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>
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8 * modification, are permitted provided that the following conditions
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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 #define LWKT_POOL_TOKENS 16384 /* must be power of 2 */
86 #define LWKT_POOL_MASK (LWKT_POOL_TOKENS - 1)
88 struct lwkt_pool_token {
89 struct lwkt_token token;
92 static struct lwkt_pool_token pool_tokens[LWKT_POOL_TOKENS];
93 struct spinlock tok_debug_spin = SPINLOCK_INITIALIZER(&tok_debug_spin,
96 #define TOKEN_STRING "REF=%p TOK=%p TD=%p"
97 #define TOKEN_ARGS lwkt_tokref_t ref, lwkt_token_t tok, struct thread *td
98 #define CONTENDED_STRING TOKEN_STRING " (contention started)"
99 #define UNCONTENDED_STRING TOKEN_STRING " (contention stopped)"
100 #if !defined(KTR_TOKENS)
101 #define KTR_TOKENS KTR_ALL
104 KTR_INFO_MASTER(tokens);
105 KTR_INFO(KTR_TOKENS, tokens, fail, 0, TOKEN_STRING, TOKEN_ARGS);
106 KTR_INFO(KTR_TOKENS, tokens, succ, 1, TOKEN_STRING, TOKEN_ARGS);
108 KTR_INFO(KTR_TOKENS, tokens, release, 2, TOKEN_STRING, TOKEN_ARGS);
109 KTR_INFO(KTR_TOKENS, tokens, remote, 3, TOKEN_STRING, TOKEN_ARGS);
110 KTR_INFO(KTR_TOKENS, tokens, reqremote, 4, TOKEN_STRING, TOKEN_ARGS);
111 KTR_INFO(KTR_TOKENS, tokens, reqfail, 5, TOKEN_STRING, TOKEN_ARGS);
112 KTR_INFO(KTR_TOKENS, tokens, drain, 6, TOKEN_STRING, TOKEN_ARGS);
113 KTR_INFO(KTR_TOKENS, tokens, contention_start, 7, CONTENDED_STRING, TOKEN_ARGS);
114 KTR_INFO(KTR_TOKENS, tokens, contention_stop, 7, UNCONTENDED_STRING, TOKEN_ARGS);
117 #define logtoken(name, ref) \
118 KTR_LOG(tokens_ ## name, ref, ref->tr_tok, curthread)
121 * Global tokens. These replace the MP lock for major subsystem locking.
122 * These tokens are initially used to lockup both global and individual
125 * Once individual structures get their own locks these tokens are used
126 * only to protect global lists & other variables and to interlock
127 * allocations and teardowns and such.
129 * The UP initializer causes token acquisition to also acquire the MP lock
130 * for maximum compatibility. The feature may be enabled and disabled at
131 * any time, the MP state is copied to the tokref when the token is acquired
132 * and will not race against sysctl changes.
134 struct lwkt_token mp_token = LWKT_TOKEN_INITIALIZER(mp_token);
135 struct lwkt_token pmap_token = LWKT_TOKEN_INITIALIZER(pmap_token);
136 struct lwkt_token dev_token = LWKT_TOKEN_INITIALIZER(dev_token);
137 struct lwkt_token vm_token = LWKT_TOKEN_INITIALIZER(vm_token);
138 struct lwkt_token vmspace_token = LWKT_TOKEN_INITIALIZER(vmspace_token);
139 struct lwkt_token kvm_token = LWKT_TOKEN_INITIALIZER(kvm_token);
140 struct lwkt_token sigio_token = LWKT_TOKEN_INITIALIZER(sigio_token);
141 struct lwkt_token tty_token = LWKT_TOKEN_INITIALIZER(tty_token);
142 struct lwkt_token vnode_token = LWKT_TOKEN_INITIALIZER(vnode_token);
143 struct lwkt_token vga_token = LWKT_TOKEN_INITIALIZER(vga_token);
144 struct lwkt_token kbd_token = LWKT_TOKEN_INITIALIZER(kbd_token);
147 * Exponential backoff (exclusive tokens) and TSC windowing (shared tokens)
148 * parameters. Remember that tokens backoff to the scheduler, large values
151 static int token_backoff_max __cachealign = 4096;
