/*- * Copyright (c) 1998 Berkeley Software Design, Inc. 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. Berkeley Software Design Inc's name may not be used to endorse or * promote products derived from this software without specific prior * written permission. * * THIS SOFTWARE IS PROVIDED BY BERKELEY SOFTWARE DESIGN INC ``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 BERKELEY SOFTWARE DESIGN INC 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. * * from BSDI $Id: mutex_witness.c,v 1.1.2.20 2000/04/27 03:10:27 cp Exp $ * and BSDI $Id: synch_machdep.c,v 2.3.2.39 2000/04/27 03:10:25 cp Exp $ */ /* * Implementation of turnstiles used to hold queue of threads blocked on * non-sleepable locks. Sleepable locks use condition variables to * implement their queues. Turnstiles differ from a sleep queue in that * turnstile queue's are assigned to a lock held by an owning thread. Thus, * when one thread is enqueued onto a turnstile, it can lend its priority * to the owning thread. * * We wish to avoid bloating locks with an embedded turnstile and we do not * want to use back-pointers in the locks for the same reason. Thus, we * use a similar approach to that of Solaris 7 as described in Solaris * Internals by Jim Mauro and Richard McDougall. Turnstiles are looked up * in a hash table based on the address of the lock. Each entry in the * hash table is a linked-lists of turnstiles and is called a turnstile * chain. Each chain contains a spin mutex that protects all of the * turnstiles in the chain. * * Each time a thread is created, a turnstile is malloc'd and attached to * that thread. When a thread blocks on a lock, if it is the first thread * to block, it lends its turnstile to the lock. If the lock already has * a turnstile, then it gives its turnstile to the lock's turnstile's free * list. When a thread is woken up, it takes a turnstile from the free list * if there are any other waiters. If it is the only thread blocked on the * lock, then it reclaims the turnstile associated with the lock and removes * it from the hash table. */ #include "opt_turnstile_profiling.h" #include __FBSDID("$FreeBSD$"); #include #include #include #include #include #include #include #include #include #include #include #include #include /* * Constants for the hash table of turnstile chains. TC_SHIFT is a magic * number chosen because the sleep queue's use the same value for the * shift. Basically, we ignore the lower 8 bits of the address. * TC_TABLESIZE must be a power of two for TC_MASK to work properly. */ #define TC_TABLESIZE 128 /* Must be power of 2. */ #define TC_MASK (TC_TABLESIZE - 1) #define TC_SHIFT 8 #define TC_HASH(lock) (((uintptr_t)(lock) >> TC_SHIFT) & TC_MASK) #define TC_LOOKUP(lock) &turnstile_chains[TC_HASH(lock)] /* * There are three different lists of turnstiles as follows. The list * connected by ts_link entries is a per-thread list of all the turnstiles * attached to locks that we own. This is used to fixup our priority when * a lock is released. The other two lists use the ts_hash entries. The * first of these two is the turnstile chain list that a turnstile is on * when it is attached to a lock. The second list to use ts_hash is the * free list hung off of a turnstile that is attached to a lock. * * Each turnstile contains two lists of threads. The ts_blocked list is * a linked list of threads blocked on the turnstile's lock. The * ts_pending