kernel - Carefully refactor contended tokens and spinlocks

[dragonfly.git] / sys / kern / lwkt_token.c

/*
 * Copyright (c) 2003,2004,2009 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * lwkt_token - Implement soft token locks.
 *
 * Tokens are locks which serialize a thread only while the thread is
 * running.  If the thread blocks all tokens are released, then reacquired
 * when the thread resumes.
 *
 * This implementation requires no critical sections or spin locks, but
 * does use atomic_cmpset_ptr().
 *
 * Tokens may be recursively acquired by the same thread.  However the
 * caller must be sure to release such tokens in reverse order.
 */
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/rtprio.h>
#include <sys/queue.h>
#include <sys/sysctl.h>
#include <sys/ktr.h>
#include <sys/kthread.h>
#include <machine/cpu.h>
#include <sys/lock.h>
#include <sys/spinlock.h>

#include <sys/thread2.h>
#include <sys/spinlock2.h>
#include <sys/mplock2.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/vm_kern.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_map.h>
#include <vm/vm_pager.h>
#include <vm/vm_extern.h>
#include <vm/vm_zone.h>

#include <machine/stdarg.h>
#include <machine/smp.h>

#include "opt_ddb.h"
#ifdef DDB
#include <ddb/ddb.h>
#endif

extern int lwkt_sched_debug;

#ifndef LWKT_NUM_POOL_TOKENS
#define LWKT_NUM_POOL_TOKENS    4001
#endif

struct lwkt_pool_token {
        struct lwkt_token       token;
} __cachealign;

static struct lwkt_pool_token   pool_tokens[LWKT_NUM_POOL_TOKENS];
struct spinlock tok_debug_spin = SPINLOCK_INITIALIZER(&tok_debug_spin,
                                                      "tok_debug_spin");

#define TOKEN_STRING    "REF=%p TOK=%p TD=%p"
#define TOKEN_ARGS      lwkt_tokref_t ref, lwkt_token_t tok, struct thread *td
#define CONTENDED_STRING        TOKEN_STRING " (contention started)"
#define UNCONTENDED_STRING      TOKEN_STRING " (contention stopped)"
#if !defined(KTR_TOKENS)
#define KTR_TOKENS      KTR_ALL
#endif

KTR_INFO_MASTER(tokens);
KTR_INFO(KTR_TOKENS, tokens, fail, 0, TOKEN_STRING, TOKEN_ARGS);
KTR_INFO(KTR_TOKENS, tokens, succ, 1, TOKEN_STRING, TOKEN_ARGS);
#if 0
KTR_INFO(KTR_TOKENS, tokens, release, 2, TOKEN_STRING, TOKEN_ARGS);
KTR_INFO(KTR_TOKENS, tokens, remote, 3, TOKEN_STRING, TOKEN_ARGS);
KTR_INFO(KTR_TOKENS, tokens, reqremote, 4, TOKEN_STRING, TOKEN_ARGS);
KTR_INFO(KTR_TOKENS, tokens, reqfail, 5, TOKEN_STRING, TOKEN_ARGS);
KTR_INFO(KTR_TOKENS, tokens, drain, 6, TOKEN_STRING, TOKEN_ARGS);
KTR_INFO(KTR_TOKENS, tokens, contention_start, 7, CONTENDED_STRING, TOKEN_ARGS);
KTR_INFO(KTR_TOKENS, tokens, contention_stop, 7, UNCONTENDED_STRING, TOKEN_ARGS);
#endif

#define logtoken(name, ref)                                             \
        KTR_LOG(tokens_ ## name, ref, ref->tr_tok, curthread)