152 SYSCTL_INT(_lwkt, OID_AUTO, token_backoff_max, CTLFLAG_RW,
153 &token_backoff_max, 0, "Tokens exponential backoff");
154 static int token_window_shift __cachealign = 8;
155 SYSCTL_INT(_lwkt, OID_AUTO, token_window_shift, CTLFLAG_RW,
156 &token_window_shift, 0, "Tokens TSC windowing shift");
159 * The collision count is bumped every time the LWKT scheduler fails
160 * to acquire needed tokens in addition to a normal lwkt_gettoken()
163 SYSCTL_LONG(_lwkt, OID_AUTO, mp_collisions, CTLFLAG_RW,
164 &mp_token.t_collisions, 0, "Collision counter of mp_token");
165 SYSCTL_LONG(_lwkt, OID_AUTO, pmap_collisions, CTLFLAG_RW,
166 &pmap_token.t_collisions, 0, "Collision counter of pmap_token");
167 SYSCTL_LONG(_lwkt, OID_AUTO, dev_collisions, CTLFLAG_RW,
168 &dev_token.t_collisions, 0, "Collision counter of dev_token");
169 SYSCTL_LONG(_lwkt, OID_AUTO, vm_collisions, CTLFLAG_RW,
170 &vm_token.t_collisions, 0, "Collision counter of vm_token");
171 SYSCTL_LONG(_lwkt, OID_AUTO, vmspace_collisions, CTLFLAG_RW,
172 &vmspace_token.t_collisions, 0, "Collision counter of vmspace_token");
173 SYSCTL_LONG(_lwkt, OID_AUTO, kvm_collisions, CTLFLAG_RW,
174 &kvm_token.t_collisions, 0, "Collision counter of kvm_token");
175 SYSCTL_LONG(_lwkt, OID_AUTO, sigio_collisions, CTLFLAG_RW,
176 &sigio_token.t_collisions, 0, "Collision counter of sigio_token");
177 SYSCTL_LONG(_lwkt, OID_AUTO, tty_collisions, CTLFLAG_RW,
178 &tty_token.t_collisions, 0, "Collision counter of tty_token");
179 SYSCTL_LONG(_lwkt, OID_AUTO, vnode_collisions, CTLFLAG_RW,
180 &vnode_token.t_collisions, 0, "Collision counter of vnode_token");
182 int tokens_debug_output;
183 SYSCTL_INT(_lwkt, OID_AUTO, tokens_debug_output, CTLFLAG_RW,
184 &tokens_debug_output, 0, "Generate stack trace N times");
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
205 #define POOL_HASH_PRIME1 66555444443333333ULL
206 #define POOL_HASH_PRIME2 989042931893ULL
210 _lwkt_token_pool_lookup(void *ptr)
215 hash1 = (uintptr_t)ptr + ((uintptr_t)ptr >> 18);
216 hash1 %= POOL_HASH_PRIME1;
217 hash2 = ((uintptr_t)ptr >> 8) + ((uintptr_t)ptr >> 24);
218 hash2 %= POOL_HASH_PRIME2;
219 return (&pool_tokens[(hash1 ^ hash2) & LWKT_POOL_MASK].token);
223 * Initialize a tokref_t prior to making it visible in the thread's
228 _lwkt_tokref_init(lwkt_tokref_t ref, lwkt_token_t tok, thread_t td, long excl)
231 ref->tr_count = excl;
236 * Attempt to acquire a shared or exclusive token. Returns TRUE on success,
239 * If TOK_EXCLUSIVE is set in mode we are attempting to get an exclusive
240 * token, otherwise are attempting to get a shared token.
242 * If TOK_EXCLREQ is set in mode this is a blocking operation, otherwise
243 * it is a non-blocking operation (for both exclusive or shared acquisions).
247 _lwkt_trytokref(lwkt_tokref_t ref, thread_t td, long mode)
254 KASSERT(((mode & TOK_EXCLREQ) == 0 || /* non blocking */
255 td->td_gd->gd_intr_nesting_level == 0 ||
256 panic_cpu_gd == mycpu),
257 ("Attempt to acquire token %p not already "
258 "held in hard code section", tok));
260 if (mode & TOK_EXCLUSIVE) {
262 * Attempt to get an exclusive token
264 count = tok->t_count;
267 oref = tok->t_ref; /* can be NULL */
269 if ((count & ~TOK_EXCLREQ) == 0) {
271 * It is possible to get the exclusive bit.
