2 * Copyright (c) 2003,2004,2009 The DragonFly Project. All rights reserved.
4 * This code is derived from software contributed to The DragonFly Project
5 * by Matthew Dillon <dillon@backplane.com>
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
11 * 1. Redistributions of source code must retain the above copyright
12 * notice, this list of conditions and the following disclaimer.
13 * 2. Redistributions in binary form must reproduce the above copyright
14 * notice, this list of conditions and the following disclaimer in
15 * the documentation and/or other materials provided with the
17 * 3. Neither the name of The DragonFly Project nor the names of its
18 * contributors may be used to endorse or promote products derived
19 * from this software without specific, prior written permission.
21 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
22 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
23 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
24 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
25 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
26 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
27 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
28 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
29 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
30 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
31 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
36 * lwkt_token - Implement soft token locks.
38 * Tokens are locks which serialize a thread only while the thread is
39 * running. If the thread blocks all tokens are released, then reacquired
40 * when the thread resumes.
42 * This implementation requires no critical sections or spin locks, but
43 * does use atomic_cmpset_ptr().
45 * Tokens may be recursively acquired by the same thread. However the
46 * caller must be sure to release such tokens in reverse order.
48 #include <sys/param.h>
49 #include <sys/systm.h>
50 #include <sys/kernel.h>
52 #include <sys/rtprio.h>
53 #include <sys/queue.h>
54 #include <sys/sysctl.h>
56 #include <sys/kthread.h>
57 #include <machine/cpu.h>
59 #include <sys/spinlock.h>
61 #include <sys/thread2.h>
62 #include <sys/spinlock2.h>
63 #include <sys/mplock2.h>
66 #include <vm/vm_param.h>
67 #include <vm/vm_kern.h>
68 #include <vm/vm_object.h>
69 #include <vm/vm_page.h>
70 #include <vm/vm_map.h>
71 #include <vm/vm_pager.h>
72 #include <vm/vm_extern.h>
73 #include <vm/vm_zone.h>
75 #include <machine/stdarg.h>
76 #include <machine/smp.h>
83 extern int lwkt_sched_debug;
85 #ifndef LWKT_NUM_POOL_TOKENS
86 #define LWKT_NUM_POOL_TOKENS 4001 /* prime number */
89 static lwkt_token pool_tokens[LWKT_NUM_POOL_TOKENS];
90 struct spinlock tok_debug_spin = SPINLOCK_INITIALIZER(&tok_debug_spin, "tok_debug_spin");
92 #define TOKEN_STRING "REF=%p TOK=%p TD=%p"
93 #define TOKEN_ARGS lwkt_tokref_t ref, lwkt_token_t tok, struct thread *td
94 #define CONTENDED_STRING TOKEN_STRING " (contention started)"
95 #define UNCONTENDED_STRING TOKEN_STRING " (contention stopped)"
96 #if !defined(KTR_TOKENS)
97 #define KTR_TOKENS KTR_ALL
100 KTR_INFO_MASTER(tokens);
101 KTR_INFO(KTR_TOKENS, tokens, fail, 0, TOKEN_STRING, TOKEN_ARGS);
102 KTR_INFO(KTR_TOKENS, tokens, succ, 1, TOKEN_STRING, TOKEN_ARGS);
104 KTR_INFO(KTR_TOKENS, tokens, release, 2, TOKEN_STRING, TOKEN_ARGS);
105 KTR_INFO(KTR_TOKENS, tokens, remote, 3, TOKEN_STRING, TOKEN_ARGS);
106 KTR_INFO(KTR_TOKENS, tokens, reqremote, 4, TOKEN_STRING, TOKEN_ARGS);
107 KTR_INFO(KTR_TOKENS, tokens, reqfail, 5, TOKEN_STRING, TOKEN_ARGS);
108 KTR_INFO(KTR_TOKENS, tokens, drain, 6, TOKEN_STRING, TOKEN_ARGS);
109 KTR_INFO(KTR_TOKENS, tokens, contention_start, 7, CONTENDED_STRING, TOKEN_ARGS);
110 KTR_INFO(KTR_TOKENS, tokens, contention_stop, 7, UNCONTENDED_STRING, TOKEN_ARGS);
113 #define logtoken(name, ref) \
114 KTR_LOG(tokens_ ## name, ref, ref->tr_tok, curthread)
117 * Global tokens. These replace the MP lock for major subsystem locking.