/*
 * Global tokens.  These replace the MP lock for major subsystem locking.
 * These tokens are initially used to lockup both global and individual
 * operations.
 *
 * Once individual structures get their own locks these tokens are used
 * only to protect global lists & other variables and to interlock
 * allocations and teardowns and such.
 *
 * The UP initializer causes token acquisition to also acquire the MP lock
 * for maximum compatibility.  The feature may be enabled and disabled at
 * any time, the MP state is copied to the tokref when the token is acquired
 * and will not race against sysctl changes.
 */
struct lwkt_token mp_token = LWKT_TOKEN_INITIALIZER(mp_token);
struct lwkt_token pmap_token = LWKT_TOKEN_INITIALIZER(pmap_token);
struct lwkt_token dev_token = LWKT_TOKEN_INITIALIZER(dev_token);
struct lwkt_token vm_token = LWKT_TOKEN_INITIALIZER(vm_token);
struct lwkt_token vmspace_token = LWKT_TOKEN_INITIALIZER(vmspace_token);
struct lwkt_token kvm_token = LWKT_TOKEN_INITIALIZER(kvm_token);
struct lwkt_token sigio_token = LWKT_TOKEN_INITIALIZER(sigio_token);
struct lwkt_token tty_token = LWKT_TOKEN_INITIALIZER(tty_token);
struct lwkt_token vnode_token = LWKT_TOKEN_INITIALIZER(vnode_token);

/*
 * Exponential backoff (exclusive tokens) and TSC windowing (shared tokens)
 * parameters.  We generally want a smaller backoff than we use for spinlocks
 * because we can fall-back to the scheduler.
 */
static int token_backoff_max __cachealign = 1024;
SYSCTL_INT(_lwkt, OID_AUTO, token_backoff_max, CTLFLAG_RW,
    &token_backoff_max, 0, "Tokens exponential backoff");
static int token_window_shift __cachealign = 8;
SYSCTL_INT(_lwkt, OID_AUTO, token_window_shift, CTLFLAG_RW,
    &token_window_shift, 0, "Tokens TSC windowing shift");

/*
 * The collision count is bumped every time the LWKT scheduler fails
 * to acquire needed tokens in addition to a normal lwkt_gettoken()
 * stall.
 */
SYSCTL_LONG(_lwkt, OID_AUTO, mp_collisions, CTLFLAG_RW,
    &mp_token.t_collisions, 0, "Collision counter of mp_token");
SYSCTL_LONG(_lwkt, OID_AUTO, pmap_collisions, CTLFLAG_RW,
    &pmap_token.t_collisions, 0, "Collision counter of pmap_token");
SYSCTL_LONG(_lwkt, OID_AUTO, dev_collisions, CTLFLAG_RW,
    &dev_token.t_collisions, 0, "Collision counter of dev_token");
SYSCTL_LONG(_lwkt, OID_AUTO, vm_collisions, CTLFLAG_RW,
    &vm_token.t_collisions, 0, "Collision counter of vm_token");
SYSCTL_LONG(_lwkt, OID_AUTO, vmspace_collisions, CTLFLAG_RW,
    &vmspace_token.t_collisions, 0, "Collision counter of vmspace_token");
SYSCTL_LONG(_lwkt, OID_AUTO, kvm_collisions, CTLFLAG_RW,
    &kvm_token.t_collisions, 0, "Collision counter of kvm_token");
SYSCTL_LONG(_lwkt, OID_AUTO, sigio_collisions, CTLFLAG_RW,
    &sigio_token.t_collisions, 0, "Collision counter of sigio_token");
SYSCTL_LONG(_lwkt, OID_AUTO, tty_collisions, CTLFLAG_RW,
    &tty_token.t_collisions, 0, "Collision counter of tty_token");
SYSCTL_LONG(_lwkt, OID_AUTO, vnode_collisions, CTLFLAG_RW,
    &vnode_token.t_collisions, 0, "Collision counter of vnode_token");

int tokens_debug_output;
SYSCTL_INT(_lwkt, OID_AUTO, tokens_debug_output, CTLFLAG_RW,
    &tokens_debug_output, 0, "Generate stack trace N times");

static int _lwkt_getalltokens_sorted(thread_t td);

/*
 * Acquire the initial mplock
 *
 * (low level boot only)
 */
void
cpu_get_initial_mplock(void)
{
        KKASSERT(mp_token.t_ref == NULL);
        if (lwkt_trytoken(&mp_token) == FALSE)
                panic("cpu_get_initial_mplock");
}

