kern_thread.c

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/*-
 * Copyright (C) 2001 Julian Elischer <julian@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(s), this list of conditions and the following disclaimer as
 *    the first lines of this file unmodified other than the possible
 *    addition of one or more copyright notices.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice(s), this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDER(S) ``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 HOLDER(S) 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.
 */

#include "opt_witness.h"
#include "opt_kdtrace.h"
#include "opt_hwpmc_hooks.h"

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

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/rangelock.h>
#include <sys/resourcevar.h>
#include <sys/sdt.h>
#include <sys/smp.h>
#include <sys/sched.h>
#include <sys/sleepqueue.h>
#include <sys/selinfo.h>
#include <sys/turnstile.h>
#include <sys/ktr.h>
#include <sys/rwlock.h>
#include <sys/umtx.h>
#include <sys/cpuset.h>
#ifdef  HWPMC_HOOKS
#include <sys/pmckern.h>
#endif

#include <security/audit/audit.h>

#include <vm/vm.h>
#include <vm/vm_extern.h>
#include <vm/uma.h>
#include <sys/eventhandler.h>

SDT_PROVIDER_DECLARE(proc);
SDT_PROBE_DEFINE(proc, , , lwp__exit);


/*
 * thread related storage.
 */
static uma_zone_t thread_zone;

TAILQ_HEAD(, thread) zombie_threads = TAILQ_HEAD_INITIALIZER(zombie_threads);
static struct mtx zombie_lock;
MTX_SYSINIT(zombie_lock, &zombie_lock, "zombie lock", MTX_SPIN);

static void thread_zombie(struct thread *);

#define TID_BUFFER_SIZE 1024

struct mtx tid_lock;
static struct unrhdr *tid_unrhdr;
static lwpid_t tid_buffer[TID_BUFFER_SIZE];
static int tid_head, tid_tail;
static MALLOC_DEFINE(M_TIDHASH, "tidhash", "thread hash");

struct  tidhashhead *tidhashtbl;
u_long  tidhash;
struct  rwlock tidhash_lock;

static lwpid_t
tid_alloc(void)
{
        lwpid_t tid;

        tid = alloc_unr(tid_unrhdr);
        if (tid != -1)
                return (tid);
        mtx_lock(&tid_lock);
        if (tid_head == tid_tail) {
                mtx_unlock(&tid_lock);
                return (-1);
        }
        tid = tid_buffer[tid_head];
        tid_head = (tid_head + 1) % TID_BUFFER_SIZE;
        mtx_unlock(&tid_lock);
        return (tid);
}

static void
tid_free(lwpid_t tid)
{
        lwpid_t tmp_tid = -1;

        mtx_lock(&tid_lock);
        if ((tid_tail + 1) % TID_BUFFER_SIZE == tid_head) {
                tmp_tid = tid_buffer[tid_head];
                tid_head = (tid_head + 1) % TID_BUFFER_SIZE;
        }
        tid_buffer[tid_tail] = tid;
        tid_tail = (tid_tail + 1) % TID_BUFFER_SIZE;
        mtx_unlock(&tid_lock);
        if (tmp_tid != -1)
                free_unr(tid_unrhdr, tmp_tid);
}

/*
 * Prepare a thread for use.
 */
static int
thread_ctor(void *mem, int size, void *arg, int flags)
{
        struct thread   *td;

        td = (struct thread *)mem;
        td->td_state = TDS_INACTIVE;
        td->td_oncpu = NOCPU;

        td->td_tid = tid_alloc();

        /*
         * Note that td_critnest begins life as 1 because the thread is not
         * running and is thereby implicitly waiting to be on the receiving
         * end of a context switch.
         */
        td->td_critnest = 1;
        td->td_lend_user_pri = PRI_MAX;
        EVENTHANDLER_INVOKE(thread_ctor, td);
#ifdef AUDIT
        audit_thread_alloc(td);
#endif
        umtx_thread_alloc(td);
        return (0);
}

/*
 * Reclaim a thread after use.
 */
static void
thread_dtor(void *mem, int size, void *arg)
{
        struct thread *td;

        td = (struct thread *)mem;