118 * These tokens are initially used to lockup both global and individual
121 * Once individual structures get their own locks these tokens are used
122 * only to protect global lists & other variables and to interlock
123 * allocations and teardowns and such.
125 * The UP initializer causes token acquisition to also acquire the MP lock
126 * for maximum compatibility. The feature may be enabled and disabled at
127 * any time, the MP state is copied to the tokref when the token is acquired
128 * and will not race against sysctl changes.
130 struct lwkt_token mp_token = LWKT_TOKEN_INITIALIZER(mp_token);
131 struct lwkt_token pmap_token = LWKT_TOKEN_INITIALIZER(pmap_token);
132 struct lwkt_token dev_token = LWKT_TOKEN_INITIALIZER(dev_token);
133 struct lwkt_token vm_token = LWKT_TOKEN_INITIALIZER(vm_token);
134 struct lwkt_token vmspace_token = LWKT_TOKEN_INITIALIZER(vmspace_token);
135 struct lwkt_token kvm_token = LWKT_TOKEN_INITIALIZER(kvm_token);
136 struct lwkt_token sigio_token = LWKT_TOKEN_INITIALIZER(sigio_token);
137 struct lwkt_token tty_token = LWKT_TOKEN_INITIALIZER(tty_token);
138 struct lwkt_token vnode_token = LWKT_TOKEN_INITIALIZER(vnode_token);
140 static int lwkt_token_spin = 5;
141 SYSCTL_INT(_lwkt, OID_AUTO, token_spin, CTLFLAG_RW,
142 &lwkt_token_spin, 0, "Decontention spin loops");
143 static int lwkt_token_delay = 0;
144 SYSCTL_INT(_lwkt, OID_AUTO, token_delay, CTLFLAG_RW,
145 &lwkt_token_delay, 0, "Decontention spin delay in ns");
148 * The collision count is bumped every time the LWKT scheduler fails
149 * to acquire needed tokens in addition to a normal lwkt_gettoken()
152 SYSCTL_LONG(_lwkt, OID_AUTO, mp_collisions, CTLFLAG_RW,
153 &mp_token.t_collisions, 0, "Collision counter of mp_token");
154 SYSCTL_LONG(_lwkt, OID_AUTO, pmap_collisions, CTLFLAG_RW,
155 &pmap_token.t_collisions, 0, "Collision counter of pmap_token");
156 SYSCTL_LONG(_lwkt, OID_AUTO, dev_collisions, CTLFLAG_RW,
157 &dev_token.t_collisions, 0, "Collision counter of dev_token");
158 SYSCTL_LONG(_lwkt, OID_AUTO, vm_collisions, CTLFLAG_RW,
159 &vm_token.t_collisions, 0, "Collision counter of vm_token");
160 SYSCTL_LONG(_lwkt, OID_AUTO, vmspace_collisions, CTLFLAG_RW,
161 &vmspace_token.t_collisions, 0, "Collision counter of vmspace_token");
162 SYSCTL_LONG(_lwkt, OID_AUTO, kvm_collisions, CTLFLAG_RW,
163 &kvm_token.t_collisions, 0, "Collision counter of kvm_token");
164 SYSCTL_LONG(_lwkt, OID_AUTO, sigio_collisions, CTLFLAG_RW,
165 &sigio_token.t_collisions, 0, "Collision counter of sigio_token");
166 SYSCTL_LONG(_lwkt, OID_AUTO, tty_collisions, CTLFLAG_RW,
167 &tty_token.t_collisions, 0, "Collision counter of tty_token");
168 SYSCTL_LONG(_lwkt, OID_AUTO, vnode_collisions, CTLFLAG_RW,
169 &vnode_token.t_collisions, 0, "Collision counter of vnode_token");
171 int tokens_debug_output;
172 SYSCTL_INT(_lwkt, OID_AUTO, tokens_debug_output, CTLFLAG_RW,
173 &tokens_debug_output, 0, "Generate stack trace N times");
176 #ifdef DEBUG_LOCKS_LATENCY
178 static long tokens_add_latency;
179 SYSCTL_LONG(_debug, OID_AUTO, tokens_add_latency, CTLFLAG_RW,
180 &tokens_add_latency, 0,
181 "Add spinlock latency");
186 static int _lwkt_getalltokens_sorted(thread_t td);
189 * Acquire the initial mplock
191 * (low level boot only)
194 cpu_get_initial_mplock(void)
196 KKASSERT(mp_token.t_ref == NULL);
197 if (lwkt_trytoken(&mp_token) == FALSE)
198 panic("cpu_get_initial_mplock");
202 * Return a pool token given an address. Use a prime number to reduce
207 _lwkt_token_pool_lookup(void *ptr)
211 i = (u_int)(uintptr_t)ptr % LWKT_NUM_POOL_TOKENS;
212 return(&pool_tokens[i]);
216 * Initialize a tokref_t prior to making it visible in the thread's
221 _lwkt_tokref_init(lwkt_tokref_t ref, lwkt_token_t tok, thread_t td, long excl)
224 ref->tr_count = excl;
229 * Attempt to acquire a shared or exclusive token. Returns TRUE on success,
232 * If TOK_EXCLUSIVE is set in mode we are attempting to get an exclusive
233 * token, otherwise are attempting to get a shared token.
