/* * CDDL HEADER START * * The contents of this file are subject to the terms of the * Common Development and Distribution License (the "License"). * You may not use this file except in compliance with the License. * * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE * or http://www.opensolaris.org/os/licensing. * See the License for the specific language governing permissions * and limitations under the License. * * When distributing Covered Code, include this CDDL HEADER in each * file and include the License file at usr/src/OPENSOLARIS.LICENSE. * If applicable, add the following below this CDDL HEADER, with the * fields enclosed by brackets "[]" replaced with your own identifying * information: Portions Copyright [yyyy] [name of copyright owner] * * CDDL HEADER END */ /* * Copyright 2007 Sun Microsystems, Inc. All rights reserved. * Use is subject to license terms. */ #pragma ident "%Z%%M% %I% %E% SMI" #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include /* * Tasks * * A task is a collection of processes, associated with a common project ID * and related by a common initial parent. The task primarily represents a * natural process sequence with known resource usage, although it can also be * viewed as a convenient grouping of processes for signal delivery, processor * binding, and administrative operations. * * Membership and observership * We can conceive of situations where processes outside of the task may wish * to examine the resource usage of the task. Similarly, a number of the * administrative operations on a task can be performed by processes who are * not members of the task. Accordingly, we must design a locking strategy * where observers of the task, who wish to examine or operate on the task, * and members of task, who can perform the mentioned operations, as well as * leave the task, see a consistent and correct representation of the task at * all times. * * Locking * Because the task membership is a new relation between processes, its * locking becomes an additional responsibility of the pidlock/p_lock locking * sequence; however, tasks closely resemble sessions and the session locking * model is mostly appropriate for the interaction of tasks, processes, and * procfs. * * kmutex_t task_hash_lock * task_hash_lock is a global lock protecting the contents of the task * ID-to-task pointer hash. Holders of task_hash_lock must not attempt to * acquire pidlock or p_lock. * uint_t tk_hold_count * tk_hold_count, the number of members and observers of the current task, * must be manipulated atomically. * proc_t *tk_memb_list * proc_t *p_tasknext * proc_t *p_taskprev * The task's membership list is protected by pidlock, and is therefore * always acquired before any of its members' p_lock mutexes. The p_task * member of the proc structure is protected by pidlock or p_lock for * reading, and by both pidlock and p_lock for modification, as is done for * p_sessp. The key point is that only the process can modify its p_task, * and not any entity on the system. (/proc will use prlock() to prevent * the process from leaving, as opposed to pidlock.) * kmutex_t tk_usage_lock * tk_usage_lock is a per-task lock protecting the contents of the task * usage structure and tk_nlwps counter for the task.max-lwps resource * control. */ int task_hash_size = 256; static kmutex_t task_hash_lock; static mod_hash_t *task_hash; static id_space_t *taskid_space; /* global taskid space */ static kmem_cache_t *task_cache; /* kmem cache for task structures */ rctl_hndl_t rc_task_lwps; rctl_hndl_t rc_task_cpu_time; /* * static rctl_qty_t task_usage_lwps(void *taskp) * * Overview * task_usage_lwps() is the usage operation for the resource control * associated with the number of LWPs in a task. * * Return values * The number of LWPs in the given task is returned. * * Caller's context * The p->p_lock must be held across the call. */ /*ARGSUSED*/ static rctl_qty_t task_lwps_usage(rctl_t *r, proc_t *p) { task_t *t; rctl_qty_t nlwps; ASSERT(MUTEX_HELD(&p->p_lock)); t = p->p_task; mutex_enter(&p->p_zone->zone_nlwps_lock); nlwps = t->tk_nlwps; mutex_exit(&p->p_zone->zone_nlwps_lock); return (nlwps); } /* * static int task_test_lwps(void *taskp, rctl_val_t *, int64_t incr, * int flags) * * Overview * task_test_lwps() is the test-if-valid-increment for the resource control * for the number of processes in a task. * * Return values * 0 if the threshold limit was not passed, 1 if the limit was passed. * * Caller's context * p->p_lock must be held across the call. */ /*ARGSUSED*/ static int task_lwps_test(rctl_t *r, proc_t *p, rctl_entity_p_t *e, rctl_val_t *rcntl, rctl_qty_t incr, uint_t flags) { rctl_qty_t nlwps; ASSERT(MUTEX_HELD(&p->p_lock)); ASSERT(e->rcep_t == RCENTITY_TASK); if (e->rcep_p.task == NULL) return (0); ASSERT(MUTEX_HELD(&(e->rcep_p.task->tk_zone->zone_nlwps_lock))); nlwps = e->rcep_p.task->tk_nlwps; if (nlwps + incr > rcntl->rcv_value) return (1); return (0); } /*ARGSUSED*/ static int task_lwps_set(rctl_t *rctl, struct proc *p, rctl_entity_p_t *e, rctl_qty_t nv) { ASSERT(MUTEX_HELD(&p->p_lock)); ASSERT(e->rcep_t == RCENTITY_TASK); if (e->rcep_p.task == NULL) return (0); e->rcep_p.task->tk_nlwps_ctl = nv; return (0); } /* * static rctl_qty_t task_usage_cpu_secs(void *taskp) * * Overview * task_usage_cpu_secs() is the usage operation for the resource control * associated with the total accrued CPU seconds for a task. * * Return values * The number of CPU seconds consumed by the task is returned. * * Caller's context * The given task must be held across the call. */ /*ARGSUSED*/ static rctl_qty_t task_cpu_time_usage(rctl_t *r, proc_t *p) { task_t *t = p->p_task; ASSERT(MUTEX_HELD(&p->p_lock)); return (t->tk_cpu_time / hz); } /* * static int task_test_cpu_secs(void *taskp, rctl_val_t *, int64_t incr, * int flags) * * Overview * task_test_cpu_secs() is the test-if-valid-increment for the resource * control for the total accrued CPU seconds for a task. * * Return values * 0 if the threshold limit was not passed, 1 if the limit was passed. * * Caller's context * The given task must be held across the call. */ /*ARGSUSED*/ static int task_cpu_time_test(rctl_t *r, proc_t *p, rctl_entity_p_t *e, struct rctl_val *rcntl, rctl_qty_t incr, uint_t flags) { task_t *t; ASSERT(MUTEX_HELD(&p->p_lock)); ASSERT(e->rcep_t == RCENTITY_TASK); if (e->rcep_p.task == NULL) return (0); t = e->rcep_p.task; if ((t->tk_cpu_time + incr) / hz >= rcntl->rcv_value) return (1); return (0); } static task_t * task_find(taskid_t id, zoneid_t zoneid) { task_t *tk; ASSERT(MUTEX_HELD(&task_hash_lock)); if (mod_hash_find(task_hash, (mod_hash_key_t)(uintptr_t)id, (mod_hash_val_t *)&tk) == MH_ERR_NOTFOUND || (zoneid != ALL_ZONES && zoneid != tk->tk_zone->zone_id)) return (NULL); return (tk); } /* * task_hold_by_id(), task_hold_by_id_zone() * * Overview * task_hold_by_id() is used to take a reference on a task by its task id, * supporting the various system call interfaces for obtaining resource data, * delivering signals, and so forth. * * Return values * Returns a pointer to the task_t with taskid_t id. The task is returned * with its hold count incremented by one. Returns NULL if there * is no task with the requested id. * * Caller's context * Caller must not be holding task_hash_lock. No restrictions on context. */ task_t * task_hold_by_id_zone(taskid_t id, zoneid_t zoneid) { task_t *tk; mutex_enter(&task_hash_lock); if ((tk = task_find(id, zoneid)) != NULL) atomic_add_32(&tk->tk_hold_count, 1); mutex_exit(&task_hash_lock); return (tk); } task_t * task_hold_by_id(taskid_t id) { zoneid_t zoneid; if (INGLOBALZONE(curproc)) zoneid = ALL_ZONES; else zoneid = getzoneid(); return (task_hold_by_id_zone(id, zoneid)); } /* * void task_hold(task_t *) * * Overview * task_hold() is used to take an additional reference to the given task. * * Return values * None. * * Caller's context * No restriction on context. */ void task_hold(task_t *tk) { atomic_add_32(&tk->tk_hold_count, 1); } /* * void task_rele(task_t *) * * Overview * task_rele() relinquishes a reference on the given task, which was acquired * via task_hold() or task_hold_by_id(). If this is the last member or * observer of the task, dispatch it for commitment via the accounting * subsystem. * * Return values * None. * * Caller's context * Caller must not be holding the task_hash_lock. * Caller's context must be acceptable for KM_SLEEP allocations. */ void task_rele(task_t *tk) { mutex_enter(&task_hash_lock); if (atomic_add_32_nv(&tk->tk_hold_count, -1) > 0) { mutex_exit(&task_hash_lock); return; } mutex_enter(&tk->tk_zone->zone_nlwps_lock); tk->tk_proj->kpj_ntasks--; mutex_exit(&tk->tk_zone->zone_nlwps_lock); if (mod_hash_destroy(task_hash, (mod_hash_key_t)(uintptr_t)tk->tk_tkid) != 0) panic("unable to delete task %d", tk->tk_tkid); mutex_exit(&task_hash_lock); /* * At this point, there are no members or observers of the task, so we * can safely send it on for commitment to the accounting subsystem. * The task will be destroyed in task_end() subsequent to commitment. */ (void) taskq_dispatch(exacct_queue, exacct_commit_task, tk, KM_SLEEP); } /* * task_t *task_create(projid_t, zone *) * * Overview * A process constructing a new task calls task_create() to construct and * preinitialize the task for the appropriate destination project. Only one * task, the primordial task0, is not created with task_create(). * * Return values * None. * * Caller's context * Caller's context should be safe for KM_SLEEP allocations. * The caller should appropriately bump the kpj_ntasks counter on the * project that contains this task. */ task_t * task_create(projid_t projid, zone_t *zone) { task_t *tk = kmem_cache_alloc(task_cache, KM_SLEEP); task_t *ancestor_tk; taskid_t tkid; task_usage_t *tu = kmem_zalloc(sizeof (task_usage_t), KM_SLEEP); mod_hash_hndl_t hndl; rctl_set_t *set = rctl_set_create(); rctl_alloc_gp_t *gp; rctl_entity_p_t e; bzero(tk, sizeof (task_t)); tk->tk_tkid = tkid = id_alloc(taskid_space); tk->tk_nlwps = 0; tk->tk_nlwps_ctl = INT_MAX; tk->tk_usage = tu; tk->tk_proj = project_hold_by_id(projid, zone, PROJECT_HOLD_INSERT); tk->tk_flags = TASK_NORMAL; /* * Copy ancestor task's resource controls. */ zone_task_hold(zone); mutex_enter(&curproc->p_lock); ancestor_tk = curproc->p_task; task_hold(ancestor_tk); tk->tk_zone = zone; mutex_exit(&curproc->p_lock); for (;;) { gp = rctl_set_dup_prealloc(ancestor_tk->tk_rctls); mutex_enter(&ancestor_tk->tk_rctls->rcs_lock); if (rctl_set_dup_ready(ancestor_tk->tk_rctls, gp)) break; mutex_exit(&ancestor_tk->tk_rctls->rcs_lock); rctl_prealloc_destroy(gp); } /* * At this point, curproc does not have the appropriate linkage * through the task to the project. So, rctl_set_dup should only * copy the rctls, and leave the callbacks for later. */ e.rcep_p.task = tk; e.rcep_t = RCENTITY_TASK; tk->tk_rctls = rctl_set_dup(ancestor_tk->tk_rctls, curproc, curproc, &e, set, gp, RCD_DUP); mutex_exit(&ancestor_tk->tk_rctls->rcs_lock); rctl_prealloc_destroy(gp); /* * Record the ancestor task's ID for use by extended accounting. */ tu->tu_anctaskid = ancestor_tk->tk_tkid; task_rele(ancestor_tk); /* * Put