#ifdef INVARIANTS
        /* Verify that this thread is in a safe state to free. */
        switch (td->td_state) {
        case TDS_INHIBITED:
        case TDS_RUNNING:
        case TDS_CAN_RUN:
        case TDS_RUNQ:
                /*
                 * We must never unlink a thread that is in one of
                 * these states, because it is currently active.
                 */
                panic("bad state for thread unlinking");
                /* NOTREACHED */
        case TDS_INACTIVE:
                break;
        default:
                panic("bad thread state");
                /* NOTREACHED */
        }
#endif
#ifdef AUDIT
        audit_thread_free(td);
#endif
        /* Free all OSD associated to this thread. */
        osd_thread_exit(td);

        EVENTHANDLER_INVOKE(thread_dtor, td);
        tid_free(td->td_tid);
}

/*
 * Initialize type-stable parts of a thread (when newly created).
 */
static int
thread_init(void *mem, int size, int flags)
{
        struct thread *td;

        td = (struct thread *)mem;

        td->td_sleepqueue = sleepq_alloc();
        td->td_turnstile = turnstile_alloc();
        td->td_rlqe = NULL;
        td->td_vp_reserv = 0;
        EVENTHANDLER_INVOKE(thread_init, td);
        td->td_sched = (struct td_sched *)&td[1];
        umtx_thread_init(td);
        td->td_kstack = 0;
        td->td_sel = NULL;
        return (0);
}

/*
 * Tear down type-stable parts of a thread (just before being discarded).
 */
static void
thread_fini(void *mem, int size)
{
        struct thread *td;

        td = (struct thread *)mem;
        EVENTHANDLER_INVOKE(thread_fini, td);
        rlqentry_free(td->td_rlqe);
        turnstile_free(td->td_turnstile);
        sleepq_free(td->td_sleepqueue);
        umtx_thread_fini(td);
        seltdfini(td);
}

/*
 * For a newly created process,
 * link up all the structures and its initial threads etc.
 * called from:
 * {arch}/{arch}/machdep.c   ia64_init(), init386() etc.
 * proc_dtor() (should go away)
 * proc_init()
 */
void
proc_linkup0(struct proc *p, struct thread *td)
{
        TAILQ_INIT(&p->p_threads);           /* all threads in proc */
        proc_linkup(p, td);
}

void
proc_linkup(struct proc *p, struct thread *td)
{

        sigqueue_init(&p->p_sigqueue, p);
        p->p_ksi = ksiginfo_alloc(1);
        if (p->p_ksi != NULL) {
                /* XXX p_ksi may be null if ksiginfo zone is not ready */
                p->p_ksi->ksi_flags = KSI_EXT | KSI_INS;
        }
        LIST_INIT(&p->p_mqnotifier);
        p->p_numthreads = 0;
        thread_link(td, p);
}

/*
 * Initialize global thread allocation resources.
 */
void
threadinit(void)
{

        mtx_init(&tid_lock, "TID lock", NULL, MTX_DEF);

        /*
         * pid_max cannot be greater than PID_MAX.
         * leave one number for thread0.
         */
        tid_unrhdr = new_unrhdr(PID_MAX + 2, INT_MAX, &tid_lock);

        thread_zone = uma_zcreate("THREAD", sched_sizeof_thread(),
            thread_ctor, thread_dtor, thread_init, thread_fini,
            16 - 1, 0);
        tidhashtbl = hashinit(maxproc / 2, M_TIDHASH, &tidhash);
        rw_init(&tidhash_lock, "tidhash");
}

/*
 * Place an unused thread on the zombie list.
 * Use the slpq as that must be unused by now.
 */
void
thread_zombie(struct thread *td)
{
        mtx_lock_spin(&zombie_lock);
        TAILQ_INSERT_HEAD(&zombie_threads, td, td_slpq);
        mtx_unlock_spin(&zombie_lock);
}

/*
 * Release a thread that has exited after cpu_throw().
 */
void
thread_stash(struct thread *td)
{
        atomic_subtract_rel_int(&td->td_proc->p_exitthreads, 1);
        thread_zombie(td);
}

