kern_rmlock.c

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/*-
 * Copyright (c) 2007 Stephan Uphoff <ups@FreeBSD.org>
 * All rights reserved.
 *
 * 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 author nor the names of any co-contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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.
 */

/*
 * Machine independent bits of reader/writer lock implementation.
 */

#include <sys/cdefs.h>
__FBSDID("$BSDSUniX$");

#include "opt_ddb.h"
#include "opt_kdtrace.h"

#include <sys/param.h>
#include <sys/systm.h>

#include <sys/kernel.h>
#include <sys/kdb.h>
#include <sys/ktr.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/rmlock.h>
#include <sys/sched.h>
#include <sys/smp.h>
#include <sys/turnstile.h>
#include <sys/lock_profile.h>
#include <machine/cpu.h>

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

/*
 * A cookie to mark destroyed rmlocks.  This is stored in the head of
 * rm_activeReaders.
 */
#define RM_DESTROYED    ((void *)0xdead)

#define rm_destroyed(rm)                                                \
        (LIST_FIRST(&(rm)->rm_activeReaders) == RM_DESTROYED)

#define RMPF_ONQUEUE    1
#define RMPF_SIGNAL     2

#ifndef INVARIANTS
#define _rm_assert(c, what, file, line)
#endif

static void     assert_rm(struct lock_object *lock, int what);
#ifdef DDB
static void     db_show_rm(struct lock_object *lock);
#endif
static void     lock_rm(struct lock_object *lock, int how);
#ifdef KDTRACE_HOOKS
static int      owner_rm(struct lock_object *lock, struct thread **owner);
#endif
static int      unlock_rm(struct lock_object *lock);

struct lock_class lock_class_rm = {
        .lc_name = "rm",
        .lc_flags = LC_SLEEPLOCK | LC_RECURSABLE,
        .lc_assert = assert_rm,
#ifdef DDB
        .lc_ddb_show = db_show_rm,
#endif
        .lc_lock = lock_rm,
        .lc_unlock = unlock_rm,
#ifdef KDTRACE_HOOKS
        .lc_owner = owner_rm,
#endif
};

struct lock_class lock_class_rm_sleepable = {
        .lc_name = "sleepable rm",
        .lc_flags = LC_SLEEPLOCK | LC_SLEEPABLE | LC_RECURSABLE,
        .lc_assert = assert_rm,
#ifdef DDB
        .lc_ddb_show = db_show_rm,
#endif
        .lc_lock = lock_rm,
        .lc_unlock = unlock_rm,
#ifdef KDTRACE_HOOKS
        .lc_owner = owner_rm,
#endif
};

static void
assert_rm(struct lock_object *lock, int what)
{

        rm_assert((struct rmlock *)lock, what);
}

/*
 * These do not support read locks because it would be hard to make
 * the tracker work correctly with the current lock_class API as you
 * would need to have the tracker pointer available when calling
 * rm_rlock() in lock_rm().
 */
static void
lock_rm(struct lock_object *lock, int how)
{
        struct rmlock *rm;

        rm = (struct rmlock *)lock;
        if (how)
                rm_wlock(rm);
#ifdef INVARIANTS
        else
                panic("lock_rm called in read mode");
#endif
}

static int
unlock_rm(struct lock_object *lock)
{
        struct rmlock *rm;

        rm = (struct rmlock *)lock;
        rm_wunlock(rm);
        return (1);
}

#ifdef KDTRACE_HOOKS
static int
owner_rm(struct lock_object *lock, struct thread **owner)
{
        struct rmlock *rm;
        struct lock_class *lc;

        rm = (struct rmlock *)lock;
        lc = LOCK_CLASS(&rm->rm_wlock_object);
        return (lc->lc_owner(&rm->rm_wlock_object, owner));
}
#endif

static struct mtx rm_spinlock;

MTX_SYSINIT(rm_spinlock, &rm_spinlock, "rm_spinlock", MTX_SPIN);

/*
 * Add or remove tracker from per-cpu list.
 *
 * The per-cpu list can be traversed at any time in forward direction from an
 * interrupt on the *local* cpu.
 */
static void inline
rm_tracker_add(struct pcpu *pc, struct rm_priotracker *tracker)
{
        struct rm_queue *next;

