER(pmap_spin, "pmap_spin");
79 #if !defined(KTR_SPIN_CONTENTION)
80 #define KTR_SPIN_CONTENTION KTR_ALL
82 #define SPIN_STRING "spin=%p type=%c"
83 #define SPIN_ARG_SIZE (sizeof(void *) + sizeof(int))
85 KTR_INFO_MASTER(spin);
87 KTR_INFO(KTR_SPIN_CONTENTION, spin, beg, 0, SPIN_STRING, SPIN_ARG_SIZE);
88 KTR_INFO(KTR_SPIN_CONTENTION, spin, end, 1, SPIN_STRING, SPIN_ARG_SIZE);
91 #define logspin(name, spin, type) \
92 KTR_LOG(spin_ ## name, spin, type)
95 static int spin_lock_test_mode;
98 #ifdef DEBUG_LOCKS_LATENCY
100 __read_frequently static long spinlocks_add_latency;
101 SYSCTL_LONG(_debug, OID_AUTO, spinlocks_add_latency, CTLFLAG_RW,
102 &spinlocks_add_latency, 0,
103 "Add spinlock latency");
107 __read_frequently static long spin_backoff_max = 4096;
108 SYSCTL_LONG(_debug, OID_AUTO, spin_backoff_max, CTLFLAG_RW,
109 &spin_backoff_max, 0,
110 "Spinlock exponential backoff limit");
112 /* 1 << n clock cycles, approx */
113 __read_frequently static long spin_window_shift = 8;
114 SYSCTL_LONG(_debug, OID_AUTO, spin_window_shift, CTLFLAG_RW,
115 &spin_window_shift, 0,
116 "Spinlock TSC windowing");
119 * We contested due to another exclusive lock holder. We lose.
121 * We have to unwind the attempt and may acquire the spinlock
122 * anyway while doing so.
125 spin_trylock_contested(struct spinlock *spin)
127 globaldata_t gd = mycpu;
130 * Handle degenerate case, else fail.
132 if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED|0, 1))
134 /*atomic_add_int(&spin->counta, -1);*/
136 crit_exit_raw(gd->gd_curthread);
142 * The spin_lock() inline was unable to acquire the lock and calls this
143 * function with spin->counta already incremented, passing (spin->counta - 1)
144 * to the function (the result of the inline's fetchadd).
146 * Note that we implement both exclusive and shared spinlocks, so we cannot
147 * use atomic_swap_int(). Instead, we try to use atomic_fetchadd_int()
148 * to put most of the burden on the cpu. Atomic_cmpset_int() (cmpxchg)
149 * can cause a lot of unnecessary looping in situations where it is just
150 * trying to increment the count.
152 * Similarly, we leave the SHARED flag intact and incur slightly more
153 * overhead when switching from shared to exclusive. This allows us to
154 * use atomic_fetchadd_int() for both spinlock types in the critical
157 * The exponential (n^1.5) backoff algorithm is designed to both reduce
158 * cache bus contention between cpu cores and sockets, and to allow some
159 * bursting of exclusive locks in heavily contended situations to improve
162 * The exclusive lock priority mechanism prevents even heavily contended
163 * exclusive locks from being starved by shared locks
166 _spin_lock_contested(struct spinlock *spin, const char *ident, int value)
168 indefinite_info_t info;
174 * WARNING! Caller has already incremented the lock. We must
175 * increment the count value (from the inline's fetch-add)
178 * Handle the degenerate case where the spinlock is flagged SHARED
179 * with only our reference. We can convert it to EXCLUSIVE.
181 if (value == (SPINLOCK_SHARED | 1) - 1) {
182 if (atomic_cmpset_int(&spin->counta, SPINLOCK_SHARED | 1, 1))
185 /* ++value; value not used after this */
186 info.type = 0; /* avoid improper gcc warning */
187 info.ident = NULL; /* avoid improper gcc warning */
191 * Transfer our exclusive request to the high bits and clear the
192 * SPINLOCK_SHARED bit if it was set. This makes the spinlock
193 * appear exclusive, preventing any NEW shared or exclusive
194 * spinlocks from being obtained while we wait for existing
195 * shared or exclusive holders to unlock.
197 * Don't tread on earlier exclusive waiters by stealing the lock
198 * away early if the low bits happen to now be 1.
200 * The shared unlock understands that this may occur.
202 ovalue = atomic_fetchadd_int(&spin->counta, SPINLOCK_EXCLWAIT - 1);
203 ovalue += SPINLOCK_EXCLWAIT - 1;
204 if (ovalue & SPINLOCK_SHARED) {
205 atomic_clear_int(&spin->counta, SPINLOCK_SHARED);
206 ovalue &= ~SPINLOCK_SHARED;
210 expbackoff = (expbackoff + 1) * 3 / 2;
211 if (expbackoff == 6) /* 1, 3, 6, 10, ... */
212 indefinite_init(&info, ident, 0, 'S');
213 if ((rdtsc() >> spin_window_shift) % ncpus != mycpuid) {
214 for (loop = expbackoff; loop; --loop)
220 * If the low bits are zero, try to acquire the exclusive lock
221 * by transfering our high bit reservation to the low bits.
223 * NOTE: Avoid unconditional atomic op by testing ovalue,
224 * otherwise we get cache bus armageddon.
226 * NOTE: We must also ensure that the SHARED bit is cleared.
