/* $NetBSD: kern_rwlock.c,v 1.66.4.2 2023/07/31 14:45:59 martin Exp $ */ /*- * Copyright (c) 2002, 2006, 2007, 2008, 2009, 2019, 2020 * The NetBSD Foundation, Inc. * All rights reserved. * * This code is derived from software contributed to The NetBSD Foundation * by Jason R. Thorpe and Andrew Doran. * * 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. * * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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. */ /* * Kernel reader/writer lock implementation, modeled after those * found in Solaris, a description of which can be found in: * * Solaris Internals: Core Kernel Architecture, Jim Mauro and * Richard McDougall. * * The NetBSD implementation differs from that described in the book, in * that the locks are partially adaptive. Lock waiters spin wait while a * lock is write held and the holder is still running on a CPU. The method * of choosing which threads to awaken when a lock is released also differs, * mainly to take account of the partially adaptive behaviour. */ #include __KERNEL_RCSID(0, "$NetBSD: kern_rwlock.c,v 1.66.4.2 2023/07/31 14:45:59 martin Exp $"); #include "opt_lockdebug.h" #define __RWLOCK_PRIVATE #include #include #include #include #include #include #include #include #include #include #include #include #include /* * LOCKDEBUG */ #define RW_DEBUG_P(rw) (((rw)->rw_owner & RW_NODEBUG) == 0) #define RW_WANTLOCK(rw, op) \ LOCKDEBUG_WANTLOCK(RW_DEBUG_P(rw), (rw), \ (uintptr_t)__builtin_return_address(0), op == RW_READER); #define RW_LOCKED(rw, op) \ LOCKDEBUG_LOCKED(RW_DEBUG_P(rw), (rw), NULL, \ (uintptr_t)__builtin_return_address(0), op == RW_READER); #define RW_UNLOCKED(rw, op) \ LOCKDEBUG_UNLOCKED(RW_DEBUG_P(rw), (rw), \ (uintptr_t)__builtin_return_address(0), op == RW_READER); /* * DIAGNOSTIC */ #if defined(DIAGNOSTIC) #define RW_ASSERT(rw, cond) \ do { \ if (__predict_false(!(cond))) \ rw_abort(__func__, __LINE__, rw, "assertion failed: " #cond);\ } while (/* CONSTCOND */ 0) #else #define RW_ASSERT(rw, cond) /* nothing */ #endif /* DIAGNOSTIC */ /* * Memory barriers. */ #ifdef __HAVE_ATOMIC_AS_MEMBAR #define RW_MEMBAR_ACQUIRE() #define RW_MEMBAR_RELEASE() #else #define RW_MEMBAR_ACQUIRE() membar_acquire() #define RW_MEMBAR_RELEASE() membar_release() #endif /* * For platforms that do not provide stubs, or for the LOCKDEBUG case. */ #ifdef LOCKDEBUG #undef __HAVE_RW_STUBS #endif #ifndef __HAVE_RW_STUBS __strong_alias(rw_enter,rw_vector_enter); __strong_alias(rw_exit,rw_vector_exit); __strong_alias(rw_tryenter,rw_vector_tryenter); #endif static void rw_abort(const char *, size_t, krwlock_t *, const char *); static void rw_dump(const volatile void *, lockop_printer_t); static lwp_t *rw_owner(wchan_t); lockops_t rwlock_lockops = { .lo_name = "Reader / writer lock", .lo_type = LOCKOPS_SLEEP, .lo_dump = rw_dump, }; syncobj_t rw_syncobj = { .sobj_flag = SOBJ_SLEEPQ_SORTED, .sobj_unsleep = turnstile_unsleep, .sobj_changepri = turnstile_changepri, .sobj_lendpri = sleepq_lendpri, .sobj_owner = rw_owner, }; /* * rw_cas: * * Do an atomic