        /* Initialize all tracker pointers */
        tracker->rmp_cpuQueue.rmq_prev = &pc->pc_rm_queue;
        next = pc->pc_rm_queue.rmq_next;
        tracker->rmp_cpuQueue.rmq_next = next;

        /* rmq_prev is not used during froward traversal. */
        next->rmq_prev = &tracker->rmp_cpuQueue;

        /* Update pointer to first element. */
        pc->pc_rm_queue.rmq_next = &tracker->rmp_cpuQueue;
}

/*
 * Return a count of the number of trackers the thread 'td' already
 * has on this CPU for the lock 'rm'.
 */
static int
rm_trackers_present(const struct pcpu *pc, const struct rmlock *rm,
    const struct thread *td)
{
        struct rm_queue *queue;
        struct rm_priotracker *tracker;
        int count;

        count = 0;
        for (queue = pc->pc_rm_queue.rmq_next; queue != &pc->pc_rm_queue;
            queue = queue->rmq_next) {
                tracker = (struct rm_priotracker *)queue;
                if ((tracker->rmp_rmlock == rm) && (tracker->rmp_thread == td))
                        count++;
        }
        return (count);
}

static void inline
rm_tracker_remove(struct pcpu *pc, struct rm_priotracker *tracker)
{
        struct rm_queue *next, *prev;

        next = tracker->rmp_cpuQueue.rmq_next;
        prev = tracker->rmp_cpuQueue.rmq_prev;

        /* Not used during forward traversal. */
        next->rmq_prev = prev;

        /* Remove from list. */
        prev->rmq_next = next;
}

static void
rm_cleanIPI(void *arg)
{
        struct pcpu *pc;
        struct rmlock *rm = arg;
        struct rm_priotracker *tracker;
        struct rm_queue *queue;
        pc = pcpu_find(curcpu);

        for (queue = pc->pc_rm_queue.rmq_next; queue != &pc->pc_rm_queue;
            queue = queue->rmq_next) {
                tracker = (struct rm_priotracker *)queue;
                if (tracker->rmp_rmlock == rm && tracker->rmp_flags == 0) {
                        tracker->rmp_flags = RMPF_ONQUEUE;
                        mtx_lock_spin(&rm_spinlock);
                        LIST_INSERT_HEAD(&rm->rm_activeReaders, tracker,
                            rmp_qentry);
                        mtx_unlock_spin(&rm_spinlock);
                }
        }
}

void
rm_init_flags(struct rmlock *rm, const char *name, int opts)
{
        struct lock_class *lc;
        int liflags;

        liflags = 0;
        if (!(opts & RM_NOWITNESS))
                liflags |= LO_WITNESS;
        if (opts & RM_RECURSE)
                liflags |= LO_RECURSABLE;
        rm->rm_writecpus = all_cpus;
        LIST_INIT(&rm->rm_activeReaders);
        if (opts & RM_SLEEPABLE) {
                liflags |= LO_SLEEPABLE;
                lc = &lock_class_rm_sleepable;
                sx_init_flags(&rm->rm_lock_sx, "rmlock_sx", SX_NOWITNESS);
        } else {
                lc = &lock_class_rm;
                mtx_init(&rm->rm_lock_mtx, name, "rmlock_mtx", MTX_NOWITNESS);
        }
        lock_init(&rm->lock_object, lc, name, NULL, liflags);
}

void
rm_init(struct rmlock *rm, const char *name)
{

        rm_init_flags(rm, name, 0);
}

void
rm_destroy(struct rmlock *rm)
{

        rm_assert(rm, RA_UNLOCKED);
        LIST_FIRST(&rm->rm_activeReaders) = RM_DESTROYED;
        if (rm->lock_object.lo_flags & LO_SLEEPABLE)
                sx_destroy(&rm->rm_lock_sx);
        else
                mtx_destroy(&rm->rm_lock_mtx);
        lock_destroy(&rm->lock_object);
}

int
rm_wowned(struct rmlock *rm)
{

        if (rm->lock_object.lo_flags & LO_SLEEPABLE)
                return (sx_xlocked(&rm->rm_lock_sx));
        else
                return (mtx_owned(&rm->rm_lock_mtx));
}

void
rm_sysinit(void *arg)
{
        struct rm_args *args = arg;

        rm_init(args->ra_rm, args->ra_desc);
}

void
rm_sysinit_flags(void *arg)
{
        struct rm_args_flags *args = arg;

        rm_init_flags(args->ra_rm, args->ra_desc, args->ra_opts);
}

static int
_rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker, int trylock)
{
        struct pcpu *pc;

        critical_enter();
        pc = pcpu_find(curcpu);