/*
 * Return a pool token given an address.  Use a prime number to reduce
 * overlaps.
 */
static __inline
lwkt_token_t
_lwkt_token_pool_lookup(void *ptr)
{
        uint32_t i;

        i = (uint32_t)(uintptr_t)ptr % LWKT_NUM_POOL_TOKENS;
        return (&pool_tokens[i].token);
}

/*
 * Initialize a tokref_t prior to making it visible in the thread's
 * token array.
 */
static __inline
void
_lwkt_tokref_init(lwkt_tokref_t ref, lwkt_token_t tok, thread_t td, long excl)
{
        ref->tr_tok = tok;
        ref->tr_count = excl;
        ref->tr_owner = td;
}

/*
 * Attempt to acquire a shared or exclusive token.  Returns TRUE on success,
 * FALSE on failure.
 *
 * If TOK_EXCLUSIVE is set in mode we are attempting to get an exclusive
 * token, otherwise are attempting to get a shared token.
 *
 * If TOK_EXCLREQ is set in mode this is a blocking operation, otherwise
 * it is a non-blocking operation (for both exclusive or shared acquisions).
 */
static __inline
int
_lwkt_trytokref(lwkt_tokref_t ref, thread_t td, long mode)
{
        lwkt_token_t tok;
        lwkt_tokref_t oref;
        long count;

        tok = ref->tr_tok;
        KASSERT(((mode & TOK_EXCLREQ) == 0 ||   /* non blocking */
                td->td_gd->gd_intr_nesting_level == 0 ||
                panic_cpu_gd == mycpu),
                ("Attempt to acquire token %p not already "
                "held in hard code section", tok));

        if (mode & TOK_EXCLUSIVE) {
                /*
                 * Attempt to get an exclusive token
                 */
                count = tok->t_count;

                for (;;) {
                        oref = tok->t_ref;      /* can be NULL */
                        cpu_ccfence();
                        if ((count & ~TOK_EXCLREQ) == 0) {
                                /*
                                 * It is possible to get the exclusive bit.
                                 * We must clear TOK_EXCLREQ on successful
                                 * acquisition.
                                 */
                                if (atomic_fcmpset_long(&tok->t_count, &count,
                                                        (count & ~TOK_EXCLREQ) |
                                                        TOK_EXCLUSIVE)) {
                                        KKASSERT(tok->t_ref == NULL);
                                        tok->t_ref = ref;
                                        return TRUE;
                                }
                                /* retry */
                        } else if ((count & TOK_EXCLUSIVE) &&
                                   oref >= &td->td_toks_base &&
                                   oref < td->td_toks_stop) {
                                /*
                                 * Our thread already holds the exclusive
                                 * bit, we treat this tokref as a shared
                                 * token (sorta) to make the token release
                                 * code easier.  Treating this as a shared
                                 * token allows us to simply increment the
                                 * count field.
                                 *
                                 * NOTE: oref cannot race above if it
                                 *       happens to be ours, so we're good.
                                 *       But we must still have a stable
                                 *       variable for both parts of the
                                 *       comparison.
                                 *
                                 * NOTE: Since we already have an exclusive
                                 *       lock and don't need to check EXCLREQ
                                 *       we can just use an atomic_add here
                                 */
                                atomic_add_long(&tok->t_count, TOK_INCR);
                                ref->tr_count &= ~TOK_EXCLUSIVE;
                                return TRUE;
                        } else if ((mode & TOK_EXCLREQ) &&
                                   (count & TOK_EXCLREQ) == 0) {
                                /*
                                 * Unable to get the exclusive bit but being
                                 * asked to set the exclusive-request bit.
                                 * Since we are going to retry anyway just
                                 * set the bit unconditionally.
                                 */
                                atomic_set_long(&tok->t_count, TOK_EXCLREQ);
                                return FALSE;
                        } else {
                                /*
                                 * Unable to get the exclusive bit and not
                                 * being asked to set the exclusive-request
                                 * (aka lwkt_trytoken()), or EXCLREQ was
                                 * already set.
                                 */
                                cpu_pause();
                                return FALSE;
                        }
                        /* retry */
                }
        } else {
                /*
                 * Attempt to get a shared token.  Note that TOK_EXCLREQ
                 * for shared tokens simply means the caller intends to
                 * block.  We never actually set the bit in tok->t_count.
                 *
                 * Due to the token's no-deadlock guarantee, and complications
                 * created by the sorted reacquisition code, we can only
                 * give exclusive requests priority over shared requests
                 * in situations where the thread holds only one token.
                 */
                count = tok->t_count;