272 * We must clear TOK_EXCLREQ on successful
275 if (atomic_fcmpset_long(&tok->t_count, &count,
276 (count & ~TOK_EXCLREQ) |
278 KKASSERT(tok->t_ref == NULL);
283 } else if ((count & TOK_EXCLUSIVE) &&
284 oref >= &td->td_toks_base &&
285 oref < td->td_toks_stop) {
287 * Our thread already holds the exclusive
288 * bit, we treat this tokref as a shared
289 * token (sorta) to make the token release
290 * code easier. Treating this as a shared
291 * token allows us to simply increment the
294 * NOTE: oref cannot race above if it
295 * happens to be ours, so we're good.
296 * But we must still have a stable
297 * variable for both parts of the
300 * NOTE: Since we already have an exclusive
301 * lock and don't need to check EXCLREQ
302 * we can just use an atomic_add here
304 atomic_add_long(&tok->t_count, TOK_INCR);
305 ref->tr_count &= ~TOK_EXCLUSIVE;
307 } else if ((mode & TOK_EXCLREQ) &&
308 (count & TOK_EXCLREQ) == 0) {
310 * Unable to get the exclusive bit but being
311 * asked to set the exclusive-request bit.
312 * Since we are going to retry anyway just
313 * set the bit unconditionally.
315 atomic_set_long(&tok->t_count, TOK_EXCLREQ);
319 * Unable to get the exclusive bit and not
320 * being asked to set the exclusive-request
321 * (aka lwkt_trytoken()), or EXCLREQ was
331 * Attempt to get a shared token. Note that TOK_EXCLREQ
332 * for shared tokens simply means the caller intends to
333 * block. We never actually set the bit in tok->t_count.
335 * Due to the token's no-deadlock guarantee, and complications
336 * created by the sorted reacquisition code, we can only
337 * give exclusive requests priority over shared requests
338 * in situations where the thread holds only one token.
340 count = tok->t_count;
343 oref = tok->t_ref; /* can be NULL */
345 if ((count & (TOK_EXCLUSIVE|mode)) == 0 ||
346 ((count & TOK_EXCLUSIVE) == 0 &&
347 td->td_toks_stop != &td->td_toks_base + 1)
350 * It may be possible to get the token shared.
352 if ((atomic_fetchadd_long(&tok->t_count, TOK_INCR) & TOK_EXCLUSIVE) == 0) {
355 count = atomic_fetchadd_long(&tok->t_count,
359 } else if ((count & TOK_EXCLUSIVE) &&
360 oref >= &td->td_toks_base &&
361 oref < td->td_toks_stop) {
363 * We own the exclusive bit on the token so
364 * we can in fact also get it shared.
366 atomic_add_long(&tok->t_count, TOK_INCR);
370 * We failed to get the token shared
381 _lwkt_trytokref_spin(lwkt_tokref_t ref, thread_t td, long mode)
383 if (_lwkt_trytokref(ref, td, mode))
386 if (mode & TOK_EXCLUSIVE) {
388 * Contested exclusive token, use exponential backoff
395 while (expbackoff < 6 + token_backoff_max) {
396 expbackoff = (expbackoff + 1) * 3 / 2;
397 if ((rdtsc() >> token_window_shift) % ncpus != mycpuid) {
398 for (loop = expbackoff; loop; --loop)
401 if (_lwkt_trytokref(ref, td, mode))
406 * Contested shared token, use TSC windowing. Note that
407 * exclusive tokens have priority over shared tokens only
408 * for the first token.
410 if ((rdtsc() >> token_window_shift) % ncpus == mycpuid) {
411 if (_lwkt_trytokref(ref, td, mode & ~TOK_EXCLREQ))
414 if (_lwkt_trytokref(ref, td, mode))
419 ++mycpu->gd_cnt.v_lock_colls;
425 * Release a token that we hold.
