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Linux kernel mirror (for testing) git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414// 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>
#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;};
/* * 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 atomic_long_set(&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 = atomic_long_add_return(adjust, &sem->count); do { if (count > 0) break; if (atomic_long_try_cmpxchg(&sem->count, &count, count - adjust)) return; } while (1);
list_for_each_entry_safe(waiter, next, &sem->read_wait, list) { tsk = waiter->task; smp_store_release(&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 = atomic_long_add_return(LDSEM_ACTIVE_BIAS, &sem->count); do { if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) return 1; if (atomic_long_try_cmpxchg(&sem->count, &count, count - LDSEM_ACTIVE_BIAS)) 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 no waiters, * and early-out if not */ do { if (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) { count += adjust; 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 (!smp_load_acquire(&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) { atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count); sem->wait_readers--; 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 (atomic_long_try_cmpxchg(&sem->count, &count, count + adjust)) 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) atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count); list_del(&waiter.list);
/* * In case of timeout, wake up every reader who gave the right of way * to writer. Prevent separation readers into two groups: * one that helds semaphore and another that sleeps. * (in case of no contention with a writer) */ if (!locked && list_empty(&sem->write_wait)) __ldsem_wake_readers(sem);
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;
rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
count = atomic_long_add_return(LDSEM_READ_BIAS, &sem->count); if (count <= 0) { lock_contended(&sem->dep_map, _RET_IP_); if (!down_read_failed(sem, count, timeout)) { rwsem_release(&sem->dep_map, _RET_IP_); return 0; } } lock_acquired(&sem->dep_map, _RET_IP_); return 1;}
static int __ldsem_down_write_nested(struct ld_semaphore *sem, int subclass, long timeout){ long count;
rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
count = atomic_long_add_return(LDSEM_WRITE_BIAS, &sem->count); if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) { lock_contended(&sem->dep_map, _RET_IP_); if (!down_write_failed(sem, count, timeout)) { rwsem_release(&sem->dep_map, _RET_IP_); return 0; } } lock_acquired(&sem->dep_map, _RET_IP_); 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 = atomic_long_read(&sem->count);
while (count >= 0) { if (atomic_long_try_cmpxchg(&sem->count, &count, count + LDSEM_READ_BIAS)) { rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_); lock_acquired(&sem->dep_map, _RET_IP_); 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);}
/* * release a read lock */void ldsem_up_read(struct ld_semaphore *sem){ long count;
rwsem_release(&sem->dep_map, _RET_IP_);
count = atomic_long_add_return(-LDSEM_READ_BIAS, &sem->count); 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;
rwsem_release(&sem->dep_map, _RET_IP_);
count = atomic_long_add_return(-LDSEM_WRITE_BIAS, &sem->count); 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