list is a linked list of threads previously awakened by * turnstile_signal() or turnstile_wait() that are waiting to be put on * the run queue. * * Locking key: * c - turnstile chain lock * q - td_contested lock */ struct turnstile { TAILQ_HEAD(, thread) ts_blocked; /* (c + q) Blocked threads. */ TAILQ_HEAD(, thread) ts_pending; /* (c) Pending threads. */ LIST_ENTRY(turnstile) ts_hash; /* (c) Chain and free list. */ LIST_ENTRY(turnstile) ts_link; /* (q) Contested locks. */ LIST_HEAD(, turnstile) ts_free; /* (c) Free turnstiles. */ struct lock_object *ts_lockobj; /* (c) Lock we reference. */ struct thread *ts_owner; /* (c + q) Who owns the lock. */ }; struct turnstile_chain { LIST_HEAD(, turnstile) tc_turnstiles; /* List of turnstiles. */ struct mtx tc_lock; /* Spin lock for this chain. */ #ifdef TURNSTILE_PROFILING u_int tc_depth; /* Length of tc_queues. */ u_int tc_max_depth; /* Max length of tc_queues. */ #endif }; #ifdef TURNSTILE_PROFILING u_int turnstile_max_depth; SYSCTL_NODE(_debug, OID_AUTO, turnstile, CTLFLAG_RD, 0, "turnstile profiling"); SYSCTL_NODE(_debug_turnstile, OID_AUTO, chains, CTLFLAG_RD, 0, "turnstile chain stats"); SYSCTL_UINT(_debug_turnstile, OID_AUTO, max_depth, CTLFLAG_RD, &turnstile_max_depth, 0, "maxmimum depth achieved of a single chain"); #endif static struct mtx td_contested_lock; static struct turnstile_chain turnstile_chains[TC_TABLESIZE]; MALLOC_DEFINE(M_TURNSTILE, "turnstiles", "turnstiles"); /* * Prototypes for non-exported routines. */ static void init_turnstile0(void *dummy); #ifdef TURNSTILE_PROFILING static void init_turnstile_profiling(void *arg); #endif static void propagate_priority(struct thread *); static void turnstile_setowner(struct turnstile *ts, struct thread *owner); /* * Walks the chain of turnstiles and their owners to propagate the priority * of the thread being blocked to all the threads holding locks that have to * release their locks before this thread can run again. */ static void propagate_priority(struct thread *td) { struct turnstile_chain *tc; struct turnstile *ts; struct thread *td1; int pri; mtx_assert(&sched_lock, MA_OWNED); pri = td->td_priority; ts = td->td_blocked; for (;;) { td = ts->ts_owner; if (td == NULL) { /* * This really isn't quite right. Really * ought to bump priority of thread that * next acquires the lock. */ return; } MPASS(td->td_proc != NULL); MPASS(td->td_proc->p_magic == P_MAGIC); /* * XXX: The owner of a turnstile can be stale if it is the * first thread to grab a slock of a sx lock. In that case * it is possible for us to be at SSLEEP or some other * weird state. We should probably just return if the state * isn't SRUN or SLOCK. */ KASSERT(!TD_IS_SLEEPING(td), ("sleeping thread (pid %d) owns a non-sleepable lock", td->td_proc->p_pid)); /* * If this thread already has higher priority than the * thread that is being blocked, we are finished. */ if (td->td_priority <= pri) return; /* * If lock holder is actually running, just bump priority. */ if (TD_IS_RUNNING(td)) { td->td_priority = pri; return; } #ifndef SMP /* * For UP, we check to see if td is curthread (this shouldn't * ever happen however as it would mean we are in a deadlock.) */ KASSERT(td != curthread, ("Deadlock detected")); #endif /* * If on run queue move to new run queue, and quit. * XXXKSE this gets a lot more complicated under threads * but try anyhow. */ if (TD_ON_RUNQ(td)) { MPASS(td->td_blocked == NULL); sched_prio(td, pri); return; } /* * Bump this thread's priority. */ td->td_priority = pri; /* * If we aren't blocked on a lock, we should be. */ KASSERT(TD_ON_LOCK(td), ( "process %d(%s):%d holds %s but isn't blocked on a lock\n", td->td_proc->p_pid, td->td_proc->p_comm, td->td_state, ts->ts_lockobj->lo_name)); /* * Pick up the lock that td is blocked on. */ ts = td->td_blocked; MPASS(ts != NULL); tc = TC_LOOKUP(ts->ts_lockobj); mtx_lock_spin(&tc->tc_lock); /* * This thread may not be blocked on this turnstile anymore * but instead might already be woken up on another CPU * that is waiting on sched_lock in turnstile_unpend() to * finish waking this thread up. We can detect this case * by checking to see if this thread has been given a * turnstile by either turnstile_signal() or * turnstile_broadcast(). In this case, treat the thread as * if it was already running. */ if (td->td_turnstile != NULL) { mtx_unlock_spin(&tc->tc_lock); return; } /* * Check if the thread needs to be moved up on * the blocked chain. It doesn't need to be moved * if it is already at the head of the list or if * the item in front of it still has a higher priority. */ if (td == TAILQ_FIRST(&ts->ts_blocked)) { mtx_unlock_spin(&tc->tc_lock); continue; } td1 = TAILQ_PREV(td, threadqueue, td_lockq); if (td1->td_priority <= pri) { mtx_unlock_spin(&tc->tc_lock); continue; } /* * Remove thread from blocked chain and determine where * it should be moved up to. Since we know that td1 has * a lower priority than td, we know that at least one * thread in the chain has a lower priority and that * td1 will thus not be NULL after the loop. */ mtx_lock_spin(&td_contested_lock); TAILQ_REMOVE(&ts->ts_blocked, td, td_lockq); TAILQ_FOREACH(td1, &ts->ts_blocked, td_lockq) { MPASS(td1->td_proc->p_magic == P_MAGIC); if (td1->td_priority > pri) break; } MPASS(td1 != NULL); TAILQ_INSERT_BEFORE(td1, td, td_lockq); mtx_unlock_spin(&td_contested_lock); CTR4(KTR_LOCK, "propagate_priority: td %p moved before %p on [%p] %s", td, td1, ts->ts_lockobj, ts->ts_lockobj->lo_name); mtx_unlock_spin(&tc->tc_lock); } } /* * Early initialization of turnstiles. This is not done via a SYSINIT() * since this needs to be initialized very early when mutexes are first * initialized. */ void init_turnstiles(void) { int i; for (i = 0; i < TC_TABLESIZE; i++) { LIST_INIT(&turnstile_chains[i].tc_turnstiles); mtx_init(&turnstile_chains[i].tc_lock, "turnstile chain", NULL, MTX_SPIN); } mtx_init(&td_contested_lock, "td_contested", NULL, MTX_SPIN); thread0.td_turnstile = NULL; } #ifdef TURNSTILE_PROFILING static void init_turnstile_profiling(void *arg) { struct sysctl_oid *chain_oid; char chain_name[10]; int i; for (i = 0; i < TC_TABLESIZE; i++) { snprintf(chain_name, sizeof(chain_name), "%d", i); chain_oid = SYSCTL_ADD_NODE(NULL, SYSCTL_STATIC_CHILDREN(_debug_turnstile_chains), OID_AUTO, chain_name, CTLFLAG_RD, NULL, "turnstile chain stats"); SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO, "depth", CTLFLAG_RD, &turnstile_chains[i].tc_depth, 0, NULL); SYSCTL_ADD_UINT(NULL, SYSCTL_CHILDREN(chain_oid), OID_AUTO, "max_depth", CTLFLAG_RD, &turnstile_chains[i].tc_max_depth, 0, NULL); } } SYSINIT(turnstile_profiling, SI_SUB_LOCK, SI_ORDER_ANY, init_turnstile_profiling, NULL); #endif static void init_turnstile0(void *dummy) { thread0.td_turnstile = turnstile_alloc(); } SYSINIT(turnstile0, SI_SUB_LOCK, SI_ORDER_ANY, init_turnstile0, NULL); /* * Set