/*
 * Reap zombie resources.
 */
void
thread_reap(void)
{
        struct thread *td_first, *td_next;

        /*
         * Don't even bother to lock if none at this instant,
         * we really don't care about the next instant..
         */
        if (!TAILQ_EMPTY(&zombie_threads)) {
                mtx_lock_spin(&zombie_lock);
                td_first = TAILQ_FIRST(&zombie_threads);
                if (td_first)
                        TAILQ_INIT(&zombie_threads);
                mtx_unlock_spin(&zombie_lock);
                while (td_first) {
                        td_next = TAILQ_NEXT(td_first, td_slpq);
                        if (td_first->td_ucred)
                                crfree(td_first->td_ucred);
                        thread_free(td_first);
                        td_first = td_next;
                }
        }
}

/*
 * Allocate a thread.
 */
struct thread *
thread_alloc(int pages)
{
        struct thread *td;

        thread_reap(); /* check if any zombies to get */

        td = (struct thread *)uma_zalloc(thread_zone, M_WAITOK);
        KASSERT(td->td_kstack == 0, ("thread_alloc got thread with kstack"));
        if (!vm_thread_new(td, pages)) {
                uma_zfree(thread_zone, td);
                return (NULL);
        }
        cpu_thread_alloc(td);
        return (td);
}

int
thread_alloc_stack(struct thread *td, int pages)
{

        KASSERT(td->td_kstack == 0,
            ("thread_alloc_stack called on a thread with kstack"));
        if (!vm_thread_new(td, pages))
                return (0);
        cpu_thread_alloc(td);
        return (1);
}

/*
 * Deallocate a thread.
 */
void
thread_free(struct thread *td)
{

        lock_profile_thread_exit(td);
        if (td->td_cpuset)
                cpuset_rel(td->td_cpuset);
        td->td_cpuset = NULL;
        cpu_thread_free(td);
        if (td->td_kstack != 0)
                vm_thread_dispose(td);
        uma_zfree(thread_zone, td);
}

/*
 * Discard the current thread and exit from its context.
 * Always called with scheduler locked.
 *
 * Because we can't free a thread while we're operating under its context,
 * push the current thread into our CPU's deadthread holder. This means
 * we needn't worry about someone else grabbing our context before we
 * do a cpu_throw().
 */
void
thread_exit(void)
{
        uint64_t runtime, new_switchtime;
        struct thread *td;
        struct thread *td2;
        struct proc *p;
        int wakeup_swapper;

        td = curthread;
        p = td->td_proc;

        PROC_SLOCK_ASSERT(p, MA_OWNED);
        mtx_assert(&Giant, MA_NOTOWNED);

        PROC_LOCK_ASSERT(p, MA_OWNED);
        KASSERT(p != NULL, ("thread exiting without a process"));
        CTR3(KTR_PROC, "thread_exit: thread %p (pid %ld, %s)", td,
            (long)p->p_pid, td->td_name);
        KASSERT(TAILQ_EMPTY(&td->td_sigqueue.sq_list), ("signal pending"));

#ifdef AUDIT
        AUDIT_SYSCALL_EXIT(0, td);
#endif
        umtx_thread_exit(td);
        /*
         * drop FPU & debug register state storage, or any other
         * architecture specific resources that
         * would not be on a new untouched process.
         */
        cpu_thread_exit(td);    /* XXXSMP */

        /*
         * The last thread is left attached to the process
         * So that the whole bundle gets recycled. Skip
         * all this stuff if we never had threads.
         * EXIT clears all sign of other threads when
         * it goes to single threading, so the last thread always
         * takes the short path.
         */
        if (p->p_flag & P_HADTHREADS) {
                if (p->p_numthreads > 1) {
                        thread_unlink(td);
                        td2 = FIRST_THREAD_IN_PROC(p);
                        sched_exit_thread(td2, td);

                        /*
                         * The test below is NOT true if we are the
                         * sole exiting thread. P_STOPPED_SINGLE is unset
                         * in exit1() after it is the only survivor.
                         */
                        if (P_SHOULDSTOP(p) == P_STOPPED_SINGLE) {
                                if (p->p_numthreads == p->p_suspcount) {
                                        thread_lock(p->p_singlethread);
                                        wakeup_swapper = thread_unsuspend_one(
                                                p->p_singlethread);
                                        thread_unlock(p->p_singlethread);
                                        if (wakeup_swapper)
                                                kick_proc0();
                                }
                        }

