// SPDX-License-Identifier: GPL-2.0 /* * Ldisc rw semaphore * * The ldisc semaphore is semantically a rw_semaphore but which enforces * an alternate policy, namely: * 1) Supports lock wait timeouts * 2) Write waiter has priority * 3) Downgrading is not supported * * Implementation notes: * 1) Upper half of semaphore count is a wait count (differs from rwsem * in that rwsem normalizes the upper half to the wait bias) * 2) Lacks overflow checking * * The generic counting was copied and modified from include/asm-generic/rwsem.h * by Paul Mackerras <paulus@samba.org>. * * The scheduling policy was copied and modified from lib/rwsem.c * Written by David Howells (dhowells@redhat.com). * * This implementation incorporates the write lock stealing work of * Michel Lespinasse <walken@google.com>. * * Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com> */ #include <linux/list.h> #include <linux/spinlock.h> #include <linux/atomic.h> #include <linux/tty.h> #include <linux/sched.h> #include <linux/sched/debug.h> #include <linux/sched/task.h> #ifdef CONFIG_DEBUG_LOCK_ALLOC # define __acq(l, s, t, r, c, n, i) \ lock_acquire(&(l)->dep_map, s, t, r, c, n, i) # define __rel(l, n, i) \ lock_release(&(l)->dep_map, n, i) #define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i) #define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i) #define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i) #define lockdep_release(l, n, i) __rel(l, n, i) #else # define lockdep_acquire(l, s, t, i) do { } while (0) # define lockdep_acquire_nest(l, s, t, n, i) do { } while (0) # define lockdep_acquire_read(l, s, t, i) do { } while (0) # define lockdep_release(l, n, i) do { } while (0) #endif #ifdef CONFIG_LOCK_STAT # define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_) #else # define lock_stat(_lock, stat) do { } while (0) #endif #if BITS_PER_LONG == 64 # define LDSEM_ACTIVE_MASK 0xffffffffL #else # define LDSEM_ACTIVE_MASK 0x0000ffffL #endif #define LDSEM_UNLOCKED 0L #define LDSEM_ACTIVE_BIAS 1L #define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1) #define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS #define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS) struct ldsem_waiter { struct list_head list; struct task_struct *task; }; static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem) { return atomic_long_add_return(delta, (atomic_long_t *)&sem->count); } /* * ldsem_cmpxchg() updates @*old with the last-known sem->count value. * Returns 1 if count was successfully changed; @*old will have @new value. * Returns 0 if count was not changed; @*old will have most recent sem->count */ static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem) { long tmp = atomic_long_cmpxchg(&sem->count, *old, new); if (tmp == *old) { *old = new; return 1; } else { *old = tmp; return 0; } } /* * Initialize an ldsem: */ void __init_ldsem(struct ld_semaphore *sem, const char *name, struct lock_class_key *key) { #ifdef CONFIG_DEBUG_LOCK_ALLOC /* * Make sure we are not reinitializing a held semaphore: */ debug_check_no_locks_freed((void *)sem, sizeof(*sem)); lockdep_init_map(&sem->dep_map, name, key, 0); #endif sem->count = LDSEM_UNLOCKED; sem->wait_readers = 0; raw_spin_lock_init(&sem->wait_lock); INIT_LIST_HEAD(&sem->read_wait); INIT_LIST_HEAD(&sem->write_wait); } static void __ldsem_wake_readers(struct ld_semaphore *sem) { struct ldsem_waiter *waiter, *next; struct task_struct *tsk; long adjust, count; /* Try to grant read locks to all readers on the read wait list. * Note the 'active part' of the count is incremented by * the number of readers before waking any processes up. */ adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS); count = ldsem_atomic_update(adjust, sem); do { if (count > 0) break; if (ldsem_cmpxchg(&count, count - adjust, sem)) return; } while (1); list_for_each_entry_safe(waiter, next, &sem->read_wait, list) { tsk = waiter->task; smp_mb(); waiter->task = NULL; wake_up_process(tsk); put_task_struct(tsk); } INIT_LIST_HEAD(&sem->read_wait); sem->wait_readers = 0; } static inline int writer_trylock(struct ld_semaphore *sem) { /* only wake this writer if the active part of the count can be * transitioned from 0 -> 1 */ long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem); do { if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) return 1; if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem)) return 0; } while (1); } static void __ldsem_wake_writer(struct ld_semaphore *sem) { struct ldsem_waiter *waiter; waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list); wake_up_process(waiter->task); } /* * handle the lock release when processes blocked on it that can now run * - if we come here from up_xxxx(), then: * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed) * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so) * - the spinlock must be held by the caller * - woken process blocks are discarded from the list after having task zeroed */ static void __ldsem_wake(struct ld_semaphore *sem) { if (!list_empty(&sem->write_wait)) __ldsem_wake_writer(sem); else if (!list_empty(&sem->read_wait)) __ldsem_wake_readers(sem); } static void ldsem_wake(struct ld_semaphore *sem) { unsigned long flags; raw_spin_lock_irqsave(&sem->wait_lock, flags); __ldsem_wake(sem); raw_spin_unlock_irqrestore(&sem->wait_lock, flags); } /* * wait for the read lock to be