427 * Since tokens are polled, we don't have to deal with wakeups and releasing
432 _lwkt_reltokref(lwkt_tokref_t ref, thread_t td)
438 if (tok->t_ref == ref) {
440 * We are an exclusive holder. We must clear tr_ref
441 * before we clear the TOK_EXCLUSIVE bit. If we are
442 * unable to clear the bit we must restore
446 KKASSERT(count & TOK_EXCLUSIVE);
449 atomic_clear_long(&tok->t_count, TOK_EXCLUSIVE);
452 * We are a shared holder
454 count = atomic_fetchadd_long(&tok->t_count, -TOK_INCR);
455 KKASSERT(count & TOK_COUNTMASK); /* count prior */
460 * Obtain all the tokens required by the specified thread on the current
461 * cpu, return 0 on failure and non-zero on success. If a failure occurs
462 * any partially acquired tokens will be released prior to return.
464 * lwkt_getalltokens is called by the LWKT scheduler to re-acquire all
465 * tokens that the thread had to release when it switched away.
467 * If spinning is non-zero this function acquires the tokens in a particular
468 * order to deal with potential deadlocks. We simply use address order for
471 * Called from a critical section.
474 lwkt_getalltokens(thread_t td, int spinning)
480 return(_lwkt_getalltokens_sorted(td));
483 * Acquire tokens in forward order, assign or validate tok->t_ref.
485 for (scan = &td->td_toks_base; scan < td->td_toks_stop; ++scan) {
489 * Only try really hard on the last token
491 if (scan == td->td_toks_stop - 1) {
492 if (_lwkt_trytokref_spin(scan, td, scan->tr_count))
495 if (_lwkt_trytokref(scan, td, scan->tr_count))
500 * Otherwise we failed to acquire all the tokens.
501 * Release whatever we did get.
504 ("token %p is not initialized", tok));
505 td->td_gd->gd_cnt.v_lock_name[0] = 't';
506 strncpy(td->td_gd->gd_cnt.v_lock_name + 1,
508 sizeof(td->td_gd->gd_cnt.v_lock_name) - 2);
509 if (lwkt_sched_debug > 0) {
511 kprintf("toka %p %s %s\n",
512 tok, tok->t_desc, td->td_comm);
514 td->td_wmesg = tok->t_desc;
516 while (--scan >= &td->td_toks_base)
517 _lwkt_reltokref(scan, td);
525 * Release all tokens owned by the specified thread on the current cpu.
527 * This code is really simple. Even in cases where we own all the tokens
528 * note that t_ref may not match the scan for recursively held tokens which
529 * are held deeper in the stack, or for the case where a lwkt_getalltokens()
532 * Tokens are released in reverse order to reduce chasing race failures.
534 * Called from a critical section.
537 lwkt_relalltokens(thread_t td)
542 * Weird order is to try to avoid a panic loop
544 if (td->td_toks_have) {
545 scan = td->td_toks_have;
546 td->td_toks_have = NULL;
548 scan = td->td_toks_stop;
550 while (--scan >= &td->td_toks_base)
551 _lwkt_reltokref(scan, td);
555 * This is the decontention version of lwkt_getalltokens(). The tokens are
556 * acquired in address-sorted order to deal with any deadlocks. Ultimately
557 * token failures will spin into the scheduler and get here.
559 * Called from critical section
563 _lwkt_getalltokens_sorted(thread_t td)
565 lwkt_tokref_t sort_array[LWKT_MAXTOKENS];
573 * Sort the token array. Yah yah, I know this isn't fun.
575 * NOTE: Recursively acquired tokens are ordered the same as in the
576 * td_toks_array so we can always get the earliest one first.
577 * This is particularly important when a token is acquired
578 * exclusively multiple times, as only the first acquisition
579 * is treated as an exclusive token.
582 scan = &td->td_toks_base;
583 while (scan < td->td_toks_stop) {
584 for (j = 0; j < i; ++j) {
585 if (scan->tr_tok < sort_array[j]->tr_tok)
589 bcopy(sort_array + j, sort_array + j + 1,
590 (i - j) * sizeof(lwkt_tokref_t));
592 sort_array[j] = scan;
599 * Acquire tokens in forward order, assign or validate tok->t_ref.