the owner of the lock this turnstile is attached to. */ static void turnstile_setowner(struct turnstile *ts, struct thread *owner) { mtx_assert(&td_contested_lock, MA_OWNED); MPASS(owner->td_proc->p_magic == P_MAGIC); MPASS(ts->ts_owner == NULL); ts->ts_owner = owner; LIST_INSERT_HEAD(&owner->td_contested, ts, ts_link); } /* * Malloc a turnstile for a new thread, initialize it and return it. */ struct turnstile * turnstile_alloc(void) { struct turnstile *ts; ts = malloc(sizeof(struct turnstile), M_TURNSTILE, M_WAITOK | M_ZERO); TAILQ_INIT(&ts->ts_blocked); TAILQ_INIT(&ts->ts_pending); LIST_INIT(&ts->ts_free); return (ts); } /* * Free a turnstile when a thread is destroyed. */ void turnstile_free(struct turnstile *ts) { MPASS(ts != NULL); MPASS(TAILQ_EMPTY(&ts->ts_blocked)); MPASS(TAILQ_EMPTY(&ts->ts_pending)); free(ts, M_TURNSTILE); } /* * Look up the turnstile for a lock in the hash table locking the associated * turnstile chain along the way. Return with the turnstile chain locked. * If no turnstile is found in the hash table, NULL is returned. */ struct turnstile * turnstile_lookup(struct lock_object *lock) { struct turnstile_chain *tc; struct turnstile *ts; tc = TC_LOOKUP(lock); mtx_lock_spin(&tc->tc_lock); LIST_FOREACH(ts, &tc->tc_turnstiles, ts_hash) if (ts->ts_lockobj == lock) return (ts); return (NULL); } /* * Unlock the turnstile chain associated with a given lock. */ void turnstile_release(struct lock_object *lock) { struct turnstile_chain *tc; tc = TC_LOOKUP(lock); mtx_unlock_spin(&tc->tc_lock); } /* * Take ownership of a turnstile and adjust the priority of the new * owner appropriately. */ void turnstile_claim(struct turnstile *ts) { struct turnstile_chain *tc; struct thread *td, *owner; tc = TC_LOOKUP(ts->ts_lockobj); mtx_assert(&tc->tc_lock, MA_OWNED); owner = curthread; mtx_lock_spin(&td_contested_lock); turnstile_setowner(ts, owner); mtx_unlock_spin(&td_contested_lock); td = TAILQ_FIRST(&ts->ts_blocked); MPASS(td != NULL); MPASS(td->td_proc->p_magic == P_MAGIC); mtx_unlock_spin(&tc->tc_lock); /* * Update the priority of the new owner if needed. */ mtx_lock_spin(&sched_lock); if (td->td_priority < owner->td_priority) owner->td_priority = td->td_priority; mtx_unlock_spin(&sched_lock); } /* * Block the current thread on the turnstile ts. This function will context * switch and not return until this thread has been woken back up. This * function must be called with the appropriate turnstile chain locked and * will return with it unlocked. */ void turnstile_wait(struct turnstile *ts, struct lock_object *lock, struct thread *owner) { struct turnstile_chain *tc; struct thread *td, *td1; td = curthread; tc = TC_LOOKUP(lock); mtx_assert(&tc->tc_lock, MA_OWNED); MPASS(td->td_turnstile != NULL); MPASS(owner != NULL); MPASS(owner->td_proc->p_magic == P_MAGIC); /* If the passed in turnstile is NULL, use this thread's turnstile. */ if (ts == NULL) { #ifdef TURNSTILE_PROFILING tc->tc_depth++; if (tc->tc_depth > tc->tc_max_depth) { tc->tc_max_depth = tc->tc_depth; if (tc->tc_max_depth > turnstile_max_depth) turnstile_max_depth = tc->tc_max_depth; } #endif ts = td->td_turnstile; LIST_INSERT_HEAD(&tc->tc_turnstiles, ts, ts_hash); KASSERT(TAILQ_EMPTY(&ts->ts_pending), ("thread's turnstile has pending threads")); KASSERT(TAILQ_EMPTY(&ts->ts_blocked), ("thread's turnstile has a non-empty queue")); KASSERT(LIST_EMPTY(&ts->ts_free), ("thread's