compare-and-swap on the lock word. */ static inline uintptr_t rw_cas(krwlock_t *rw, uintptr_t o, uintptr_t n) { return (uintptr_t)atomic_cas_ptr((volatile void *)&rw->rw_owner, (void *)o, (void *)n); } /* * rw_swap: * * Do an atomic swap of the lock word. This is used only when it's * known that the lock word is set up such that it can't be changed * behind us (assert this), so there's no point considering the result. */ static inline void rw_swap(krwlock_t *rw, uintptr_t o, uintptr_t n) { n = (uintptr_t)atomic_swap_ptr((volatile void *)&rw->rw_owner, (void *)n); RW_ASSERT(rw, n == o); RW_ASSERT(rw, (o & RW_HAS_WAITERS) != 0); } /* * rw_dump: * * Dump the contents of a rwlock structure. */ static void rw_dump(const volatile void *cookie, lockop_printer_t pr) { const volatile krwlock_t *rw = cookie; pr("owner/count : %#018lx flags : %#018x\n", (long)RW_OWNER(rw), (int)RW_FLAGS(rw)); } /* * rw_abort: * * Dump information about an error and panic the system. This * generates a lot of machine code in the DIAGNOSTIC case, so * we ask the compiler to not inline it. */ static void __noinline rw_abort(const char *func, size_t line, krwlock_t *rw, const char *msg) { if (__predict_false(panicstr != NULL)) return; LOCKDEBUG_ABORT(func, line, rw, &rwlock_lockops, msg); } /* * rw_init: * * Initialize a rwlock for use. */ void _rw_init(krwlock_t *rw, uintptr_t return_address) { #ifdef LOCKDEBUG /* XXX only because the assembly stubs can't handle RW_NODEBUG */ if (LOCKDEBUG_ALLOC(rw, &rwlock_lockops, return_address)) rw->rw_owner = 0; else rw->rw_owner = RW_NODEBUG; #else rw->rw_owner = 0; #endif } void rw_init(krwlock_t *rw) { _rw_init(rw, (uintptr_t)__builtin_return_address(0)); } /* * rw_destroy: * * Tear down a rwlock. */ void rw_destroy(krwlock_t *rw) { RW_ASSERT(rw, (rw->rw_owner & ~RW_NODEBUG) == 0); LOCKDEBUG_FREE((rw->rw_owner & RW_NODEBUG) == 0, rw); } /* * rw_oncpu: * * Return true if an rwlock owner is running on a CPU in the system. * If the target is waiting on the kernel big lock, then we must * release it. This is necessary to avoid deadlock. */ static bool rw_oncpu(uintptr_t owner) { #ifdef MULTIPROCESSOR struct cpu_info *ci; lwp_t *l; KASSERT(kpreempt_disabled()); if ((owner & (RW_WRITE_LOCKED|RW_HAS_WAITERS)) != RW_WRITE_LOCKED) { return false; } /* * See lwp_dtor() why dereference of the LWP pointer is safe. * We must have kernel preemption disabled for that. */ l = (lwp_t *)(owner & RW_THREAD); ci = l->l_cpu; if (ci && ci->ci_curlwp == l) { /* Target is running; do we need to block? */ return (ci->ci_biglock_wanted != l); } #endif /* Not running. It may be safe to block now. */ return false; } /* * rw_vector_enter: * * Acquire a rwlock. */ void rw_vector_enter(krwlock_t *rw, const krw_t op) { uintptr_t owner, incr, need_wait, set_wait, curthread, next; turnstile_t *ts; int queue; lwp_t *l; LOCKSTAT_TIMER(slptime); LOCKSTAT_TIMER(slpcnt); LOCKSTAT_TIMER(spintime); LOCKSTAT_COUNTER(spincnt); LOCKSTAT_FLAG(lsflag); l = curlwp; curthread = (uintptr_t)l; RW_ASSERT(rw, !cpu_intr_p()); RW_ASSERT(rw, curthread != 0); RW_WANTLOCK(rw, op); if (__predict_true(panicstr == NULL)) { KDASSERT(pserialize_not_in_read_section()); LOCKDEBUG_BARRIER(&kernel_lock, 1); } /* * We play a slight trick here. If we're a reader, we want * increment the read count. If we're a writer, we want to * set the owner field and the WRITE_LOCKED bit. * * In the latter case, we expect those bits to be zero, * therefore we can use an add operation to set them, which * means an add operation for both cases. */ if (__predict_true(op == RW_READER)) { incr = RW_READ_INCR; set_wait = RW_HAS_WAITERS; need_wait = RW_WRITE_LOCKED | RW_WRITE_WANTED; queue = TS_READER_Q; } else { RW_ASSERT(rw, op == RW_WRITER); incr = curthread | RW_WRITE_LOCKED; set_wait = RW_HAS_WAITERS | RW_WRITE_WANTED; need_wait = RW_WRITE_LOCKED | RW_THREAD; queue = TS_WRITER_Q; } LOCKSTAT_ENTER(lsflag); KPREEMPT_DISABLE(curlwp); for (owner = rw->rw_owner;;) { /* * Read the lock owner field. If the need-to-wait * indicator is clear, then try to acquire the lock. */ if ((owner & need_wait) == 0) { next = rw_cas(rw, owner, (owner + incr) & ~RW_WRITE_WANTED); if (__predict_true(next == owner)) { /* Got it! */ RW_MEMBAR_ACQUIRE(); break; } /* * Didn't get it -- spin around again (we'll * probably sleep on the next iteration). */ owner = next; continue; } if (__predict_false(RW_OWNER(rw) == curthread)) { rw_abort(__func__, __LINE__, rw, "locking against myself"); } /* * If the lock owner is running on another CPU, and * there are no existing waiters, then spin. */ if (rw_oncpu(owner)) { LOCKSTAT_START_TIMER(lsflag, spintime); u_int count = SPINLOCK_BACKOFF_MIN; do { KPREEMPT_ENABLE(curlwp); SPINLOCK_BACKOFF(count); KPREEMPT_DISABLE(curlwp); owner = rw->rw_owner; } while (rw_oncpu(owner)); LOCKSTAT_STOP_TIMER(lsflag, spintime); LOCKSTAT_COUNT(spincnt, 1); if ((owner & need_wait) == 0) continue; } /* * Grab the turnstile chain lock. Once we have that, we * can adjust the waiter bits and sleep queue. */ ts = turnstile_lookup(rw); /* * Mark the rwlock as having waiters. If the set fails, * then we may not need to sleep and should spin again. * Reload rw_owner because turnstile_lookup() may have * spun on the turnstile chain lock. */ owner = rw->rw_owner; if ((owner & need_wait) == 0 || rw_oncpu(owner)) { turnstile_exit(rw); continue; } next = rw_cas(rw, owner, owner | set_wait); /* XXX membar? */ if (__predict_false(next != owner)) { turnstile_exit(rw); owner = next; continue; } LOCKSTAT_START_TIMER(lsflag, slptime); turnstile_block(ts, queue, rw, &rw_syncobj); LOCKSTAT_STOP_TIMER(lsflag, slptime); LOCKSTAT_COUNT(slpcnt, 1); /* * No need for a memory barrier because of context switch. * If not handed the lock, then spin again. */ if (op == RW_READER || (rw->rw_owner & RW_THREAD) == curthread) break; owner = rw->rw_owner; } KPREEMPT_ENABLE(curlwp); LOCKSTAT_EVENT_RA(lsflag, rw, LB_RWLOCK | (op == RW_WRITER ? LB_SLEEP1 : LB_SLEEP2), slpcnt, slptime, (l->l_rwcallsite != 0 ? l->l_rwcallsite : (uintptr_t)__builtin_return_address(0))); LOCKSTAT_EVENT_RA(lsflag, rw, LB_RWLOCK | LB_SPIN, spincnt, spintime, (l->l_rwcallsite != 0 ? l->l_rwcallsite : (uintptr_t)__builtin_return_address(0))); LOCKSTAT_EXIT(lsflag); RW_ASSERT(rw, (op != RW_READER && RW_OWNER(rw) == curthread) || (op == RW_READER && RW_COUNT(rw) != 0)); RW_LOCKED(rw, op); } /* * rw_vector_exit: * * Release a rwlock. */ void rw_vector_exit(krwlock_t *rw) { uintptr_t curthread, owner, decr, newown, next; turnstile_t *ts; int rcnt, wcnt; lwp_t *l; l = curlwp; curthread = (uintptr_t)l; RW_ASSERT(rw, curthread != 0); /* * Again, we use a trick. Since we used an add operation to * set the required lock bits, we can use a subtract to clear * them, which makes the read-release and write-release path * the same. */ owner = rw->rw_owner; if (__predict_false((owner & RW_WRITE_LOCKED) != 0)) { RW_UNLOCKED(rw, RW_WRITER); RW_ASSERT(rw, RW_OWNER(rw) == curthread); decr = curthread | RW_WRITE_LOCKED; } else { RW_UNLOCKED(rw, RW_READER); RW_ASSERT(rw, RW_COUNT(rw) != 0); decr = RW_READ_INCR; } /* * Compute what we expect the new value of the lock to be. Only * proceed to do direct handoff if there are waiters, and if the * lock would become unowned. */ RW_MEMBAR_RELEASE(); for (;;) { newown = (owner - decr); if ((newown & (RW_THREAD | RW_HAS_WAITERS)) == RW_HAS_WAITERS) break; next = rw_cas(rw, owner, newown); if (__predict_true(next == owner)) return; owner = next; } /* * Grab the turnstile chain lock. This gets the interlock * on the sleep queue. Once we have that, we can adjust the * waiter bits. */ ts = turnstile_lookup(rw); owner = rw->rw_owner; RW_ASSERT(rw, ts != NULL); RW_ASSERT(rw, (owner & RW_HAS_WAITERS) != 0); wcnt = TS_WAITERS(ts, TS_WRITER_Q); rcnt = TS_WAITERS(ts, TS_READER_Q); /* * Give the lock away. * * If we are releasing a write lock, then prefer to wake all * outstanding readers. Otherwise, wake one writer if there * are outstanding readers, or all writers if there are no * pending readers. If waking one specific writer, the writer * is handed the lock here. If waking multiple writers, we * set WRITE_WANTED to block out new readers, and let them * do the work of acquiring the lock in rw_vector_enter(). */ if (rcnt == 0 || decr == RW_READ_INCR) { RW_ASSERT(rw, wcnt != 0); RW_ASSERT(rw, (owner & RW_WRITE_WANTED) != 0); if (rcnt != 0) { /* Give the lock to the longest waiting writer. */ l = TS_FIRST(ts, TS_WRITER_Q); newown = (uintptr_t)l | (owner & RW_NODEBUG); newown |= RW_WRITE_LOCKED | RW_HAS_WAITERS; if (wcnt > 1) newown |= RW_WRITE_WANTED; rw_swap(rw, owner, newown); turnstile_wakeup(ts, TS_WRITER_Q, 1, l); } else { /* Wake all writers and let them fight it out. */ newown = owner & RW_NODEBUG; newown |= RW_WRITE_WANTED; rw_swap(rw, owner, newown); turnstile_wakeup(ts, TS_WRITER_Q, wcnt, NULL); } } else { RW_ASSERT(rw, rcnt != 0); /* * Give the lock to all blocked readers. If there * is a writer waiting, new readers that arrive * after the release will be blocked out. */ newown = owner & RW_NODEBUG; newown += rcnt << RW_READ_COUNT_SHIFT; if (wcnt != 0) newown |= RW_HAS_WAITERS | RW_WRITE_WANTED; /* Wake up all sleeping readers. */ rw_swap(rw, owner, newown); turnstile_wakeup(ts, TS_READER_Q, rcnt, NULL); } } /* * rw_vector_tryenter: * * Try to acquire a rwlock. */ int rw_vector_tryenter(krwlock_t *rw, const krw_t op) { uintptr_t curthread, owner, incr, need_wait, next; lwp_t *l; l = curlwp; curthread = (uintptr_t)l; RW_ASSERT(rw, curthread != 0); if (op == RW_READER) { incr = RW_READ_INCR; need_wait = RW_WRITE_LOCKED | RW_WRITE_WANTED; } else { RW_ASSERT(rw, op == RW_WRITER); incr = curthread | RW_WRITE_LOCKED; need_wait = RW_WRITE_LOCKED | RW_THREAD; } for (owner = rw->rw_owner;; owner = next) { if (__predict_false((owner & need_wait) != 0)) return 0; next = rw_cas(rw, owner, owner + incr); if (__predict_true(next == owner)) { /* Got it! */ break; } } RW_WANTLOCK(rw, op); RW_LOCKED(rw, op); RW_ASSERT(rw, (op != RW_READER && RW_OWNER(rw) == curthread) || (op == RW_READER && RW_COUNT(rw) != 0)); RW_MEMBAR_ACQUIRE(); return 1; } /* * rw_downgrade: * * Downgrade a write lock to a read lock. */ void rw_downgrade(krwlock_t *rw) { uintptr_t owner, curthread, newown, next; turnstile_t *ts; int rcnt, wcnt; lwp_t *l; l = curlwp; curthread = (uintptr_t)l; RW_ASSERT(rw, curthread != 0); RW_ASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) != 0); RW_ASSERT(rw, RW_OWNER(rw) == curthread); RW_UNLOCKED(rw, RW_WRITER); #if !defined(DIAGNOSTIC) __USE(curthread); #endif RW_MEMBAR_RELEASE(); for (owner = rw->rw_owner;; owner = next) { /* * If there are no waiters we can do this the easy way. Try * swapping us down to one read hold. If it fails, the lock * condition has changed and we most likely now have * waiters. */ if ((owner & RW_HAS_WAITERS) == 0) { newown = (owner & RW_NODEBUG); next = rw_cas(rw, owner, newown + RW_READ_INCR); if (__predict_true(next == owner)) { RW_LOCKED(rw, RW_READER); RW_ASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0); RW_ASSERT(rw, RW_COUNT(rw) != 0); return; } continue; } /* * Grab the turnstile chain lock. This gets the interlock * on the sleep queue. Once we have that, we can adjust the * waiter bits. */ ts = turnstile_lookup(rw); RW_ASSERT(rw, ts != NULL); rcnt = TS_WAITERS(ts, TS_READER_Q); wcnt = TS_WAITERS(ts, TS_WRITER_Q); if (rcnt == 0) { /* * If there are no readers, just preserve the * waiters bits, swap us down to one read hold and * return. */ RW_ASSERT(rw, wcnt != 0); RW_ASSERT(rw, (rw->rw_owner & RW_WRITE_WANTED) != 0); RW_ASSERT(rw, (rw->rw_owner & RW_HAS_WAITERS) != 0); newown = owner & RW_NODEBUG; newown |= RW_READ_INCR | RW_HAS_WAITERS | RW_WRITE_WANTED; next = rw_cas(rw, owner, newown); turnstile_exit(rw); if (__predict_true(next == owner)) break; } else { /* * Give the lock to all blocked readers. We may * retain one read hold if downgrading. If there is * a writer waiting, new readers will be blocked * out. */ newown = owner & RW_NODEBUG; newown += (rcnt << RW_READ_COUNT_SHIFT) + RW_READ_INCR; if (wcnt != 0) newown |= RW_HAS_WAITERS | RW_WRITE_WANTED; next = rw_cas(rw, owner, newown); if (__predict_true(next == owner)) { /* Wake up all sleeping readers. */ turnstile_wakeup(ts, TS_READER_Q, rcnt, NULL); break; } turnstile_exit(rw); } } RW_WANTLOCK(rw, RW_READER); RW_LOCKED(rw, RW_READER); RW_ASSERT(rw, (rw->rw_owner & RW_WRITE_LOCKED) == 0); RW_ASSERT(rw, RW_COUNT(rw) != 0); } /* * rw_tryupgrade: * * Try to upgrade a read lock to a write lock. We must be the only * reader. */ int rw_tryupgrade(krwlock_t *rw) { uintptr_t owner, curthread, newown, next; struct lwp *l; l = curlwp; curthread = (uintptr_t)l; RW_ASSERT(rw, curthread != 0); RW_ASSERT(rw, rw_read_held(rw)); for (owner = RW_READ_INCR;; owner = next) { newown = curthread | RW_WRITE_LOCKED | (owner & ~RW_THREAD); next = rw_cas(rw, owner, newown); if (__predict_true(next == owner)) { RW_MEMBAR_ACQUIRE(); break; } RW_ASSERT(rw, (next & RW_WRITE_LOCKED) == 0); if (__predict_false((next & RW_THREAD) != RW_READ_INCR)) { RW_ASSERT(rw, (next & RW_THREAD) != 0); return 0; } } RW_UNLOCKED(rw, RW_READER); RW_WANTLOCK(rw, RW_WRITER); RW_LOCKED(rw, RW_WRITER); RW_ASSERT(rw, rw->rw_owner & RW_WRITE_LOCKED); RW_ASSERT(rw, RW_OWNER(rw) == curthread); return 1; } /* * rw_read_held: * * Returns true if the rwlock is held for reading. Must only be * used for diagnostic assertions, and never be used to make * decisions about how to use a rwlock. */ int rw_read_held(krwlock_t *rw) { uintptr_t owner; if (rw == NULL) return 0; owner = rw->rw_owner; return (owner & RW_WRITE_LOCKED) == 0 && (owner & RW_THREAD) != 0; } /* * rw_write_held: * * Returns true if the rwlock is held for writing. Must only be * used for diagnostic assertions, and never be used to make * decisions about how to use a rwlock. */ int rw_write_held(krwlock_t *rw) { if (rw == NULL) return 0; return (rw->rw_owner & (RW_WRITE_LOCKED | RW_THREAD)) == (RW_WRITE_LOCKED | (uintptr_t)curlwp); } /* * rw_lock_held: * * Returns true if the rwlock is held for reading or writing. Must * only be used for diagnostic assertions, and never be used to make * decisions about how to use a rwlock. */ int rw_lock_held(krwlock_t *rw) { if (rw == NULL) return 0; return (rw->rw_owner & RW_THREAD) != 0; } /* * rw_lock_op: * * For a rwlock that is known to be held by the caller, return * RW_READER or RW_WRITER to describe the hold type. */ krw_t rw_lock_op(krwlock_t *rw) { RW_ASSERT(rw, rw_lock_held(rw)); return (rw->rw_owner & RW_WRITE_LOCKED) != 0 ? RW_WRITER : RW_READER; } /* * rw_owner: * * Return the current owner of an RW lock, but only if it is write * held. Used for priority inheritance. */ static lwp_t * rw_owner(wchan_t obj) { krwlock_t *rw = (void *)(uintptr_t)obj; /* discard qualifiers */ uintptr_t owner = rw->rw_owner; if ((owner & RW_WRITE_LOCKED) == 0) return NULL; return (void *)(owner & RW_THREAD); } /* * rw_owner_running: * * Return true if a RW lock is unheld, or write held and the owner is * running on a CPU. For the pagedaemon. */ bool rw_owner_running(const krwlock_t *rw) { #ifdef MULTIPROCESSOR uintptr_t owner; bool rv; kpreempt_disable(); owner = rw->rw_owner; rv = (owner & RW_THREAD) == 0 || rw_oncpu(owner); kpreempt_enable(); return rv; #else return rw_owner(rw) == curlwp; #endif }