                        atomic_add_int(&td->td_proc->p_exitthreads, 1);
                        PCPU_SET(deadthread, td);
                } else {
                        /*
                         * The last thread is exiting.. but not through exit()
                         */
                        panic ("thread_exit: Last thread exiting on its own");
                }
        } 
#ifdef  HWPMC_HOOKS
        /*
         * If this thread is part of a process that is being tracked by hwpmc(4),
         * inform the module of the thread's impending exit.
         */
        if (PMC_PROC_IS_USING_PMCS(td->td_proc))
                PMC_SWITCH_CONTEXT(td, PMC_FN_CSW_OUT);
#endif
        PROC_UNLOCK(p);

        /* Do the same timestamp bookkeeping that mi_switch() would do. */
        new_switchtime = cpu_ticks();
        runtime = new_switchtime - PCPU_GET(switchtime);
        td->td_runtime += runtime;
        td->td_incruntime += runtime;
        PCPU_SET(switchtime, new_switchtime);
        PCPU_SET(switchticks, ticks);
        PCPU_INC(cnt.v_swtch);

        /* Save our resource usage in our process. */
        td->td_ru.ru_nvcsw++;
        ruxagg(p, td);
        rucollect(&p->p_ru, &td->td_ru);

        thread_lock(td);
        PROC_SUNLOCK(p);
        td->td_state = TDS_INACTIVE;
#ifdef WITNESS
        witness_thread_exit(td);
#endif
        CTR1(KTR_PROC, "thread_exit: cpu_throw() thread %p", td);
        sched_throw(td);
        panic("I'm a teapot!");
        /* NOTREACHED */
}

/*
 * Do any thread specific cleanups that may be needed in wait()
 * called with Giant, proc and schedlock not held.
 */
void
thread_wait(struct proc *p)
{
        struct thread *td;

        mtx_assert(&Giant, MA_NOTOWNED);
        KASSERT((p->p_numthreads == 1), ("Multiple threads in wait1()"));
        td = FIRST_THREAD_IN_PROC(p);
        /* Lock the last thread so we spin until it exits cpu_throw(). */
        thread_lock(td);
        thread_unlock(td);
        /* Wait for any remaining threads to exit cpu_throw(). */
        while (p->p_exitthreads)
                sched_relinquish(curthread);
        lock_profile_thread_exit(td);
        cpuset_rel(td->td_cpuset);
        td->td_cpuset = NULL;
        cpu_thread_clean(td);
        crfree(td->td_ucred);
        thread_reap();  /* check for zombie threads etc. */
}

/*
 * Link a thread to a process.
 * set up anything that needs to be initialized for it to
 * be used by the process.
 */
void
thread_link(struct thread *td, struct proc *p)
{

        /*
         * XXX This can't be enabled because it's called for proc0 before
         * its lock has been created.
         * PROC_LOCK_ASSERT(p, MA_OWNED);
         */
        td->td_state    = TDS_INACTIVE;
        td->td_proc     = p;
        td->td_flags    = TDF_INMEM;

        LIST_INIT(&td->td_contested);
        LIST_INIT(&td->td_lprof[0]);
        LIST_INIT(&td->td_lprof[1]);
        sigqueue_init(&td->td_sigqueue, p);
        callout_init(&td->td_slpcallout, CALLOUT_MPSAFE);
        TAILQ_INSERT_HEAD(&p->p_threads, td, td_plist);
        p->p_numthreads++;
}

/*
 * Convert a process with one thread to an unthreaded process.
 */
void
thread_unthread(struct thread *td)
{
        struct proc *p = td->td_proc;

        KASSERT((p->p_numthreads == 1), ("Unthreading with >1 threads"));
        p->p_flag &= ~P_HADTHREADS;
}

/*
 * Called from:
 *  thread_exit()
 */
void
thread_unlink(struct thread *td)
{
        struct proc *p = td->td_proc;