turnstile has a non-empty free list")); KASSERT(ts->ts_lockobj == NULL, ("stale ts_lockobj pointer")); ts->ts_lockobj = lock; mtx_lock_spin(&td_contested_lock); TAILQ_INSERT_TAIL(&ts->ts_blocked, td, td_lockq); turnstile_setowner(ts, owner); mtx_unlock_spin(&td_contested_lock); } else { TAILQ_FOREACH(td1, &ts->ts_blocked, td_lockq) if (td1->td_priority > td->td_priority) break; mtx_lock_spin(&td_contested_lock); if (td1 != NULL) TAILQ_INSERT_BEFORE(td1, td, td_lockq); else TAILQ_INSERT_TAIL(&ts->ts_blocked, td, td_lockq); mtx_unlock_spin(&td_contested_lock); MPASS(td->td_turnstile != NULL); LIST_INSERT_HEAD(&ts->ts_free, td->td_turnstile, ts_hash); MPASS(owner == ts->ts_owner); } td->td_turnstile = NULL; mtx_unlock_spin(&tc->tc_lock); mtx_lock_spin(&sched_lock); /* * Handle race condition where a thread on another CPU that owns * lock 'lock' could have woken us in between us dropping the * turnstile chain lock and acquiring the sched_lock. */ if (td->td_flags & TDF_TSNOBLOCK) { td->td_flags &= ~TDF_TSNOBLOCK; mtx_unlock_spin(&sched_lock); return; } #ifdef notyet /* * If we're borrowing an interrupted thread's VM context, we * must clean up before going to sleep. */ if (td->td_ithd != NULL) { struct ithd *it = td->td_ithd; if (it->it_interrupted) { if (LOCK_LOG_TEST(lock, 0)) CTR3(KTR_LOCK, "%s: %p interrupted %p", __func__, it, it->it_interrupted); intr_thd_fixup(it); } } #endif /* Save who we are blocked on and switch. */ td->td_blocked = ts; td->td_lockname = lock->lo_name; TD_SET_LOCK(td); propagate_priority(td); if (LOCK_LOG_TEST(lock, 0)) CTR4(KTR_LOCK, "%s: td %p blocked on [%p] %s", __func__, td, lock, lock->lo_name); mi_switch(SW_VOL, NULL); if (LOCK_LOG_TEST(lock, 0)) CTR4(KTR_LOCK, "%s: td %p free from blocked on [%p] %s", __func__, td, lock, lock->lo_name); mtx_unlock_spin(&sched_lock); } /* * Pick the highest priority thread on this turnstile and put it on the * pending list. This must be called with the turnstile chain locked. */ int turnstile_signal(struct turnstile *ts) { struct turnstile_chain *tc; struct thread *td; int empty; MPASS(ts != NULL); MPASS(curthread->td_proc->p_magic == P_MAGIC); MPASS(ts->ts_owner == curthread); tc = TC_LOOKUP(ts->ts_lockobj); mtx_assert(&tc->tc_lock, MA_OWNED); /* * Pick the highest priority thread blocked on this lock and * move it to the pending list. */ td = TAILQ_FIRST(&ts->ts_blocked); MPASS(td->td_proc->p_magic == P_MAGIC); mtx_lock_spin(&td_contested_lock); TAILQ_REMOVE(&ts->ts_blocked, td, td_lockq); mtx_unlock_spin(&td_contested_lock); TAILQ_INSERT_TAIL(&ts->ts_pending, td, td_lockq); /* * If the turnstile is now empty, remove it from its chain and * give it to the about-to-be-woken thread. Otherwise take a * turnstile from the free list and give it to the thread. */ empty = TAILQ_EMPTY(&ts->ts_blocked); if (empty) { MPASS(LIST_EMPTY(&ts->ts_free)); #ifdef TURNSTILE_PROFILING tc->tc_depth--; #endif } else ts = LIST_FIRST(&ts->ts_free); MPASS(ts != NULL); LIST_REMOVE(ts, ts_hash); td->td_turnstile = ts; return (empty); } /* * Put all blocked threads on the pending list. This must be called with * the turnstile chain locked. */ void turnstile_broadcast(struct turnstile *ts) { struct turnstile_chain *tc; struct turnstile *ts1; struct thread *td; MPASS(ts != NULL); MPASS(curthread->td_proc->p_magic == P_MAGIC); MPASS(ts->ts_owner == curthread); tc = TC_LOOKUP(ts->ts_lockobj); mtx_assert(&tc->tc_lock, MA_OWNED); /* * Transfer the blocked list to the pending list. */ mtx_lock_spin(&td_contested_lock); TAILQ_CONCAT(&ts->ts_pending, &ts->ts_blocked, td_lockq); mtx_unlock_spin(&td_contested_lock); /* * Give a turnstile to each thread. The last thread gets * this turnstile. */ TAILQ_FOREACH(td, &ts->ts_pending, td_lockq) { if (LIST_EMPTY(&ts->ts_free)) { MPASS(TAILQ_NEXT(td, td_lockq) == NULL); ts1 = ts; #ifdef TURNSTILE_PROFILING tc->tc_depth--; #endif } else ts1 = LIST_FIRST(&ts->ts_free); MPASS(ts1 != NULL); LIST_REMOVE(ts1, ts_hash); td->td_turnstile = ts1; } } /* * Wakeup all threads on the pending list and adjust the priority of the * current thread appropriately. This must be called with the turnstile * chain locked. */ void turnstile_unpend(struct turnstile *ts) { TAILQ_HEAD( ,thread) pending_threads; struct turnstile_chain *tc; struct thread *td; int cp, pri; MPASS(ts != NULL); MPASS(ts->ts_owner == curthread); tc = TC_LOOKUP(ts->ts_lockobj); mtx_assert(&tc->tc_lock, MA_OWNED); MPASS(!TAILQ_EMPTY(&ts->ts_pending)); /* * Move the list of pending threads out of the turnstile and * into a local variable. */ TAILQ_INIT(&pending_threads); TAILQ_CONCAT(&pending_threads, &ts->ts_pending, td_lockq); #ifdef INVARIANTS if (TAILQ_EMPTY(&ts->ts_blocked)) ts->ts_lockobj = NULL; #endif /* * Remove the turnstile from this thread's list of contested locks * since this thread doesn't own it anymore. New threads will * not be blocking on the turnstile until it is claimed by a new * owner. */ mtx_lock_spin(&td_contested_lock); ts->ts_owner = NULL; LIST_REMOVE(ts, ts_link); mtx_unlock_spin(&td_contested_lock); mtx_unlock_spin(&tc->tc_lock); /* * Adjust the priority of curthread based on other contested * locks it owns. Don't lower the priority below the base * priority however. */ td = curthread; pri = PRI_MAX; mtx_lock_spin(&sched_lock); mtx_lock_spin(&td_contested_lock); LIST_FOREACH(ts, &td->td_contested, ts_link) { cp = TAILQ_FIRST(&ts->ts_blocked)->td_priority; if (cp < pri) pri = cp; } mtx_unlock_spin(&td_contested_lock); if (pri > td->td_base_pri) pri = td->td_base_pri; td->td_priority = pri; /* * Wake up all the pending threads. If a thread is not blocked * on a lock, then it is currently executing on another CPU in * turnstile_wait() or sitting on a run queue waiting to resume * in turnstile_wait(). Set a flag to force it to try to acquire * the lock again instead of blocking. */ while (!TAILQ_EMPTY(&pending_threads)) { td = TAILQ_FIRST(&pending_threads); TAILQ_REMOVE(&pending_threads, td, td_lockq); MPASS(td->td_proc->p_magic == P_MAGIC); if (TD_ON_LOCK(td)) { td->td_blocked = NULL; td->td_lockname = NULL; TD_CLR_LOCK(td); MPASS(TD_CAN_RUN(td)); setrunqueue(td, SRQ_BORING); } else { td->td_flags |= TDF_TSNOBLOCK; MPASS(TD_IS_RUNNING(td) || TD_ON_RUNQ(td)); } } mtx_unlock_spin(&sched_lock); } /* * Return the first thread in a turnstile. */ struct thread * turnstile_head(struct turnstile *ts) { #ifdef INVARIANTS struct turnstile_chain *tc; MPASS(ts != NULL); tc = TC_LOOKUP(ts->ts_lockobj); mtx_assert(&tc->tc_lock, MA_OWNED); #endif return (TAILQ_FIRST(&ts->ts_blocked)); } /* * Returns true if a turnstile is empty. */ int turnstile_empty(struct turnstile *ts) { #ifdef INVARIANTS struct turnstile_chain *tc; MPASS(ts != NULL); tc = TC_LOOKUP(ts->ts_lockobj); mtx_assert(&tc->tc_lock, MA_OWNED); #endif return (TAILQ_EMPTY(&ts->ts_blocked)); }