601 for (i = 0; i < n; ++i) {
602 scan = sort_array[i];
606 * Only try really hard on the last token
608 if (scan == td->td_toks_stop - 1) {
609 if (_lwkt_trytokref_spin(scan, td, scan->tr_count))
612 if (_lwkt_trytokref(scan, td, scan->tr_count))
617 * Otherwise we failed to acquire all the tokens.
618 * Release whatever we did get.
620 td->td_gd->gd_cnt.v_lock_name[0] = 't';
621 strncpy(td->td_gd->gd_cnt.v_lock_name + 1,
623 sizeof(td->td_gd->gd_cnt.v_lock_name) - 2);
624 if (lwkt_sched_debug > 0) {
626 kprintf("tokb %p %s %s\n",
627 tok, tok->t_desc, td->td_comm);
629 td->td_wmesg = tok->t_desc;
632 scan = sort_array[i];
633 _lwkt_reltokref(scan, td);
640 * We were successful, there is no need for another core to signal
647 * Get a serializing token. This routine can block.
650 lwkt_gettoken(lwkt_token_t tok)
652 thread_t td = curthread;
655 ref = td->td_toks_stop;
656 KKASSERT(ref < &td->td_toks_end);
659 _lwkt_tokref_init(ref, tok, td, TOK_EXCLUSIVE|TOK_EXCLREQ);
663 * Taking an exclusive token after holding it shared will
664 * livelock. Scan for that case and assert.
668 for (tk = &td->td_toks_base; tk < ref; tk++) {
669 if (tk->tr_tok != tok)
673 if (tk->tr_count & TOK_EXCLUSIVE)
676 /* We found only shared instances of this token if found >0 here */
677 KASSERT((found == 0), ("Token %p s/x livelock", tok));
681 if (_lwkt_trytokref_spin(ref, td, TOK_EXCLUSIVE|TOK_EXCLREQ))
685 * Give up running if we can't acquire the token right now.
687 * Since the tokref is already active the scheduler now
688 * takes care of acquisition, so we need only call
691 * Since we failed this was not a recursive token so upon
692 * return tr_tok->t_ref should be assigned to this specific
695 td->td_wmesg = tok->t_desc;
698 td->td_toks_have = td->td_toks_stop - 1;
700 if (tokens_debug_output > 0) {
701 --tokens_debug_output;
702 spin_lock(&tok_debug_spin);
703 kprintf("Excl Token %p thread %p %s %s\n",
704 tok, td, tok->t_desc, td->td_comm);
707 spin_unlock(&tok_debug_spin);
710 atomic_set_int(&td->td_mpflags, TDF_MP_DIDYIELD);
713 KKASSERT(tok->t_ref == ref);
717 * Similar to gettoken but we acquire a shared token instead of an exclusive
721 lwkt_gettoken_shared(lwkt_token_t tok)
723 thread_t td = curthread;
726 ref = td->td_toks_stop;
727 KKASSERT(ref < &td->td_toks_end);
730 _lwkt_tokref_init(ref, tok, td, TOK_EXCLREQ);
734 * Taking a pool token in shared mode is a bad idea; other
735 * addresses deeper in the call stack may hash to the same pool
736 * token and you may end up with an exclusive-shared livelock.
737 * Warn in this condition.
739 if ((tok >= &pool_tokens[0].token) &&
740 (tok < &pool_tokens[LWKT_POOL_TOKENS].token))
741 kprintf("Warning! Taking pool token %p in shared mode\n", tok);
745 if (_lwkt_trytokref_spin(ref, td, TOK_EXCLREQ))
749 * Give up running if we can't acquire the token right now.
751 * Since the tokref is already active the scheduler now
752 * takes care of acquisition, so we need only call
755 * Since we failed this was not a recursive token so upon
756 * return tr_tok->t_ref should be assigned to this specific
759 td->td_wmesg = tok->t_desc;
762 td->td_toks_have = td->td_toks_stop - 1;
764 if (tokens_debug_output > 0) {
765 --tokens_debug_output;
766 spin_lock(&tok_debug_spin);
767 kprintf("Shar Token %p thread %p %s %s\n",
768 tok, td, tok->t_desc, td->td_comm);
771 spin_unlock(&tok_debug_spin);
774 atomic_set_int(&td->td_mpflags, TDF_MP_DIDYIELD);
780 * Attempt to acquire a token, return TRUE on success, FALSE on failure.