235 * If TOK_EXCLREQ is set in mode this is a blocking operation, otherwise
236 * it is a non-blocking operation (for both exclusive or shared acquisions).
240 _lwkt_trytokref(lwkt_tokref_t ref, thread_t td, long mode)
247 KASSERT(((mode & TOK_EXCLREQ) == 0 || /* non blocking */
248 td->td_gd->gd_intr_nesting_level == 0 ||
249 panic_cpu_gd == mycpu),
250 ("Attempt to acquire token %p not already "
251 "held in hard code section", tok));
253 if (mode & TOK_EXCLUSIVE) {
255 * Attempt to get an exclusive token
258 count = tok->t_count;
259 oref = tok->t_ref; /* can be NULL */
261 if ((count & ~TOK_EXCLREQ) == 0) {
263 * It is possible to get the exclusive bit.
264 * We must clear TOK_EXCLREQ on successful
267 if (atomic_cmpset_long(&tok->t_count, count,
268 (count & ~TOK_EXCLREQ) |
270 KKASSERT(tok->t_ref == NULL);
275 } else if ((count & TOK_EXCLUSIVE) &&
276 oref >= &td->td_toks_base &&
277 oref < td->td_toks_stop) {
279 * Our thread already holds the exclusive
280 * bit, we treat this tokref as a shared
281 * token (sorta) to make the token release
284 * NOTE: oref cannot race above if it
285 * happens to be ours, so we're good.
286 * But we must still have a stable
287 * variable for both parts of the
290 * NOTE: Since we already have an exclusive
291 * lock and don't need to check EXCLREQ
292 * we can just use an atomic_add here
294 atomic_add_long(&tok->t_count, TOK_INCR);
295 ref->tr_count &= ~TOK_EXCLUSIVE;
297 } else if ((mode & TOK_EXCLREQ) &&
298 (count & TOK_EXCLREQ) == 0) {
300 * Unable to get the exclusive bit but being
301 * asked to set the exclusive-request bit.
302 * Since we are going to retry anyway just
303 * set the bit unconditionally.
305 atomic_set_long(&tok->t_count, TOK_EXCLREQ);
309 * Unable to get the exclusive bit and not
310 * being asked to set the exclusive-request
311 * (aka lwkt_trytoken()), or EXCLREQ was
321 * Attempt to get a shared token. Note that TOK_EXCLREQ
322 * for shared tokens simply means the caller intends to
323 * block. We never actually set the bit in tok->t_count.
326 count = tok->t_count;
327 oref = tok->t_ref; /* can be NULL */
329 if ((count & (TOK_EXCLUSIVE/*|TOK_EXCLREQ*/)) == 0) {
330 /* XXX EXCLREQ should work */
332 * It is possible to get the token shared.
334 if (atomic_cmpset_long(&tok->t_count, count,
339 } else if ((count & TOK_EXCLUSIVE) &&
340 oref >= &td->td_toks_base &&
341 oref < td->td_toks_stop) {
343 * We own the exclusive bit on the token so
344 * we can in fact also get it shared.