        /* Check if we just need to do a proper critical_exit. */
        if (!CPU_ISSET(pc->pc_cpuid, &rm->rm_writecpus)) {
                critical_exit();
                return (1);
        }

        /* Remove our tracker from the per-cpu list. */
        rm_tracker_remove(pc, tracker);

        /* Check to see if the IPI granted us the lock after all. */
        if (tracker->rmp_flags) {
                /* Just add back tracker - we hold the lock. */
                rm_tracker_add(pc, tracker);
                critical_exit();
                return (1);
        }

        /*
         * We allow readers to aquire a lock even if a writer is blocked if
         * the lock is recursive and the reader already holds the lock.
         */
        if ((rm->lock_object.lo_flags & LO_RECURSABLE) != 0) {
                /*
                 * Just grant the lock if this thread already has a tracker
                 * for this lock on the per-cpu queue.
                 */
                if (rm_trackers_present(pc, rm, curthread) != 0) {
                        mtx_lock_spin(&rm_spinlock);
                        LIST_INSERT_HEAD(&rm->rm_activeReaders, tracker,
                            rmp_qentry);
                        tracker->rmp_flags = RMPF_ONQUEUE;
                        mtx_unlock_spin(&rm_spinlock);
                        rm_tracker_add(pc, tracker);
                        critical_exit();
                        return (1);
                }
        }

        sched_unpin();
        critical_exit();

        if (trylock) {
                if (rm->lock_object.lo_flags & LO_SLEEPABLE) {
                        if (!sx_try_xlock(&rm->rm_lock_sx))
                                return (0);
                } else {
                        if (!mtx_trylock(&rm->rm_lock_mtx))
                                return (0);
                }
        } else {
                if (rm->lock_object.lo_flags & LO_SLEEPABLE) {
                        THREAD_SLEEPING_OK();
                        sx_xlock(&rm->rm_lock_sx);
                        THREAD_NO_SLEEPING();
                } else
                        mtx_lock(&rm->rm_lock_mtx);
        }

        critical_enter();
        pc = pcpu_find(curcpu);
        CPU_CLR(pc->pc_cpuid, &rm->rm_writecpus);
        rm_tracker_add(pc, tracker);
        sched_pin();
        critical_exit();

        if (rm->lock_object.lo_flags & LO_SLEEPABLE)
                sx_xunlock(&rm->rm_lock_sx);
        else
                mtx_unlock(&rm->rm_lock_mtx);

        return (1);
}

int
_rm_rlock(struct rmlock *rm, struct rm_priotracker *tracker, int trylock)
{
        struct thread *td = curthread;
        struct pcpu *pc;

        if (SCHEDULER_STOPPED())
                return (1);

        tracker->rmp_flags  = 0;
        tracker->rmp_thread = td;
        tracker->rmp_rmlock = rm;

        td->td_critnest++;      /* critical_enter(); */

        __compiler_membar();

        pc = cpuid_to_pcpu[td->td_oncpu]; /* pcpu_find(td->td_oncpu); */

        rm_tracker_add(pc, tracker);

        sched_pin();

        __compiler_membar();

        td->td_critnest--;

        /*
         * Fast path to combine two common conditions into a single
         * conditional jump.
         */
        if (0 == (td->td_owepreempt |
            CPU_ISSET(pc->pc_cpuid, &rm->rm_writecpus)))
                return (1);