                for (;;) {
                        oref = tok->t_ref;      /* can be NULL */
                        cpu_ccfence();
                        if ((count & (TOK_EXCLUSIVE|mode)) == 0 ||
                            ((count & TOK_EXCLUSIVE) == 0 &&
                            td->td_toks_stop != &td->td_toks_base + 1)
                        ) {
                                /*
                                 * It may be possible to get the token shared.
                                 */
                                if ((atomic_fetchadd_long(&tok->t_count, TOK_INCR) & TOK_EXCLUSIVE) == 0) {
                                        return TRUE;
                                }
                                count = atomic_fetchadd_long(&tok->t_count,
                                                             -TOK_INCR);
                                count -= TOK_INCR;
                                /* retry */
                        } else if ((count & TOK_EXCLUSIVE) &&
                                   oref >= &td->td_toks_base &&
                                   oref < td->td_toks_stop) {
                                /*
                                 * We own the exclusive bit on the token so
                                 * we can in fact also get it shared.
                                 */
                                atomic_add_long(&tok->t_count, TOK_INCR);
                                return TRUE;
                        } else {
                                /*
                                 * We failed to get the token shared
                                 */
                                return FALSE;
                        }
                        /* retry */
                }
        }
}

static __inline
int
_lwkt_trytokref_spin(lwkt_tokref_t ref, thread_t td, long mode)
{
        if (_lwkt_trytokref(ref, td, mode))
                return TRUE;

        if (mode & TOK_EXCLUSIVE) {
                /*
                 * Contested exclusive token, use exponential backoff
                 * algorithm.
                 */
                long expbackoff;
                long loop;

                expbackoff = 0;
                while (expbackoff < 6 + token_backoff_max) {
                        expbackoff = (expbackoff + 1) * 3 / 2;
                        if ((rdtsc() >> token_window_shift) % ncpus != mycpuid)  {
                                for (loop = expbackoff; loop; --loop)
                                        cpu_pause();
                        }
                        if (_lwkt_trytokref(ref, td, mode))
                                return TRUE;
                }
        } else {
                /*
                 * Contested shared token, use TSC windowing.  Note that
                 * exclusive tokens have priority over shared tokens only
                 * for the first token.
                 */
                if ((rdtsc() >> token_window_shift) % ncpus == mycpuid) {
                        if (_lwkt_trytokref(ref, td, mode & ~TOK_EXCLREQ))
                                return TRUE;
                } else {
                        if (_lwkt_trytokref(ref, td, mode))
                                return TRUE;
                }

        }
        ++mycpu->gd_cnt.v_lock_colls;

        return FALSE;
}

/*
 * Release a token that we hold.
 *
 * Since tokens are polled, we don't have to deal with wakeups and releasing
 * is really easy.
 */
static __inline
void
_lwkt_reltokref(lwkt_tokref_t ref, thread_t td)
{
        lwkt_token_t tok;
        long count;

        tok = ref->tr_tok;
        if (tok->t_ref == ref) {
                /*
                 * We are an exclusive holder.  We must clear tr_ref
                 * before we clear the TOK_EXCLUSIVE bit.  If we are
                 * unable to clear the bit we must restore
                 * tok->t_ref.
                 */
#if 0
                KKASSERT(count & TOK_EXCLUSIVE);
#endif
                tok->t_ref = NULL;
                atomic_clear_long(&tok->t_count, TOK_EXCLUSIVE);
        } else {
                /*
                 * We are a shared holder
                 */
                count = atomic_fetchadd_long(&tok->t_count, -TOK_INCR);
                KKASSERT(count & TOK_COUNTMASK);        /* count prior */
        }
}