782 * We setup the tokref in case we actually get the token (if we switch later
783 * it becomes mandatory so we set TOK_EXCLREQ), but we call trytokref without
784 * TOK_EXCLREQ in case we fail.
787 lwkt_trytoken(lwkt_token_t tok)
789 thread_t td = curthread;
792 ref = td->td_toks_stop;
793 KKASSERT(ref < &td->td_toks_end);
796 _lwkt_tokref_init(ref, tok, td, TOK_EXCLUSIVE|TOK_EXCLREQ);
798 if (_lwkt_trytokref(ref, td, TOK_EXCLUSIVE))
802 * Failed, unpend the request
811 lwkt_getpooltoken(void *ptr)
815 tok = _lwkt_token_pool_lookup(ptr);
821 * Release a serializing token.
823 * WARNING! All tokens must be released in reverse order. This will be
827 lwkt_reltoken(lwkt_token_t tok)
829 thread_t td = curthread;
833 * Remove ref from thread token list and assert that it matches
834 * the token passed in. Tokens must be released in reverse order.
836 ref = td->td_toks_stop - 1;
837 KKASSERT(ref >= &td->td_toks_base && ref->tr_tok == tok);
838 _lwkt_reltokref(ref, td);
840 td->td_toks_stop = ref;
844 * It is faster for users of lwkt_getpooltoken() to use the returned
845 * token and just call lwkt_reltoken(), but for convenience we provide
846 * this function which looks the token up based on the ident.
849 lwkt_relpooltoken(void *ptr)
851 lwkt_token_t tok = _lwkt_token_pool_lookup(ptr);
856 * Return a count of the number of token refs the thread has to the
857 * specified token, whether it currently owns the token or not.
860 lwkt_cnttoken(lwkt_token_t tok, thread_t td)
865 for (scan = &td->td_toks_base; scan < td->td_toks_stop; ++scan) {
866 if (scan->tr_tok == tok)
873 * Pool tokens are used to provide a type-stable serializing token
874 * pointer that does not race against disappearing data structures.
876 * This routine is called in early boot just after we setup the BSP's
877 * globaldata structure.
880 lwkt_token_pool_init(void)
884 for (i = 0; i < LWKT_POOL_TOKENS; ++i)
885 lwkt_token_init(&pool_tokens[i].token, "pool");
889 lwkt_token_pool_lookup(void *ptr)
891 return (_lwkt_token_pool_lookup(ptr));
895 * Initialize a token.
898 lwkt_token_init(lwkt_token_t tok, const char *desc)
902 tok->t_collisions = 0;
907 lwkt_token_uninit(lwkt_token_t tok)
913 * Exchange the two most recent tokens on the tokref stack. This allows
914 * you to release a token out of order.
916 * We have to be careful about the case where the top two tokens are
917 * the same token. In this case tok->t_ref will point to the deeper
918 * ref and must remain pointing to the deeper ref. If we were to swap
919 * it the first release would clear the token even though a second
920 * ref is still present.
922 * Only exclusively held tokens contain a reference to the tokref which
923 * has to be flipped along with the swap.
926 lwkt_token_swap(void)
928 lwkt_tokref_t ref1, ref2;
929 lwkt_token_t tok1, tok2;
931 thread_t td = curthread;
935 ref1 = td->td_toks_stop - 1;
936 ref2 = td->td_toks_stop - 2;
937 KKASSERT(ref1 >= &td->td_toks_base);
938 KKASSERT(ref2 >= &td->td_toks_base);
942 count1 = ref1->tr_count;
943 count2 = ref2->tr_count;
947 ref1->tr_count = count2;
949 ref2->tr_count = count1;
950 if (tok1->t_ref == ref1)
952 if (tok2->t_ref == ref2)
960 DB_SHOW_COMMAND(tokens, db_tok_all)
962 struct lwkt_token *tok, **ptr;
963 struct lwkt_token *toklist[16] = {
977 for (tok = *ptr; tok; tok = *(++ptr)) {
978 db_printf("tok=%p tr_owner=%p t_colissions=%ld t_desc=%s\n", tok,
979 (tok->t_ref ? tok->t_ref->tr_owner : NULL),
980 tok->t_collisions, tok->t_desc);