        PROC_LOCK_ASSERT(p, MA_OWNED);
        TAILQ_REMOVE(&p->p_threads, td, td_plist);
        p->p_numthreads--;
        /* could clear a few other things here */
        /* Must  NOT clear links to proc! */
}

static int
calc_remaining(struct proc *p, int mode)
{
        int remaining;

        PROC_LOCK_ASSERT(p, MA_OWNED);
        PROC_SLOCK_ASSERT(p, MA_OWNED);
        if (mode == SINGLE_EXIT)
                remaining = p->p_numthreads;
        else if (mode == SINGLE_BOUNDARY)
                remaining = p->p_numthreads - p->p_boundary_count;
        else if (mode == SINGLE_NO_EXIT)
                remaining = p->p_numthreads - p->p_suspcount;
        else
                panic("calc_remaining: wrong mode %d", mode);
        return (remaining);
}

/*
 * Enforce single-threading.
 *
 * Returns 1 if the caller must abort (another thread is waiting to
 * exit the process or similar). Process is locked!
 * Returns 0 when you are successfully the only thread running.
 * A process has successfully single threaded in the suspend mode when
 * There are no threads in user mode. Threads in the kernel must be
 * allowed to continue until they get to the user boundary. They may even
 * copy out their return values and data before suspending. They may however be
 * accelerated in reaching the user boundary as we will wake up
 * any sleeping threads that are interruptable. (PCATCH).
 */
int
thread_single(int mode)
{
        struct thread *td;
        struct thread *td2;
        struct proc *p;
        int remaining, wakeup_swapper;

        td = curthread;
        p = td->td_proc;
        mtx_assert(&Giant, MA_NOTOWNED);
        PROC_LOCK_ASSERT(p, MA_OWNED);

        if ((p->p_flag & P_HADTHREADS) == 0)
                return (0);

        /* Is someone already single threading? */
        if (p->p_singlethread != NULL && p->p_singlethread != td)
                return (1);

        if (mode == SINGLE_EXIT) {
                p->p_flag |= P_SINGLE_EXIT;
                p->p_flag &= ~P_SINGLE_BOUNDARY;
        } else {
                p->p_flag &= ~P_SINGLE_EXIT;
                if (mode == SINGLE_BOUNDARY)
                        p->p_flag |= P_SINGLE_BOUNDARY;
                else
                        p->p_flag &= ~P_SINGLE_BOUNDARY;
        }
        p->p_flag |= P_STOPPED_SINGLE;
        PROC_SLOCK(p);
        p->p_singlethread = td;
        remaining = calc_remaining(p, mode);
        while (remaining != 1) {
                if (P_SHOULDSTOP(p) != P_STOPPED_SINGLE)
                        goto stopme;
                wakeup_swapper = 0;
                FOREACH_THREAD_IN_PROC(p, td2) {
                        if (td2 == td)
                                continue;
                        thread_lock(td2);
                        td2->td_flags |= TDF_ASTPENDING | TDF_NEEDSUSPCHK;
                        if (TD_IS_INHIBITED(td2)) {
                                switch (mode) {
                                case SINGLE_EXIT:
                                        if (TD_IS_SUSPENDED(td2))
                                                wakeup_swapper |=
                                                    thread_unsuspend_one(td2);
                                        if (TD_ON_SLEEPQ(td2) &&
                                            (td2->td_flags & TDF_SINTR))
                                                wakeup_swapper |=
                                                    sleepq_abort(td2, EINTR);
                                        break;
                                case SINGLE_BOUNDARY:
                                        if (TD_IS_SUSPENDED(td2) &&
                                            !(td2->td_flags & TDF_BOUNDARY))
                                                wakeup_swapper |=
                                                    thread_unsuspend_one(td2);
                                        if (TD_ON_SLEEPQ(td2) &&
                                            (td2->td_flags & TDF_SINTR))
                                                wakeup_swapper |=
                                                    sleepq_abort(td2, ERESTART);
                                        break;
                                case SINGLE_NO_EXIT:
                                        if (TD_IS_SUSPENDED(td2) &&
                                            !(td2->td_flags & TDF_BOUNDARY))
                                                wakeup_swapper |=
                                                    thread_unsuspend_one(td2);
                                        if (TD_ON_SLEEPQ(td2) &&
                                            (td2->td_flags & TDF_SINTR))
                                                wakeup_swapper |=
                                                    sleepq_abort(td2, ERESTART);
                                        break;
                                default:
                                        break;
                                }
                        }
#ifdef SMP
                        else if (TD_IS_RUNNING(td2) && td != td2) {
                                forward_signal(td2);
                        }
#endif
                        thread_unlock(td2);
                }
                if (wakeup_swapper)
                        kick_proc0();
                remaining = calc_remaining(p, mode);