346 atomic_add_long(&tok->t_count, TOK_INCR);
350 * We failed to get the token shared
361 _lwkt_trytokref_spin(lwkt_tokref_t ref, thread_t td, long mode)
365 if (_lwkt_trytokref(ref, td, mode)) {
366 #ifdef DEBUG_LOCKS_LATENCY
368 for (j = tokens_add_latency; j > 0; --j)
373 for (spin = lwkt_token_spin; spin > 0; --spin) {
374 if (lwkt_token_delay)
375 tsc_delay(lwkt_token_delay);
378 if (_lwkt_trytokref(ref, td, mode)) {
379 #ifdef DEBUG_LOCKS_LATENCY
381 for (j = tokens_add_latency; j > 0; --j)
391 * Release a token that we hold.
395 _lwkt_reltokref(lwkt_tokref_t ref, thread_t td)
402 count = tok->t_count;
404 if (tok->t_ref == ref) {
406 * We are an exclusive holder. We must clear tr_ref
407 * before we clear the TOK_EXCLUSIVE bit. If we are
408 * unable to clear the bit we must restore
411 KKASSERT(count & TOK_EXCLUSIVE);
413 if (atomic_cmpset_long(&tok->t_count, count,
414 count & ~TOK_EXCLUSIVE)) {
421 * We are a shared holder
423 KKASSERT(count & TOK_COUNTMASK);
424 if (atomic_cmpset_long(&tok->t_count, count,
435 * Obtain all the tokens required by the specified thread on the current
436 * cpu, return 0 on failure and non-zero on success. If a failure occurs
437 * any partially acquired tokens will be released prior to return.
439 * lwkt_getalltokens is called by the LWKT scheduler to re-acquire all
440 * tokens that the thread had to release when it switched away.
442 * If spinning is non-zero this function acquires the tokens in a particular
443 * order to deal with potential deadlocks. We simply use address order for
446 * Called from a critical section.
449 lwkt_getalltokens(thread_t td, int spinning)
455 return(_lwkt_getalltokens_sorted(td));
458 * Acquire tokens in forward order, assign or validate tok->t_ref.
460 for (scan = &td->td_toks_base; scan < td->td_toks_stop; ++scan) {
464 * Only try really hard on the last token
466 if (scan == td->td_toks_stop - 1) {
467 if (_lwkt_trytokref_spin(scan, td, scan->tr_count))
470 if (_lwkt_trytokref(scan, td, scan->tr_count))
475 * Otherwise we failed to acquire all the tokens.
476 * Release whatever we did get.
479 ("token %p is not initialized", tok));
480 strncpy(td->td_gd->gd_cnt.v_lock_name,
482 sizeof(td->td_gd->gd_cnt.v_lock_name) - 1);
484 if (lwkt_sched_debug > 0) {
486 kprintf("toka %p %s %s\n",
487 tok, tok->t_desc, td->td_comm);
489 td->td_wmesg = tok->t_desc;
491 while (--scan >= &td->td_toks_base)
492 _lwkt_reltokref(scan, td);
500 * Release all tokens owned by the specified thread on the current cpu.
502 * This code is really simple. Even in cases where we own all the tokens
503 * note that t_ref may not match the scan for recursively held tokens which
504 * are held deeper in the stack, or for the case where a lwkt_getalltokens()
507 * Tokens are released in reverse order to reduce chasing race failures.
509 * Called from a critical section.
512 lwkt_relalltokens(thread_t td)
517 * Weird order is to try to avoid a panic loop
519 if (td->td_toks_have) {
520 scan = td->td_toks_have;
521 td->td_toks_have = NULL;
523 scan = td->td_toks_stop;
525 while (--scan >= &td->td_toks_base)
526 _lwkt_reltokref(scan, td);
530 * This is the decontention version of lwkt_getalltokens(). The tokens are
531 * acquired in address-sorted order to deal with any deadlocks. Ultimately
532 * token failures will spin into the scheduler and get here.
534 * Called from critical section
538 _lwkt_getalltokens_sorted(thread_t td)
540 lwkt_tokref_t sort_array[LWKT_MAXTOKENS];
548 * Sort the token array. Yah yah, I know this isn't fun.
550 * NOTE: Recursively acquired tokens are ordered the same as in the
551 * td_toks_array so we can always get the earliest one first.