/*
 * Obtain all the tokens required by the specified thread on the current
 * cpu, return 0 on failure and non-zero on success.  If a failure occurs
 * any partially acquired tokens will be released prior to return.
 *
 * lwkt_getalltokens is called by the LWKT scheduler to re-acquire all
 * tokens that the thread had to release when it switched away.
 *
 * If spinning is non-zero this function acquires the tokens in a particular
 * order to deal with potential deadlocks.  We simply use address order for
 * the case.
 *
 * Called from a critical section.
 */
int
lwkt_getalltokens(thread_t td, int spinning)
{
        lwkt_tokref_t scan;
        lwkt_token_t tok;

        if (spinning)
                return(_lwkt_getalltokens_sorted(td));

        /*
         * Acquire tokens in forward order, assign or validate tok->t_ref.
         */
        for (scan = &td->td_toks_base; scan < td->td_toks_stop; ++scan) {
                tok = scan->tr_tok;
                for (;;) {
                        /*
                         * Only try really hard on the last token
                         */
                        if (scan == td->td_toks_stop - 1) {
                            if (_lwkt_trytokref_spin(scan, td, scan->tr_count))
                                    break;
                        } else {
                            if (_lwkt_trytokref(scan, td, scan->tr_count))
                                    break;
                        }

                        /*
                         * Otherwise we failed to acquire all the tokens.
                         * Release whatever we did get.
                         */
                        KASSERT(tok->t_desc,
                                ("token %p is not initialized", tok));
                        td->td_gd->gd_cnt.v_lock_name[0] = 't';
                        strncpy(td->td_gd->gd_cnt.v_lock_name + 1,
                                tok->t_desc,
                                sizeof(td->td_gd->gd_cnt.v_lock_name) - 2);
                        if (lwkt_sched_debug > 0) {
                                --lwkt_sched_debug;
                                kprintf("toka %p %s %s\n",
                                        tok, tok->t_desc, td->td_comm);
                        }
                        td->td_wmesg = tok->t_desc;
                        ++tok->t_collisions;
                        while (--scan >= &td->td_toks_base)
                                _lwkt_reltokref(scan, td);
                        return(FALSE);
                }
        }
        return (TRUE);
}

/*
 * Release all tokens owned by the specified thread on the current cpu.
 *
 * This code is really simple.  Even in cases where we own all the tokens
 * note that t_ref may not match the scan for recursively held tokens which
 * are held deeper in the stack, or for the case where a lwkt_getalltokens()
 * failed.
 *
 * Tokens are released in reverse order to reduce chasing race failures.
 * 
 * Called from a critical section.
 */
void
lwkt_relalltokens(thread_t td)
{
        lwkt_tokref_t scan;

        /*
         * Weird order is to try to avoid a panic loop
         */
        if (td->td_toks_have) {
                scan = td->td_toks_have;
                td->td_toks_have = NULL;
        } else {
                scan = td->td_toks_stop;
        }
        while (--scan >= &td->td_toks_base)
                _lwkt_reltokref(scan, td);
}

/*
 * This is the decontention version of lwkt_getalltokens().  The tokens are
 * acquired in address-sorted order to deal with any deadlocks.  Ultimately
 * token failures will spin into the scheduler and get here.
 *
 * Called from critical section
 */
static
int
_lwkt_getalltokens_sorted(thread_t td)
{
        lwkt_tokref_t sort_array[LWKT_MAXTOKENS];
        lwkt_tokref_t scan;
        lwkt_token_t tok;
        int i;
        int j;
        int n;