                /*
                 * Maybe we suspended some threads.. was it enough?
                 */
                if (remaining == 1)
                        break;

stopme:
                /*
                 * Wake us up when everyone else has suspended.
                 * In the mean time we suspend as well.
                 */
                thread_suspend_switch(td);
                remaining = calc_remaining(p, mode);
        }
        if (mode == SINGLE_EXIT) {
                /*
                 * We have gotten rid of all the other threads and we
                 * are about to either exit or exec. In either case,
                 * we try our utmost  to revert to being a non-threaded
                 * process.
                 */
                p->p_singlethread = NULL;
                p->p_flag &= ~(P_STOPPED_SINGLE | P_SINGLE_EXIT);
                thread_unthread(td);
        }
        PROC_SUNLOCK(p);
        return (0);
}

/*
 * Called in from locations that can safely check to see
 * whether we have to suspend or at least throttle for a
 * single-thread event (e.g. fork).
 *
 * Such locations include userret().
 * If the "return_instead" argument is non zero, the thread must be able to
 * accept 0 (caller may continue), or 1 (caller must abort) as a result.
 *
 * The 'return_instead' argument tells the function if it may do a
 * thread_exit() or suspend, or whether the caller must abort and back
 * out instead.
 *
 * If the thread that set the single_threading request has set the
 * P_SINGLE_EXIT bit in the process flags then this call will never return
 * if 'return_instead' is false, but will exit.
 *
 * P_SINGLE_EXIT | return_instead == 0| return_instead != 0
 *---------------+--------------------+---------------------
 *       0       | returns 0          |   returns 0 or 1
 *               | when ST ends       |   immediatly
 *---------------+--------------------+---------------------
 *       1       | thread exits       |   returns 1
 *               |                    |  immediatly
 * 0 = thread_exit() or suspension ok,
 * other = return error instead of stopping the thread.
 *
 * While a full suspension is under effect, even a single threading
 * thread would be suspended if it made this call (but it shouldn't).
 * This call should only be made from places where
 * thread_exit() would be safe as that may be the outcome unless
 * return_instead is set.
 */
int
thread_suspend_check(int return_instead)
{
        struct thread *td;
        struct proc *p;
        int wakeup_swapper;

        td = curthread;
        p = td->td_proc;
        mtx_assert(&Giant, MA_NOTOWNED);
        PROC_LOCK_ASSERT(p, MA_OWNED);
        while (P_SHOULDSTOP(p) ||
              ((p->p_flag & P_TRACED) && (td->td_dbgflags & TDB_SUSPEND))) {
                if (P_SHOULDSTOP(p) == P_STOPPED_SINGLE) {
                        KASSERT(p->p_singlethread != NULL,
                            ("singlethread not set"));
                        /*
                         * The only suspension in action is a
                         * single-threading. Single threader need not stop.
                         * XXX Should be safe to access unlocked
                         * as it can only be set to be true by us.
                         */
                        if (p->p_singlethread == td)
                                return (0);     /* Exempt from stopping. */
                }
                if ((p->p_flag & P_SINGLE_EXIT) && return_instead)
                        return (EINTR);

                /* Should we goto user boundary if we didn't come from there? */
                if (P_SHOULDSTOP(p) == P_STOPPED_SINGLE &&
                    (p->p_flag & P_SINGLE_BOUNDARY) && return_instead)
                        return (ERESTART);