554 scan = &td->td_toks_base;
555 while (scan < td->td_toks_stop) {
556 for (j = 0; j < i; ++j) {
557 if (scan->tr_tok < sort_array[j]->tr_tok)
561 bcopy(sort_array + j, sort_array + j + 1,
562 (i - j) * sizeof(lwkt_tokref_t));
564 sort_array[j] = scan;
571 * Acquire tokens in forward order, assign or validate tok->t_ref.
573 for (i = 0; i < n; ++i) {
574 scan = sort_array[i];
578 * Only try really hard on the last token
580 if (scan == td->td_toks_stop - 1) {
581 if (_lwkt_trytokref_spin(scan, td, scan->tr_count))
584 if (_lwkt_trytokref(scan, td, scan->tr_count))
589 * Otherwise we failed to acquire all the tokens.
590 * Release whatever we did get.
592 if (lwkt_sched_debug > 0) {
594 kprintf("tokb %p %s %s\n",
595 tok, tok->t_desc, td->td_comm);
597 td->td_wmesg = tok->t_desc;
600 scan = sort_array[i];
601 _lwkt_reltokref(scan, td);
608 * We were successful, there is no need for another core to signal
615 * Get a serializing token. This routine can block.
618 lwkt_gettoken(lwkt_token_t tok)
620 thread_t td = curthread;
623 ref = td->td_toks_stop;
624 KKASSERT(ref < &td->td_toks_end);
627 _lwkt_tokref_init(ref, tok, td, TOK_EXCLUSIVE|TOK_EXCLREQ);
631 * Taking an exclusive token after holding it shared will
632 * livelock. Scan for that case and assert.
636 for (tk = &td->td_toks_base; tk < ref; tk++) {
637 if (tk->tr_tok != tok)
641 if (tk->tr_count & TOK_EXCLUSIVE)
644 /* We found only shared instances of this token if found >0 here */
645 KASSERT((found == 0), ("Token %p s/x livelock", tok));
649 if (_lwkt_trytokref_spin(ref, td, TOK_EXCLUSIVE|TOK_EXCLREQ))
653 * Give up running if we can't acquire the token right now.
655 * Since the tokref is already active the scheduler now
656 * takes care of acquisition, so we need only call
659 * Since we failed this was not a recursive token so upon
660 * return tr_tok->t_ref should be assigned to this specific
663 td->td_wmesg = tok->t_desc;
666 td->td_toks_have = td->td_toks_stop - 1;
668 if (tokens_debug_output > 0) {
669 --tokens_debug_output;
670 spin_lock(&tok_debug_spin);
671 kprintf("Excl Token thread %p %s %s\n",
672 td, tok->t_desc, td->td_comm);
675 spin_unlock(&tok_debug_spin);
680 KKASSERT(tok->t_ref == ref);
684 * Similar to gettoken but we acquire a shared token instead of an exclusive
688 lwkt_gettoken_shared(lwkt_token_t tok)
690 thread_t td = curthread;
693 ref = td->td_toks_stop;
694 KKASSERT(ref < &td->td_toks_end);
697 _lwkt_tokref_init(ref, tok, td, TOK_EXCLREQ);
701 * Taking a pool token in shared mode is a bad idea; other
702 * addresses deeper in the call stack may hash to the same pool
703 * token and you may end up with an exclusive-shared livelock.
704 * Warn in this condition.
706 if ((tok >= &pool_tokens[0]) &&
707 (tok < &pool_tokens[LWKT_NUM_POOL_TOKENS]))
708 kprintf("Warning! Taking pool token %p in shared mode\n", tok);
712 if (_lwkt_trytokref_spin(ref, td, TOK_EXCLREQ))
716 * Give up running if we can't acquire the token right now.
718 * Since the tokref is already active the scheduler now
719 * takes care of acquisition, so we need only call
722 * Since we failed this was not a recursive token so upon
723 * return tr_tok->t_ref should be assigned to this specific
726 td->td_wmesg = tok->t_desc;
729 td->td_toks_have = td->td_toks_stop - 1;
731 if (tokens_debug_output > 0) {
732 --tokens_debug_output;
733 spin_lock(&tok_debug_spin);
734 kprintf("Shar Token thread %p %s %s\n",
735 td, tok->t_desc, td->td_comm);
738 spin_unlock(&tok_debug_spin);
746 * Attempt to acquire a token, return TRUE on success, FALSE on failure.