        /*
         * Sort the token array.  Yah yah, I know this isn't fun.
         *
         * NOTE: Recursively acquired tokens are ordered the same as in the
         *       td_toks_array so we can always get the earliest one first.
         *       This is particularly important when a token is acquired
         *       exclusively multiple times, as only the first acquisition
         *       is treated as an exclusive token.
         */
        i = 0;
        scan = &td->td_toks_base;
        while (scan < td->td_toks_stop) {
                for (j = 0; j < i; ++j) {
                        if (scan->tr_tok < sort_array[j]->tr_tok)
                                break;
                }
                if (j != i) {
                        bcopy(sort_array + j, sort_array + j + 1,
                              (i - j) * sizeof(lwkt_tokref_t));
                }
                sort_array[j] = scan;
                ++scan;
                ++i;
        }
        n = i;

        /*
         * Acquire tokens in forward order, assign or validate tok->t_ref.
         */
        for (i = 0; i < n; ++i) {
                scan = sort_array[i];
                tok = scan->tr_tok;
                for (;;) {
                        /*
                         * Only try really hard on the last token
                         */
                        if (scan == td->td_toks_stop - 1) {
                            if (_lwkt_trytokref_spin(scan, td, scan->tr_count))
                                    break;
                        } else {
                            if (_lwkt_trytokref(scan, td, scan->tr_count))
                                    break;
                        }

                        /*
                         * Otherwise we failed to acquire all the tokens.
                         * Release whatever we did get.
                         */
                        td->td_gd->gd_cnt.v_lock_name[0] = 't';
                        strncpy(td->td_gd->gd_cnt.v_lock_name + 1,
                                tok->t_desc,
                                sizeof(td->td_gd->gd_cnt.v_lock_name) - 2);
                        if (lwkt_sched_debug > 0) {
                                --lwkt_sched_debug;
                                kprintf("tokb %p %s %s\n",
                                        tok, tok->t_desc, td->td_comm);
                        }
                        td->td_wmesg = tok->t_desc;
                        ++tok->t_collisions;
                        while (--i >= 0) {
                                scan = sort_array[i];
                                _lwkt_reltokref(scan, td);
                        }
                        return(FALSE);
                }
        }

        /*
         * We were successful, there is no need for another core to signal
         * us.
         */
        return (TRUE);
}

/*
 * Get a serializing token.  This routine can block.
 */
void
lwkt_gettoken(lwkt_token_t tok)
{
        thread_t td = curthread;
        lwkt_tokref_t ref;

        ref = td->td_toks_stop;
        KKASSERT(ref < &td->td_toks_end);
        ++td->td_toks_stop;
        cpu_ccfence();
        _lwkt_tokref_init(ref, tok, td, TOK_EXCLUSIVE|TOK_EXCLREQ);

#ifdef DEBUG_LOCKS
        /*
         * Taking an exclusive token after holding it shared will
         * livelock. Scan for that case and assert.
         */
        lwkt_tokref_t tk;
        int found = 0;
        for (tk = &td->td_toks_base; tk < ref; tk++) {
                if (tk->tr_tok != tok)
                        continue;
                
                found++;
                if (tk->tr_count & TOK_EXCLUSIVE) 
                        goto good;
        }
        /* We found only shared instances of this token if found >0 here */
        KASSERT((found == 0), ("Token %p s/x livelock", tok));
good:
#endif

        if (_lwkt_trytokref_spin(ref, td, TOK_EXCLUSIVE|TOK_EXCLREQ))
                return;

        /*
         * Give up running if we can't acquire the token right now.
         *
         * Since the tokref is already active the scheduler now
         * takes care of acquisition, so we need only call
         * lwkt_switch().
         *
         * Since we failed this was not a recursive token so upon
         * return tr_tok->t_ref should be assigned to this specific
         * ref.
         */
        td->td_wmesg = tok->t_desc;
        ++tok->t_collisions;
        logtoken(fail, ref);
        td->td_toks_have = td->td_toks_stop - 1;

        if (tokens_debug_output > 0) {
                --tokens_debug_output;
                spin_lock(&tok_debug_spin);
                kprintf("Excl Token thread %p %s %s\n",
                        td, tok->t_desc, td->td_comm);
                print_backtrace(6);
                kprintf("\n");
                spin_unlock(&tok_debug_spin);
        }

        lwkt_switch();
        logtoken(succ, ref);
        KKASSERT(tok->t_ref == ref);
}