227 * It is possible for it to wind up being set on a
228 * shared lock override of the EXCLWAIT bits.
230 ovalue = spin->counta;
232 if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0) {
235 nvalue= ((ovalue - SPINLOCK_EXCLWAIT) | 1) &
237 if (atomic_fcmpset_int(&spin->counta, &ovalue, nvalue))
241 if (expbackoff > 6 + spin_backoff_max)
242 expbackoff = 6 + spin_backoff_max;
243 if (expbackoff >= 6) {
244 if (indefinite_check(&info))
249 indefinite_done(&info);
253 * The spin_lock_shared() inline was unable to acquire the lock and calls
254 * this function with spin->counta already incremented.
256 * This is not in the critical path unless there is contention between
257 * shared and exclusive holders.
259 * Exclusive locks have priority over shared locks. However, this can
260 * cause shared locks to be starved when large numbers of threads are
261 * competing for exclusive locks so the shared lock code uses TSC-windowing
262 * to selectively ignore the exclusive priority mechanism. This has the
263 * effect of allowing a limited number of shared locks to compete against
264 * exclusive waiters at any given moment.
266 * Note that shared locks do not implement exponential backoff. Instead,
267 * the shared lock simply polls the lock value. One cpu_pause() is built
268 * into indefinite_check().
271 _spin_lock_shared_contested(struct spinlock *spin, const char *ident)
273 indefinite_info_t info;
277 * Undo the inline's increment.
279 ovalue = atomic_fetchadd_int(&spin->counta, -1) - 1;
281 indefinite_init(&info, ident, 0, 's');
284 #ifdef DEBUG_LOCKS_LATENCY
286 for (j = spinlocks_add_latency; j > 0; --j)
292 * Loop until we can acquire the shared spinlock. Note that
293 * the low bits can be zero while the high EXCLWAIT bits are
294 * non-zero. In this situation exclusive requesters have
295 * priority (otherwise shared users on multiple cpus can hog
298 * NOTE: Reading spin->counta prior to the swap is extremely
299 * important on multi-chip/many-core boxes. On 48-core
300 * this one change improves fully concurrent all-cores
301 * compiles by 100% or better.
303 * I can't emphasize enough how important the pre-read
304 * is in preventing hw cache bus armageddon on
305 * multi-chip systems. And on single-chip/multi-core
306 * systems it just doesn't hurt.
311 * Ignore the EXCLWAIT bits if we are inside our window.
313 if ((ovalue & (SPINLOCK_EXCLWAIT - 1)) == 0 &&
314 (rdtsc() >> spin_window_shift) % ncpus == mycpuid) {
315 if (atomic_fcmpset_int(&spin->counta, &ovalue,
316 ovalue | SPINLOCK_SHARED | 1)) {
323 * Check ovalue tightly (no exponential backoff for shared
324 * locks, that would result in horrible performance. Instead,
325 * shared locks depend on the exclusive priority mechanism
326 * to avoid starving exclusive locks).
329 if (atomic_fcmpset_int(&spin->counta, &ovalue,
330 SPINLOCK_SHARED | 1)) {
337 * If SHARED is already set, go for the increment, improving
338 * the exclusive to multiple-readers transition.
340 if (ovalue & SPINLOCK_SHARED) {
341 ovalue = atomic_fetchadd_int(&spin->counta, 1);
342 /* ovalue += 1; NOT NEEDED */
343 if (ovalue & SPINLOCK_SHARED)
345 ovalue = atomic_fetchadd_int(&spin->counta, -1);
349 if (indefinite_check(&info))
352 * ovalue was wrong anyway, just reload
354 ovalue = spin->counta;
356 indefinite_done(&info);
360 * If INVARIANTS is enabled various spinlock timing tests can be run
361 * by setting debug.spin_lock_test:
363 * 1 Test the indefinite wait code
364 * 2 Time the best-case exclusive lock overhead (spin_test_count)
365 * 3 Time the best-case shared lock overhead (spin_test_count)
370 static int spin_test_count = 10000000;
371 SYSCTL_INT(_debug, OID_AUTO, spin_test_count, CTLFLAG_RW, &spin_test_count, 0,
372 "Number of iterations to use for spinlock wait code test");
375 sysctl_spin_lock_test(SYSCTL_HANDLER_ARGS)
377 struct spinlock spin;
382 if ((error = priv_check(curthread, PRIV_ROOT)) != 0)
384 if ((error = SYSCTL_IN(req, &value, sizeof(value))) != 0)
388 * Indefinite wait test
391 spin_init(&spin, "sysctllock");
392 spin_lock(&spin); /* force an indefinite wait */
393 spin_lock_test_mode = 1;
395 spin_unlock(&spin); /* Clean up the spinlock count */
397 spin_lock_test_mode = 0;
401 * Time best-case exclusive spinlocks
404 globaldata_t gd = mycpu;
406 spin_init(&spin, "sysctllocktest");
407 for (i = spin_test_count; i > 0; --i) {
408 _spin_lock_quick(gd, &spin, "test");
409 spin_unlock_quick(gd, &spin);
416 SYSCTL_PROC(_debug, KERN_PROC_ALL, spin_lock_test, CTLFLAG_RW|CTLTYPE_INT,
417 0, 0, sysctl_spin_lock_test, "I", "Test spinlock wait code");
419 #endif /* INVARIANTS */