748 * We setup the tokref in case we actually get the token (if we switch later
749 * it becomes mandatory so we set TOK_EXCLREQ), but we call trytokref without
750 * TOK_EXCLREQ in case we fail.
753 lwkt_trytoken(lwkt_token_t tok)
755 thread_t td = curthread;
758 ref = td->td_toks_stop;
759 KKASSERT(ref < &td->td_toks_end);
762 _lwkt_tokref_init(ref, tok, td, TOK_EXCLUSIVE|TOK_EXCLREQ);
764 if (_lwkt_trytokref(ref, td, TOK_EXCLUSIVE))
768 * Failed, unpend the request
778 lwkt_gettoken_hard(lwkt_token_t tok)
785 lwkt_getpooltoken(void *ptr)
789 tok = _lwkt_token_pool_lookup(ptr);
795 * Release a serializing token.
797 * WARNING! All tokens must be released in reverse order. This will be
801 lwkt_reltoken(lwkt_token_t tok)
803 thread_t td = curthread;
807 * Remove ref from thread token list and assert that it matches
808 * the token passed in. Tokens must be released in reverse order.
810 ref = td->td_toks_stop - 1;
811 KKASSERT(ref >= &td->td_toks_base && ref->tr_tok == tok);
812 _lwkt_reltokref(ref, td);
814 td->td_toks_stop = ref;
818 lwkt_reltoken_hard(lwkt_token_t tok)
825 * It is faster for users of lwkt_getpooltoken() to use the returned
826 * token and just call lwkt_reltoken(), but for convenience we provide
827 * this function which looks the token up based on the ident.
830 lwkt_relpooltoken(void *ptr)
832 lwkt_token_t tok = _lwkt_token_pool_lookup(ptr);
837 * Return a count of the number of token refs the thread has to the
838 * specified token, whether it currently owns the token or not.
841 lwkt_cnttoken(lwkt_token_t tok, thread_t td)
846 for (scan = &td->td_toks_base; scan < td->td_toks_stop; ++scan) {
847 if (scan->tr_tok == tok)
854 * Pool tokens are used to provide a type-stable serializing token
855 * pointer that does not race against disappearing data structures.
857 * This routine is called in early boot just after we setup the BSP's
858 * globaldata structure.
861 lwkt_token_pool_init(void)
865 for (i = 0; i < LWKT_NUM_POOL_TOKENS; ++i)
866 lwkt_token_init(&pool_tokens[i], "pool");
870 lwkt_token_pool_lookup(void *ptr)
872 return (_lwkt_token_pool_lookup(ptr));
876 * Initialize a token.
879 lwkt_token_init(lwkt_token_t tok, const char *desc)
883 tok->t_collisions = 0;
888 lwkt_token_uninit(lwkt_token_t tok)
894 * Exchange the two most recent tokens on the tokref stack. This allows
895 * you to release a token out of order.
897 * We have to be careful about the case where the top two tokens are
898 * the same token. In this case tok->t_ref will point to the deeper
899 * ref and must remain pointing to the deeper ref. If we were to swap
900 * it the first release would clear the token even though a second
901 * ref is still present.
903 * Only exclusively held tokens contain a reference to the tokref which
904 * has to be flipped along with the swap.
907 lwkt_token_swap(void)
909 lwkt_tokref_t ref1, ref2;
910 lwkt_token_t tok1, tok2;
912 thread_t td = curthread;
916 ref1 = td->td_toks_stop - 1;
917 ref2 = td->td_toks_stop - 2;
918 KKASSERT(ref1 >= &td->td_toks_base);
919 KKASSERT(ref2 >= &td->td_toks_base);
923 count1 = ref1->tr_count;
924 count2 = ref2->tr_count;
928 ref1->tr_count = count2;
930 ref2->tr_count = count1;
931 if (tok1->t_ref == ref1)
933 if (tok2->t_ref == ref2)
941 DB_SHOW_COMMAND(tokens, db_tok_all)
943 struct lwkt_token *tok, **ptr;
944 struct lwkt_token *toklist[16] = {
958 for (tok = *ptr; tok; tok = *(++ptr)) {
959 db_printf("tok=%p tr_owner=%p t_colissions=%ld t_desc=%s\n", tok,
960 (tok->t_ref ? tok->t_ref->tr_owner : NULL),
961 tok->t_collisions, tok->t_desc);