new task structure in the hash table. */ (void) mod_hash_reserve(task_hash, &hndl); mutex_enter(&task_hash_lock); ASSERT(task_find(tkid, getzoneid()) == NULL); if (mod_hash_insert_reserve(task_hash, (mod_hash_key_t)(uintptr_t)tkid, (mod_hash_val_t *)tk, hndl) != 0) { mod_hash_cancel(task_hash, &hndl); panic("unable to insert task %d(%p)", tkid, (void *)tk); } mutex_exit(&task_hash_lock); return (tk); } /* * void task_attach(task_t *, proc_t *) * * Overview * task_attach() is used to attach a process to a task; this operation is only * performed as a result of a fork() or settaskid() system call. The proc_t's * p_tasknext and p_taskprev fields will be set such that the proc_t is a * member of the doubly-linked list of proc_t's that make up the task. * * Return values * None. * * Caller's context * pidlock and p->p_lock must be held on entry. */ void task_attach(task_t *tk, proc_t *p) { proc_t *first, *prev; rctl_entity_p_t e; ASSERT(tk != NULL); ASSERT(p != NULL); ASSERT(MUTEX_HELD(&pidlock)); ASSERT(MUTEX_HELD(&p->p_lock)); if (tk->tk_memb_list == NULL) { p->p_tasknext = p; p->p_taskprev = p; } else { first = tk->tk_memb_list; prev = first->p_taskprev; first->p_taskprev = p; p->p_tasknext = first; p->p_taskprev = prev; prev->p_tasknext = p; } tk->tk_memb_list = p; task_hold(tk); p->p_task = tk; /* * Now that the linkage from process to task and project is * complete, do the required callbacks for the task and project * rctl sets. */ e.rcep_p.proj = tk->tk_proj; e.rcep_t = RCENTITY_PROJECT; (void) rctl_set_dup(NULL, NULL, p, &e, tk->tk_proj->kpj_rctls, NULL, RCD_CALLBACK); e.rcep_p.task = tk; e.rcep_t = RCENTITY_TASK; (void) rctl_set_dup(NULL, NULL, p, &e, tk->tk_rctls, NULL, RCD_CALLBACK); } /* * task_begin() * * Overview * A process constructing a new task calls task_begin() to initialize the * task, by attaching itself as a member. * * Return values * None. * * Caller's context * pidlock and p_lock must be held across the call to task_begin(). */ void task_begin(task_t *tk, proc_t *p) { timestruc_t ts; task_usage_t *tu; ASSERT(MUTEX_HELD(&pidlock)); ASSERT(MUTEX_HELD(&p->p_lock)); mutex_enter(&tk->tk_usage_lock); tu = tk->tk_usage; gethrestime(&ts); tu->tu_startsec = (uint64_t)ts.tv_sec; tu->tu_startnsec = (uint64_t)ts.tv_nsec; mutex_exit(&tk->tk_usage_lock); /* * Join process to the task as a member. */ task_attach(tk, p); } /* * void task_detach(proc_t *) * * Overview * task_detach() removes the specified process from its task. task_detach * sets the process's task membership to NULL, in anticipation of a final exit * or of joining a new task. Because task_rele() requires a context safe for * KM_SLEEP allocations, a task_detach() is followed by a subsequent * task_rele() once appropriate context is available. * * Because task_detach() involves relinquishing the process's membership in * the project, any observational rctls the process may have had on the task * or project are destroyed. * * Return values * None. * * Caller's context * pidlock and p_lock held across task_detach(). */ void task_detach(proc_t *p) { task_t *tk = p->p_task; ASSERT(MUTEX_HELD(&pidlock)); ASSERT(MUTEX_HELD(&p->p_lock)); ASSERT(p->p_task != NULL); ASSERT(tk->tk_memb_list != NULL); if (tk->tk_memb_list == p) tk->tk_memb_list = p->p_tasknext; if (tk->tk_memb_list == p) tk->tk_memb_list = NULL; p->p_taskprev->p_tasknext = p->p_tasknext; p->p_tasknext->p_taskprev = p->p_taskprev; rctl_set_tearoff(p->p_task->tk_rctls, p); rctl_set_tearoff(p->p_task->tk_proj->kpj_rctls, p); p->p_task = NULL; p->p_tasknext = p->p_taskprev = NULL; } /* * task_change(task_t *, proc_t *) * * Overview * task_change() removes the specified process from its current task. The * process is then attached to the specified task. This routine is called * from settaskid() when process is being moved to a new task. * * Return values * None. * * Caller's context * pidlock and p_lock held across task_change() */ void task_change(task_t *newtk, proc_t *p) { task_t *oldtk = p->p_task; ASSERT(MUTEX_HELD(&pidlock)); ASSERT(MUTEX_HELD(&p->p_lock)); ASSERT(oldtk != NULL); ASSERT(oldtk->tk_memb_list != NULL); mutex_enter(&p->p_zone->zone_nlwps_lock); oldtk->tk_nlwps -= p->p_lwpcnt; mutex_exit(&p->p_zone->zone_nlwps_lock); mutex_enter(&newtk->tk_zone->zone_nlwps_lock); newtk->tk_nlwps += p->p_lwpcnt; mutex_exit(&newtk->tk_zone->zone_nlwps_lock); task_detach(p); task_begin(newtk, p); } /* * task_end() * * Overview * task_end() contains the actions executed once the final member of * a task has released the task, and all actions connected with the task, such * as committing an accounting record to a file, are completed. It is called * by the known last consumer of the task information. Additionally, * task_end() must never refer to any process in the system. * * Return values * None. * * Caller's context * No restrictions on context, beyond that given above. */ void task_end(task_t *tk) { ASSERT(tk->tk_hold_count == 0); project_rele(tk->tk_proj); kmem_free(tk->tk_usage, sizeof (task_usage_t)); if (tk->tk_prevusage != NULL) kmem_free(tk->tk_prevusage, sizeof (task_usage_t)); if (tk->tk_zoneusage != NULL) kmem_free(tk->tk_zoneusage, sizeof (task_usage_t)); rctl_set_free(tk->tk_rctls); id_free(taskid_space, tk->tk_tkid); zone_task_rele(tk->tk_zone); kmem_cache_free(task_cache, tk); } static void changeproj(proc_t *p, kproject_t *kpj, zone_t *zone, void *projbuf, void *zonebuf) { kproject_t *oldkpj; kthread_t *t; ASSERT(MUTEX_HELD(&pidlock)); ASSERT(MUTEX_HELD(&p->p_lock)); if ((t = p->p_tlist) != NULL) { do { (void) project_hold(kpj); thread_lock(t); oldkpj = ttoproj(t); /* * Kick this thread so that he doesn't sit * on a wrong wait queue. */ if (ISWAITING(t)) setrun_locked(t); /* * The thread wants to go on the project wait queue, but * the waitq is changing. */ if (t->t_schedflag & TS_PROJWAITQ) t->t_schedflag &= ~ TS_PROJWAITQ; t->t_proj = kpj; t->t_pre_sys = 1; /* For cred update */ thread_unlock(t); fss_changeproj(t, kpj, zone, projbuf, zonebuf); project_rele(oldkpj); } while ((t = t->t_forw) != p->p_tlist); } } /* * task_join() * * Overview * task_join() contains the actions that must be executed when the first * member (curproc) of a newly created task joins it. It may never fail. * * The caller must make sure holdlwps() is called so that all other lwps are * stopped prior to calling this function. * * NB: It returns with curproc->p_lock held. * * Return values * Pointer to the old task. * * Caller's context * cpu_lock must be held entering the function. It will acquire pidlock, * p_crlock and p_lock during execution. */ task_t * task_join(task_t *tk, uint_t flags) { proc_t *p = ttoproc(curthread); task_t *prev_tk; void *projbuf, *zonebuf; zone_t *zone = tk->tk_zone; projid_t projid = tk->tk_proj->kpj_id; cred_t *oldcr; /* * We can't know for sure if holdlwps() was called, but we can check to * ensure we're single-threaded. */ ASSERT(curthread == p->p_agenttp || p->p_lwprcnt == 1); /* * Changing the credential is always hard because we cannot * allocate memory when holding locks but we don't know whether * we need to change it. We first get a reference to the current * cred if we need to change it. Then we create a credential * with an updated project id. Finally we install it, first * releasing the reference we had on the p_cred at the time we * acquired the lock the first time and later we release the * reference to p_cred