                /*
                 * Ignore suspend requests for stop signals if they
                 * are deferred.
                 */
                if (P_SHOULDSTOP(p) == P_STOPPED_SIG &&
                    td->td_flags & TDF_SBDRY) {
                        KASSERT(return_instead,
                            ("TDF_SBDRY set for unsafe thread_suspend_check"));
                        return (0);
                }

                /*
                 * If the process is waiting for us to exit,
                 * this thread should just suicide.
                 * Assumes that P_SINGLE_EXIT implies P_STOPPED_SINGLE.
                 */
                if ((p->p_flag & P_SINGLE_EXIT) && (p->p_singlethread != td)) {
                        PROC_UNLOCK(p);
                        tidhash_remove(td);
                        PROC_LOCK(p);
                        tdsigcleanup(td);
                        PROC_SLOCK(p);
                        thread_stopped(p);
                        thread_exit();
                }

                PROC_SLOCK(p);
                thread_stopped(p);
                if (P_SHOULDSTOP(p) == P_STOPPED_SINGLE) {
                        if (p->p_numthreads == p->p_suspcount + 1) {
                                thread_lock(p->p_singlethread);
                                wakeup_swapper =
                                    thread_unsuspend_one(p->p_singlethread);
                                thread_unlock(p->p_singlethread);
                                if (wakeup_swapper)
                                        kick_proc0();
                        }
                }
                PROC_UNLOCK(p);
                thread_lock(td);
                /*
                 * When a thread suspends, it just
                 * gets taken off all queues.
                 */
                thread_suspend_one(td);
                if (return_instead == 0) {
                        p->p_boundary_count++;
                        td->td_flags |= TDF_BOUNDARY;
                }
                PROC_SUNLOCK(p);
                mi_switch(SW_INVOL | SWT_SUSPEND, NULL);
                if (return_instead == 0)
                        td->td_flags &= ~TDF_BOUNDARY;
                thread_unlock(td);
                PROC_LOCK(p);
                if (return_instead == 0) {
                        PROC_SLOCK(p);
                        p->p_boundary_count--;
                        PROC_SUNLOCK(p);
                }
        }
        return (0);
}

void
thread_suspend_switch(struct thread *td)
{
        struct proc *p;

        p = td->td_proc;
        KASSERT(!TD_IS_SUSPENDED(td), ("already suspended"));
        PROC_LOCK_ASSERT(p, MA_OWNED);
        PROC_SLOCK_ASSERT(p, MA_OWNED);
        /*
         * We implement thread_suspend_one in stages here to avoid
         * dropping the proc lock while the thread lock is owned.
         */
        thread_stopped(p);
        p->p_suspcount++;
        PROC_UNLOCK(p);
        thread_lock(td);
        td->td_flags &= ~TDF_NEEDSUSPCHK;
        TD_SET_SUSPENDED(td);
        sched_sleep(td, 0);
        PROC_SUNLOCK(p);
        DROP_GIANT();
        mi_switch(SW_VOL | SWT_SUSPEND, NULL);
        thread_unlock(td);
        PICKUP_GIANT();
        PROC_LOCK(p);
        PROC_SLOCK(p);
}

void
thread_suspend_one(struct thread *td)
{
        struct proc *p = td->td_proc;

        PROC_SLOCK_ASSERT(p, MA_OWNED);
        THREAD_LOCK_ASSERT(td, MA_OWNED);
        KASSERT(!TD_IS_SUSPENDED(td), ("already suspended"));
        p->p_suspcount++;
        td->td_flags &= ~TDF_NEEDSUSPCHK;
        TD_SET_SUSPENDED(td);
        sched_sleep(td, 0);
}

int
thread_unsuspend_one(struct thread *td)
{
        struct proc *p = td->td_proc;

        PROC_SLOCK_ASSERT(p, MA_OWNED);
        THREAD_LOCK_ASSERT(td, MA_OWNED);
        KASSERT(TD_IS_SUSPENDED(td), ("Thread not suspended"));
        TD_CLR_SUSPENDED(td);
        p->p_suspcount--;
        return (setrunnable(td));
}

/*
 * Allow all threads blocked by single threading to continue running.
 */
void
thread_unsuspend(struct proc *p)
{
        struct thread *td;
        int wakeup_swapper;