/*
 * Similar to gettoken but we acquire a shared token instead of an exclusive
 * token.
 */
void
lwkt_gettoken_shared(lwkt_token_t tok)
{
        thread_t td = curthread;
        lwkt_tokref_t ref;

        ref = td->td_toks_stop;
        KKASSERT(ref < &td->td_toks_end);
        ++td->td_toks_stop;
        cpu_ccfence();
        _lwkt_tokref_init(ref, tok, td, TOK_EXCLREQ);

#ifdef DEBUG_LOCKS
        /*
         * Taking a pool token in shared mode is a bad idea; other
         * addresses deeper in the call stack may hash to the same pool
         * token and you may end up with an exclusive-shared livelock.
         * Warn in this condition.
         */
        if ((tok >= &pool_tokens[0].token) &&
            (tok < &pool_tokens[LWKT_NUM_POOL_TOKENS].token))
                kprintf("Warning! Taking pool token %p in shared mode\n", tok);
#endif


        if (_lwkt_trytokref_spin(ref, td, TOK_EXCLREQ))
                return;

        /*
         * Give up running if we can't acquire the token right now.
         *
         * Since the tokref is already active the scheduler now
         * takes care of acquisition, so we need only call
         * lwkt_switch().
         *
         * Since we failed this was not a recursive token so upon
         * return tr_tok->t_ref should be assigned to this specific
         * ref.
         */
        td->td_wmesg = tok->t_desc;
        ++tok->t_collisions;
        logtoken(fail, ref);
        td->td_toks_have = td->td_toks_stop - 1;

        if (tokens_debug_output > 0) {
                --tokens_debug_output;
                spin_lock(&tok_debug_spin);
                kprintf("Shar Token thread %p %s %s\n",
                        td, tok->t_desc, td->td_comm);
                print_backtrace(6);
                kprintf("\n");
                spin_unlock(&tok_debug_spin);
        }

        lwkt_switch();
        logtoken(succ, ref);
}

/*
 * Attempt to acquire a token, return TRUE on success, FALSE on failure.
 *
 * We setup the tokref in case we actually get the token (if we switch later
 * it becomes mandatory so we set TOK_EXCLREQ), but we call trytokref without
 * TOK_EXCLREQ in case we fail.
 */
int
lwkt_trytoken(lwkt_token_t tok)
{
        thread_t td = curthread;
        lwkt_tokref_t ref;

        ref = td->td_toks_stop;
        KKASSERT(ref < &td->td_toks_end);
        ++td->td_toks_stop;
        cpu_ccfence();
        _lwkt_tokref_init(ref, tok, td, TOK_EXCLUSIVE|TOK_EXCLREQ);

        if (_lwkt_trytokref(ref, td, TOK_EXCLUSIVE))
                return TRUE;

        /*
         * Failed, unpend the request
         */
        cpu_ccfence();
        --td->td_toks_stop;
        ++tok->t_collisions;
        return FALSE;
}

lwkt_token_t
lwkt_getpooltoken(void *ptr)
{
        lwkt_token_t tok;

        tok = _lwkt_token_pool_lookup(ptr);
        lwkt_gettoken(tok);
        return (tok);
}

/*
 * Release a serializing token.
 *
 * WARNING!  All tokens must be released in reverse order.  This will be
 *           asserted.
 */
void
lwkt_reltoken(lwkt_token_t tok)
{
        thread_t td = curthread;
        lwkt_tokref_t ref;

        /*
         * Remove ref from thread token list and assert that it matches
         * the token passed in.  Tokens must be released in reverse order.
         */
        ref = td->td_toks_stop - 1;
        KKASSERT(ref >= &td->td_toks_base && ref->tr_tok == tok);
        _lwkt_reltokref(ref, td);
        cpu_sfence();
        td->td_toks_stop = ref;
}