        /* We do not have a read token and need to acquire one. */
        return _rm_rlock_hard(rm, tracker, trylock);
}

static void
_rm_unlock_hard(struct thread *td,struct rm_priotracker *tracker)
{

        if (td->td_owepreempt) {
                td->td_critnest++;
                critical_exit();
        }

        if (!tracker->rmp_flags)
                return;

        mtx_lock_spin(&rm_spinlock);
        LIST_REMOVE(tracker, rmp_qentry);

        if (tracker->rmp_flags & RMPF_SIGNAL) {
                struct rmlock *rm;
                struct turnstile *ts;

                rm = tracker->rmp_rmlock;

                turnstile_chain_lock(&rm->lock_object);
                mtx_unlock_spin(&rm_spinlock);

                ts = turnstile_lookup(&rm->lock_object);

                turnstile_signal(ts, TS_EXCLUSIVE_QUEUE);
                turnstile_unpend(ts, TS_EXCLUSIVE_LOCK);
                turnstile_chain_unlock(&rm->lock_object);
        } else
                mtx_unlock_spin(&rm_spinlock);
}

void
_rm_runlock(struct rmlock *rm, struct rm_priotracker *tracker)
{
        struct pcpu *pc;
        struct thread *td = tracker->rmp_thread;

        if (SCHEDULER_STOPPED())
                return;

        td->td_critnest++;      /* critical_enter(); */
        pc = cpuid_to_pcpu[td->td_oncpu]; /* pcpu_find(td->td_oncpu); */
        rm_tracker_remove(pc, tracker);
        td->td_critnest--;
        sched_unpin();

        if (0 == (td->td_owepreempt | tracker->rmp_flags))
                return;

        _rm_unlock_hard(td, tracker);
}

void
_rm_wlock(struct rmlock *rm)
{
        struct rm_priotracker *prio;
        struct turnstile *ts;
        cpuset_t readcpus;

        if (SCHEDULER_STOPPED())
                return;

        if (rm->lock_object.lo_flags & LO_SLEEPABLE)
                sx_xlock(&rm->rm_lock_sx);
        else
                mtx_lock(&rm->rm_lock_mtx);

        if (CPU_CMP(&rm->rm_writecpus, &all_cpus)) {
                /* Get all read tokens back */
                readcpus = all_cpus;
                CPU_NAND(&readcpus, &rm->rm_writecpus);
                rm->rm_writecpus = all_cpus;

                /*
                 * Assumes rm->rm_writecpus update is visible on other CPUs
                 * before rm_cleanIPI is called.
                 */
#ifdef SMP
                smp_rendezvous_cpus(readcpus,
                    smp_no_rendevous_barrier,
                    rm_cleanIPI,
                    smp_no_rendevous_barrier,
                    rm);

#else
                rm_cleanIPI(rm);
#endif

                mtx_lock_spin(&rm_spinlock);
                while ((prio = LIST_FIRST(&rm->rm_activeReaders)) != NULL) {
                        ts = turnstile_trywait(&rm->lock_object);
                        prio->rmp_flags = RMPF_ONQUEUE | RMPF_SIGNAL;
                        mtx_unlock_spin(&rm_spinlock);
                        turnstile_wait(ts, prio->rmp_thread,
                            TS_EXCLUSIVE_QUEUE);
                        mtx_lock_spin(&rm_spinlock);
                }
                mtx_unlock_spin(&rm_spinlock);
        }
}

void
_rm_wunlock(struct rmlock *rm)
{

        if (rm->lock_object.lo_flags & LO_SLEEPABLE)
                sx_xunlock(&rm->rm_lock_sx);
        else
                mtx_unlock(&rm->rm_lock_mtx);
}

#ifdef LOCK_DEBUG

void
_rm_wlock_debug(struct rmlock *rm, const char *file, int line)
{

        if (SCHEDULER_STOPPED())
                return;

        KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
            ("rm_wlock() by idle thread %p on rmlock %s @ %s:%d",
            curthread, rm->lock_object.lo_name, file, line));
        KASSERT(!rm_destroyed(rm),
            ("rm_wlock() of destroyed rmlock @ %s:%d", file, line));
        _rm_assert(rm, RA_UNLOCKED, file, line);

        WITNESS_CHECKORDER(&rm->lock_object, LOP_NEWORDER | LOP_EXCLUSIVE,
            file, line, NULL);

        _rm_wlock(rm);

        LOCK_LOG_LOCK("RMWLOCK", &rm->lock_object, 0, 0, file, line);