        PROC_LOCK_ASSERT(p, MA_OWNED);
        PROC_SLOCK_ASSERT(p, MA_OWNED);
        wakeup_swapper = 0;
        if (!P_SHOULDSTOP(p)) {
                FOREACH_THREAD_IN_PROC(p, td) {
                        thread_lock(td);
                        if (TD_IS_SUSPENDED(td)) {
                                wakeup_swapper |= thread_unsuspend_one(td);
                        }
                        thread_unlock(td);
                }
        } else if ((P_SHOULDSTOP(p) == P_STOPPED_SINGLE) &&
            (p->p_numthreads == p->p_suspcount)) {
                /*
                 * Stopping everything also did the job for the single
                 * threading request. Now we've downgraded to single-threaded,
                 * let it continue.
                 */
                thread_lock(p->p_singlethread);
                wakeup_swapper = thread_unsuspend_one(p->p_singlethread);
                thread_unlock(p->p_singlethread);
        }
        if (wakeup_swapper)
                kick_proc0();
}

/*
 * End the single threading mode..
 */
void
thread_single_end(void)
{
        struct thread *td;
        struct proc *p;
        int wakeup_swapper;

        td = curthread;
        p = td->td_proc;
        PROC_LOCK_ASSERT(p, MA_OWNED);
        p->p_flag &= ~(P_STOPPED_SINGLE | P_SINGLE_EXIT | P_SINGLE_BOUNDARY);
        PROC_SLOCK(p);
        p->p_singlethread = NULL;
        wakeup_swapper = 0;
        /*
         * If there are other threads they may now run,
         * unless of course there is a blanket 'stop order'
         * on the process. The single threader must be allowed
         * to continue however as this is a bad place to stop.
         */
        if ((p->p_numthreads != 1) && (!P_SHOULDSTOP(p))) {
                FOREACH_THREAD_IN_PROC(p, td) {
                        thread_lock(td);
                        if (TD_IS_SUSPENDED(td)) {
                                wakeup_swapper |= thread_unsuspend_one(td);
                        }
                        thread_unlock(td);
                }
        }
        PROC_SUNLOCK(p);
        if (wakeup_swapper)
                kick_proc0();
}

struct thread *
thread_find(struct proc *p, lwpid_t tid)
{
        struct thread *td;

        PROC_LOCK_ASSERT(p, MA_OWNED);
        FOREACH_THREAD_IN_PROC(p, td) {
                if (td->td_tid == tid)
                        break;
        }
        return (td);
}

/* Locate a thread by number; return with proc lock held. */
struct thread *
tdfind(lwpid_t tid, pid_t pid)
{
#define RUN_THRESH      16
        struct thread *td;
        int run = 0;

        rw_rlock(&tidhash_lock);
        LIST_FOREACH(td, TIDHASH(tid), td_hash) {
                if (td->td_tid == tid) {
                        if (pid != -1 && td->td_proc->p_pid != pid) {
                                td = NULL;
                                break;
                        }
                        PROC_LOCK(td->td_proc);
                        if (td->td_proc->p_state == PRS_NEW) {
                                PROC_UNLOCK(td->td_proc);
                                td = NULL;
                                break;
                        }
                        if (run > RUN_THRESH) {
                                if (rw_try_upgrade(&tidhash_lock)) {
                                        LIST_REMOVE(td, td_hash);
                                        LIST_INSERT_HEAD(TIDHASH(td->td_tid),
                                                td, td_hash);
                                        rw_wunlock(&tidhash_lock);
                                        return (td);
                                }
                        }
                        break;
                }
                run++;
        }
        rw_runlock(&tidhash_lock);
        return (td);
}

void
tidhash_add(struct thread *td)
{
        rw_wlock(&tidhash_lock);
        LIST_INSERT_HEAD(TIDHASH(td->td_tid), td, td_hash);
        rw_wunlock(&tidhash_lock);
}

void
tidhash_remove(struct thread *td)
{
        rw_wlock(&tidhash_lock);
        LIST_REMOVE(td, td_hash);
        rw_wunlock(&tidhash_lock);
}