/*
 * It is faster for users of lwkt_getpooltoken() to use the returned
 * token and just call lwkt_reltoken(), but for convenience we provide
 * this function which looks the token up based on the ident.
 */
void
lwkt_relpooltoken(void *ptr)
{
        lwkt_token_t tok = _lwkt_token_pool_lookup(ptr);
        lwkt_reltoken(tok);
}

/*
 * Return a count of the number of token refs the thread has to the
 * specified token, whether it currently owns the token or not.
 */
int
lwkt_cnttoken(lwkt_token_t tok, thread_t td)
{
        lwkt_tokref_t scan;
        int count = 0;

        for (scan = &td->td_toks_base; scan < td->td_toks_stop; ++scan) {
                if (scan->tr_tok == tok)
                        ++count;
        }
        return(count);
}

/*
 * Pool tokens are used to provide a type-stable serializing token
 * pointer that does not race against disappearing data structures.
 *
 * This routine is called in early boot just after we setup the BSP's
 * globaldata structure.
 */
void
lwkt_token_pool_init(void)
{
        int i;

        for (i = 0; i < LWKT_NUM_POOL_TOKENS; ++i)
                lwkt_token_init(&pool_tokens[i].token, "pool");
}

lwkt_token_t
lwkt_token_pool_lookup(void *ptr)
{
        return (_lwkt_token_pool_lookup(ptr));
}

/*
 * Initialize a token.  
 */
void
lwkt_token_init(lwkt_token_t tok, const char *desc)
{
        tok->t_count = 0;
        tok->t_ref = NULL;
        tok->t_collisions = 0;
        tok->t_desc = desc;
}

void
lwkt_token_uninit(lwkt_token_t tok)
{
        /* empty */
}

/*
 * Exchange the two most recent tokens on the tokref stack.  This allows
 * you to release a token out of order.
 *
 * We have to be careful about the case where the top two tokens are
 * the same token.  In this case tok->t_ref will point to the deeper
 * ref and must remain pointing to the deeper ref.  If we were to swap
 * it the first release would clear the token even though a second
 * ref is still present.
 *
 * Only exclusively held tokens contain a reference to the tokref which
 * has to be flipped along with the swap.
 */
void
lwkt_token_swap(void)
{
        lwkt_tokref_t ref1, ref2;
        lwkt_token_t tok1, tok2;
        long count1, count2;
        thread_t td = curthread;

        crit_enter();

        ref1 = td->td_toks_stop - 1;
        ref2 = td->td_toks_stop - 2;
        KKASSERT(ref1 >= &td->td_toks_base);
        KKASSERT(ref2 >= &td->td_toks_base);

        tok1 = ref1->tr_tok;
        tok2 = ref2->tr_tok;
        count1 = ref1->tr_count;
        count2 = ref2->tr_count;

        if (tok1 != tok2) {
                ref1->tr_tok = tok2;
                ref1->tr_count = count2;
                ref2->tr_tok = tok1;
                ref2->tr_count = count1;
                if (tok1->t_ref == ref1)
                        tok1->t_ref = ref2;
                if (tok2->t_ref == ref2)
                        tok2->t_ref = ref1;
        }

        crit_exit();
}

#ifdef DDB
DB_SHOW_COMMAND(tokens, db_tok_all)
{
        struct lwkt_token *tok, **ptr;
        struct lwkt_token *toklist[16] = {
                &mp_token,
                &pmap_token,
                &dev_token,
                &vm_token,
                &vmspace_token,
                &kvm_token,
                &sigio_token,
                &tty_token,
                &vnode_token,
                NULL
        };

        ptr = toklist;
        for (tok = *ptr; tok; tok = *(++ptr)) {
                db_printf("tok=%p tr_owner=%p t_colissions=%ld t_desc=%s\n", tok,
                    (tok->t_ref ? tok->t_ref->tr_owner : NULL),
                    tok->t_collisions, tok->t_desc);
        }
}
#endif /* DDB */