granted */ static struct ld_semaphore __sched * down_read_failed(struct ld_semaphore *sem, long count, long timeout) { struct ldsem_waiter waiter; long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS; /* set up my own style of waitqueue */ raw_spin_lock_irq(&sem->wait_lock); /* Try to reverse the lock attempt but if the count has changed * so that reversing fails, check if there are are no waiters, * and early-out if not */ do { if (ldsem_cmpxchg(&count, count + adjust, sem)) break; if (count > 0) { raw_spin_unlock_irq(&sem->wait_lock); return sem; } } while (1); list_add_tail(&waiter.list, &sem->read_wait); sem->wait_readers++; waiter.task = current; get_task_struct(current); /* if there are no active locks, wake the new lock owner(s) */ if ((count & LDSEM_ACTIVE_MASK) == 0) __ldsem_wake(sem); raw_spin_unlock_irq(&sem->wait_lock); /* wait to be given the lock */ for (;;) { set_current_state(TASK_UNINTERRUPTIBLE); if (!waiter.task) break; if (!timeout) break; timeout = schedule_timeout(timeout); } __set_current_state(TASK_RUNNING); if (!timeout) { /* lock timed out but check if this task was just * granted lock ownership - if so, pretend there * was no timeout; otherwise, cleanup lock wait */ raw_spin_lock_irq(&sem->wait_lock); if (waiter.task) { ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); list_del(&waiter.list); raw_spin_unlock_irq(&sem->wait_lock); put_task_struct(waiter.task); return NULL; } raw_spin_unlock_irq(&sem->wait_lock); } return sem; } /* * wait for the write lock to be granted */ static struct ld_semaphore __sched * down_write_failed(struct ld_semaphore *sem, long count, long timeout) { struct ldsem_waiter waiter; long adjust = -LDSEM_ACTIVE_BIAS; int locked = 0; /* set up my own style of waitqueue */ raw_spin_lock_irq(&sem->wait_lock); /* Try to reverse the lock attempt but if the count has changed * so that reversing fails, check if the lock is now owned, * and early-out if so */ do { if (ldsem_cmpxchg(&count, count + adjust, sem)) break; if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) { raw_spin_unlock_irq(&sem->wait_lock); return sem; } } while (1); list_add_tail(&waiter.list, &sem->write_wait); waiter.task = current; set_current_state(TASK_UNINTERRUPTIBLE); for (;;) { if (!timeout) break; raw_spin_unlock_irq(&sem->wait_lock); timeout = schedule_timeout(timeout); raw_spin_lock_irq(&sem->wait_lock); set_current_state(TASK_UNINTERRUPTIBLE); locked = writer_trylock(sem); if (locked) break; } if (!locked) ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem); list_del(&waiter.list); raw_spin_unlock_irq(&sem->wait_lock); __set_current_state(TASK_RUNNING); /* lock wait may have timed out */ if (!locked) return NULL; return sem; } static int __ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout) { long count; lockdep_acquire_read(sem, subclass, 0, _RET_IP_); count = ldsem_atomic_update(LDSEM_READ_BIAS, sem); if (count <= 0) { lock_stat(sem, contended); if (!down_read_failed(sem, count, timeout)) { lockdep_release(sem, 1, _RET_IP_); return 0; } } lock_stat(sem, acquired); return 1; } static int __ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, long timeout) { long count; lockdep_acquire(sem, subclass, 0, _RET_IP_); count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem); if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) { lock_stat(sem, contended); if (!down_write_failed(sem, count, timeout)) { lockdep_release(sem, 1, _RET_IP_); return 0; } } lock_stat(sem, acquired); return 1; } /* * lock for reading -- returns 1 if successful, 0 if timed out */ int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout) { might_sleep(); return __ldsem_down_read_nested(sem, 0, timeout); } /* * trylock for reading -- returns 1 if successful, 0 if contention */ int ldsem_down_read_trylock(struct ld_semaphore *sem) { long count = sem->count; while (count >= 0) { if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) { lockdep_acquire_read(sem, 0, 1, _RET_IP_); lock_stat(sem, acquired); return 1; } } return 0; } /* * lock for writing -- returns 1 if successful, 0 if timed out */ int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout) { might_sleep(); return __ldsem_down_write_nested(sem, 0, timeout); } /* * trylock for writing -- returns 1 if successful, 0 if contention */ int ldsem_down_write_trylock(struct ld_semaphore *sem) { long count = sem->count; while ((count & LDSEM_ACTIVE_MASK) == 0) { if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) { lockdep_acquire(sem, 0, 1, _RET_IP_); lock_stat(sem, acquired); return 1; } } return 0; } /* * release a read lock */ void ldsem_up_read(struct ld_semaphore *sem) { long count; lockdep_release(sem, 1, _RET_IP_); count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem); if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0) ldsem_wake(sem); } /* * release a write lock */ void ldsem_up_write(struct ld_semaphore *sem) { long count; lockdep_release(sem, 1, _RET_IP_); count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem); if (count < 0) ldsem_wake(sem); } #ifdef CONFIG_DEBUG_LOCK_ALLOC int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout) { might_sleep(); return __ldsem_down_read_nested(sem, subclass, timeout); } int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, long timeout) { might_sleep(); return __ldsem_down_write_nested(sem, subclass, timeout); } #endif