at the time we acquired the lock the * second time. */ mutex_enter(&p->p_crlock); if (crgetprojid(p->p_cred) == projid) oldcr = NULL; else crhold(oldcr = p->p_cred); mutex_exit(&p->p_crlock); if (oldcr != NULL) { cred_t *newcr = crdup(oldcr); crsetprojid(newcr, projid); crfree(oldcr); mutex_enter(&p->p_crlock); oldcr = p->p_cred; p->p_cred = newcr; mutex_exit(&p->p_crlock); crfree(oldcr); } /* * Make sure that the number of processor sets is constant * across this operation. */ ASSERT(MUTEX_HELD(&cpu_lock)); projbuf = fss_allocbuf(FSS_NPSET_BUF, FSS_ALLOC_PROJ); zonebuf = fss_allocbuf(FSS_NPSET_BUF, FSS_ALLOC_ZONE); mutex_enter(&pidlock); mutex_enter(&p->p_lock); prev_tk = p->p_task; task_change(tk, p); /* * Now move threads one by one to their new project. */ changeproj(p, tk->tk_proj, zone, projbuf, zonebuf); if (flags & TASK_FINAL) p->p_task->tk_flags |= TASK_FINAL; mutex_exit(&pidlock); fss_freebuf(zonebuf, FSS_ALLOC_ZONE); fss_freebuf(projbuf, FSS_ALLOC_PROJ); return (prev_tk); } /* * rctl ops vectors */ static rctl_ops_t task_lwps_ops = { rcop_no_action, task_lwps_usage, task_lwps_set, task_lwps_test }; static rctl_ops_t task_cpu_time_ops = { rcop_no_action, task_cpu_time_usage, rcop_no_set, task_cpu_time_test }; /*ARGSUSED*/ /* * void task_init(void) * * Overview * task_init() initializes task-related hashes, caches, and the task id * space. Additionally, task_init() establishes p0 as a member of task0. * Called by main(). * * Return values * None. * * Caller's context * task_init() must be called prior to MP startup. */ void task_init(void) { proc_t *p = &p0; mod_hash_hndl_t hndl; rctl_set_t *set; rctl_alloc_gp_t *gp; rctl_entity_p_t e; /* * Initialize task_cache and taskid_space. */ task_cache = kmem_cache_create("task_cache", sizeof (task_t), 0, NULL, NULL, NULL, NULL, NULL, 0); taskid_space = id_space_create("taskid_space", 0, MAX_TASKID); /* * Initialize task hash table. */ task_hash = mod_hash_create_idhash("task_hash", task_hash_size, mod_hash_null_valdtor); /* * Initialize task-based rctls. */ rc_task_lwps = rctl_register("task.max-lwps", RCENTITY_TASK, RCTL_GLOBAL_NOACTION | RCTL_GLOBAL_COUNT, INT_MAX, INT_MAX, &task_lwps_ops); rc_task_cpu_time = rctl_register("task.max-cpu-time", RCENTITY_TASK, RCTL_GLOBAL_NOACTION | RCTL_GLOBAL_DENY_NEVER | RCTL_GLOBAL_CPU_TIME | RCTL_GLOBAL_INFINITE | RCTL_GLOBAL_UNOBSERVABLE | RCTL_GLOBAL_SECONDS, UINT64_MAX, UINT64_MAX, &task_cpu_time_ops); /* * Create task0 and place p0 in it as a member. */ task0p = kmem_cache_alloc(task_cache, KM_SLEEP); bzero(task0p, sizeof (task_t)); task0p->tk_tkid = id_alloc(taskid_space); task0p->tk_usage = kmem_zalloc(sizeof (task_usage_t), KM_SLEEP); task0p->tk_proj = project_hold_by_id(0, &zone0, PROJECT_HOLD_INSERT); task0p->tk_flags = TASK_NORMAL; task0p->tk_nlwps = p->p_lwpcnt; task0p->tk_zone = global_zone; set = rctl_set_create(); gp = rctl_set_init_prealloc(RCENTITY_TASK); mutex_enter(&curproc->p_lock); e.rcep_p.task = task0p; e.rcep_t = RCENTITY_TASK; task0p->tk_rctls = rctl_set_init(RCENTITY_TASK, curproc, &e, set, gp); mutex_exit(&curproc->p_lock); rctl_prealloc_destroy(gp); (void) mod_hash_reserve(task_hash, &hndl); mutex_enter(&task_hash_lock); ASSERT(task_find(task0p->tk_tkid, GLOBAL_ZONEID) == NULL); if (mod_hash_insert_reserve(task_hash, (mod_hash_key_t)(uintptr_t)task0p->tk_tkid, (mod_hash_val_t *)task0p, hndl) != 0) { mod_hash_cancel(task_hash, &hndl); panic("unable to insert task %d(%p)", task0p->tk_tkid, (void *)task0p); } mutex_exit(&task_hash_lock); task0p->tk_memb_list = p; /* * Initialize task pointers for p0, including doubly linked list of task * members. */ p->p_task = task0p; p->p_taskprev = p->p_tasknext = p; task_hold(task0p); }