        WITNESS_LOCK(&rm->lock_object, LOP_EXCLUSIVE, file, line);

        curthread->td_locks++;

}

void
_rm_wunlock_debug(struct rmlock *rm, const char *file, int line)
{

        if (SCHEDULER_STOPPED())
                return;

        KASSERT(!rm_destroyed(rm),
            ("rm_wunlock() of destroyed rmlock @ %s:%d", file, line));
        _rm_assert(rm, RA_WLOCKED, file, line);
        WITNESS_UNLOCK(&rm->lock_object, LOP_EXCLUSIVE, file, line);
        LOCK_LOG_LOCK("RMWUNLOCK", &rm->lock_object, 0, 0, file, line);
        _rm_wunlock(rm);
        curthread->td_locks--;
}

int
_rm_rlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
    int trylock, const char *file, int line)
{

        if (SCHEDULER_STOPPED())
                return (1);

#ifdef INVARIANTS
        if (!(rm->lock_object.lo_flags & LO_RECURSABLE) && !trylock) {
                critical_enter();
                KASSERT(rm_trackers_present(pcpu_find(curcpu), rm,
                    curthread) == 0,
                    ("rm_rlock: recursed on non-recursive rmlock %s @ %s:%d\n",
                    rm->lock_object.lo_name, file, line));
                critical_exit();
        }
#endif
        KASSERT(kdb_active != 0 || !TD_IS_IDLETHREAD(curthread),
            ("rm_rlock() by idle thread %p on rmlock %s @ %s:%d",
            curthread, rm->lock_object.lo_name, file, line));
        KASSERT(!rm_destroyed(rm),
            ("rm_rlock() of destroyed rmlock @ %s:%d", file, line));
        if (!trylock) {
                KASSERT(!rm_wowned(rm),
                    ("rm_rlock: wlock already held for %s @ %s:%d",
                    rm->lock_object.lo_name, file, line));
                WITNESS_CHECKORDER(&rm->lock_object, LOP_NEWORDER, file, line,
                    NULL);
        }

        if (_rm_rlock(rm, tracker, trylock)) {
                if (trylock)
                        LOCK_LOG_TRY("RMRLOCK", &rm->lock_object, 0, 1, file,
                            line);
                else
                        LOCK_LOG_LOCK("RMRLOCK", &rm->lock_object, 0, 0, file,
                            line);
                WITNESS_LOCK(&rm->lock_object, 0, file, line);

                curthread->td_locks++;

                return (1);
        } else if (trylock)
                LOCK_LOG_TRY("RMRLOCK", &rm->lock_object, 0, 0, file, line);

        return (0);
}

void
_rm_runlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
    const char *file, int line)
{

        if (SCHEDULER_STOPPED())
                return;

        KASSERT(!rm_destroyed(rm),
            ("rm_runlock() of destroyed rmlock @ %s:%d", file, line));
        _rm_assert(rm, RA_RLOCKED, file, line);
        WITNESS_UNLOCK(&rm->lock_object, 0, file, line);
        LOCK_LOG_LOCK("RMRUNLOCK", &rm->lock_object, 0, 0, file, line);
        _rm_runlock(rm, tracker);
        curthread->td_locks--;
}

#else

/*
 * Just strip out file and line arguments if no lock debugging is enabled in
 * the kernel - we are called from a kernel module.
 */
void
_rm_wlock_debug(struct rmlock *rm, const char *file, int line)
{

        _rm_wlock(rm);
}

void
_rm_wunlock_debug(struct rmlock *rm, const char *file, int line)
{

        _rm_wunlock(rm);
}

int
_rm_rlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
    int trylock, const char *file, int line)
{

        return _rm_rlock(rm, tracker, trylock);
}

void
_rm_runlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
    const char *file, int line)
{

        _rm_runlock(rm, tracker);
}

#endif

#ifdef INVARIANT_SUPPORT
#ifndef INVARIANTS
#undef _rm_assert
#endif

/*
 * Note that this does not need to use witness_assert() for read lock
 * assertions since an exact count of read locks held by this thread
 * is computable.
 */
void
_rm_assert(struct rmlock *rm, int what, const char *file, int line)
{
        int count;

        if (panicstr != NULL)
                return;
        switch (what) {
        case RA_LOCKED:
        case RA_LOCKED | RA_RECURSED:
        case RA_LOCKED | RA_NOTRECURSED:
        case RA_RLOCKED:
        case RA_RLOCKED | RA_RECURSED:
        case RA_RLOCKED | RA_NOTRECURSED:
                /*
                 * Handle the write-locked case.  Unlike other
                 * primitives, writers can never recurse.
                 */
                if (rm_wowned(rm)) {
                        if (what & RA_RLOCKED)
                                panic("Lock %s exclusively locked @ %s:%d\n",
                                    rm->lock_object.lo_name, file, line);
                        if (what & RA_RECURSED)
                                panic("Lock %s not recursed @ %s:%d\n",
                                    rm->lock_object.lo_name, file, line);
                        break;
                }

                critical_enter();
                count = rm_trackers_present(pcpu_find(curcpu), rm, curthread);
                critical_exit();

                if (count == 0)
                        panic("Lock %s not %slocked @ %s:%d\n",
                            rm->lock_object.lo_name, (what & RA_RLOCKED) ?
                            "read " : "", file, line);
                if (count > 1) {
                        if (what & RA_NOTRECURSED)
                                panic("Lock %s recursed @ %s:%d\n",
                                    rm->lock_object.lo_name, file, line);
                } else if (what & RA_RECURSED)
                        panic("Lock %s not recursed @ %s:%d\n",
                            rm->lock_object.lo_name, file, line);
                break;
        case RA_WLOCKED:
                if (!rm_wowned(rm))
                        panic("Lock %s not exclusively locked @ %s:%d\n",
                            rm->lock_object.lo_name, file, line);
                break;
        case RA_UNLOCKED:
                if (rm_wowned(rm))
                        panic("Lock %s exclusively locked @ %s:%d\n",
                            rm->lock_object.lo_name, file, line);

                critical_enter();
                count = rm_trackers_present(pcpu_find(curcpu), rm, curthread);
                critical_exit();

                if (count != 0)
                        panic("Lock %s read locked @ %s:%d\n",
                            rm->lock_object.lo_name, file, line);
                break;
        default:
                panic("Unknown rm lock assertion: %d @ %s:%d", what, file,
                    line);
        }
}
#endif /* INVARIANT_SUPPORT */

#ifdef DDB
static void
print_tracker(struct rm_priotracker *tr)
{
        struct thread *td;

        td = tr->rmp_thread;
        db_printf("   thread %p (tid %d, pid %d, \"%s\") {", td, td->td_tid,
            td->td_proc->p_pid, td->td_name);
        if (tr->rmp_flags & RMPF_ONQUEUE) {
                db_printf("ONQUEUE");
                if (tr->rmp_flags & RMPF_SIGNAL)
                        db_printf(",SIGNAL");
        } else
                db_printf("0");
        db_printf("}\n");
}

static void
db_show_rm(struct lock_object *lock)
{
        struct rm_priotracker *tr;
        struct rm_queue *queue;
        struct rmlock *rm;
        struct lock_class *lc;
        struct pcpu *pc;

        rm = (struct rmlock *)lock;
        db_printf(" writecpus: ");
        ddb_display_cpuset(__DEQUALIFY(const cpuset_t *, &rm->rm_writecpus));
        db_printf("\n");
        db_printf(" per-CPU readers:\n");
        STAILQ_FOREACH(pc, &cpuhead, pc_allcpu)
                for (queue = pc->pc_rm_queue.rmq_next;
                    queue != &pc->pc_rm_queue; queue = queue->rmq_next) {
                        tr = (struct rm_priotracker *)queue;
                        if (tr->rmp_rmlock == rm)
                                print_tracker(tr);
                }
        db_printf(" active readers:\n");
        LIST_FOREACH(tr, &rm->rm_activeReaders, rmp_qentry)
                print_tracker(tr);
        lc = LOCK_CLASS(&rm->rm_wlock_object);
        db_printf("Backing write-lock (%s):\n", lc->lc_name);
        lc->lc_ddb_show(&rm->rm_wlock_object);
}
#endif