467 lines
12 KiB
C
467 lines
12 KiB
C
/*
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* Ldisc rw semaphore
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*
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* The ldisc semaphore is semantically a rw_semaphore but which enforces
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* an alternate policy, namely:
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* 1) Supports lock wait timeouts
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* 2) Write waiter has priority
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* 3) Downgrading is not supported
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*
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* Implementation notes:
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* 1) Upper half of semaphore count is a wait count (differs from rwsem
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* in that rwsem normalizes the upper half to the wait bias)
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* 2) Lacks overflow checking
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*
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* The generic counting was copied and modified from include/asm-generic/rwsem.h
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* by Paul Mackerras <paulus@samba.org>.
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*
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* The scheduling policy was copied and modified from lib/rwsem.c
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* Written by David Howells (dhowells@redhat.com).
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*
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* This implementation incorporates the write lock stealing work of
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* Michel Lespinasse <walken@google.com>.
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*
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* Copyright (C) 2013 Peter Hurley <peter@hurleysoftware.com>
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*
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* This file may be redistributed under the terms of the GNU General Public
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* License v2.
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*/
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#include <linux/list.h>
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#include <linux/spinlock.h>
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#include <linux/atomic.h>
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#include <linux/tty.h>
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#include <linux/sched.h>
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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# define __acq(l, s, t, r, c, n, i) \
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lock_acquire(&(l)->dep_map, s, t, r, c, n, i)
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# define __rel(l, n, i) \
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lock_release(&(l)->dep_map, n, i)
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#define lockdep_acquire(l, s, t, i) __acq(l, s, t, 0, 1, NULL, i)
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#define lockdep_acquire_nest(l, s, t, n, i) __acq(l, s, t, 0, 1, n, i)
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#define lockdep_acquire_read(l, s, t, i) __acq(l, s, t, 1, 1, NULL, i)
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#define lockdep_release(l, n, i) __rel(l, n, i)
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#else
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# define lockdep_acquire(l, s, t, i) do { } while (0)
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# define lockdep_acquire_nest(l, s, t, n, i) do { } while (0)
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# define lockdep_acquire_read(l, s, t, i) do { } while (0)
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# define lockdep_release(l, n, i) do { } while (0)
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#endif
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#ifdef CONFIG_LOCK_STAT
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# define lock_stat(_lock, stat) lock_##stat(&(_lock)->dep_map, _RET_IP_)
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#else
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# define lock_stat(_lock, stat) do { } while (0)
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#endif
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#if BITS_PER_LONG == 64
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# define LDSEM_ACTIVE_MASK 0xffffffffL
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#else
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# define LDSEM_ACTIVE_MASK 0x0000ffffL
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#endif
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#define LDSEM_UNLOCKED 0L
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#define LDSEM_ACTIVE_BIAS 1L
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#define LDSEM_WAIT_BIAS (-LDSEM_ACTIVE_MASK-1)
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#define LDSEM_READ_BIAS LDSEM_ACTIVE_BIAS
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#define LDSEM_WRITE_BIAS (LDSEM_WAIT_BIAS + LDSEM_ACTIVE_BIAS)
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struct ldsem_waiter {
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struct list_head list;
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struct task_struct *task;
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};
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static inline long ldsem_atomic_update(long delta, struct ld_semaphore *sem)
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{
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return atomic_long_add_return(delta, (atomic_long_t *)&sem->count);
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}
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/*
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* ldsem_cmpxchg() updates @*old with the last-known sem->count value.
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* Returns 1 if count was successfully changed; @*old will have @new value.
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* Returns 0 if count was not changed; @*old will have most recent sem->count
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*/
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static inline int ldsem_cmpxchg(long *old, long new, struct ld_semaphore *sem)
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{
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long tmp = atomic_long_cmpxchg(&sem->count, *old, new);
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if (tmp == *old) {
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*old = new;
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return 1;
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} else {
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*old = tmp;
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return 0;
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}
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}
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/*
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* Initialize an ldsem:
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*/
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void __init_ldsem(struct ld_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* Make sure we are not reinitializing a held semaphore:
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*/
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debug_check_no_locks_freed((void *)sem, sizeof(*sem));
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lockdep_init_map(&sem->dep_map, name, key, 0);
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#endif
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sem->count = LDSEM_UNLOCKED;
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sem->wait_readers = 0;
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raw_spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->read_wait);
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INIT_LIST_HEAD(&sem->write_wait);
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}
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static void __ldsem_wake_readers(struct ld_semaphore *sem)
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{
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struct ldsem_waiter *waiter, *next;
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struct task_struct *tsk;
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long adjust, count;
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/* Try to grant read locks to all readers on the read wait list.
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* Note the 'active part' of the count is incremented by
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* the number of readers before waking any processes up.
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*/
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adjust = sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS);
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count = ldsem_atomic_update(adjust, sem);
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do {
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if (count > 0)
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break;
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if (ldsem_cmpxchg(&count, count - adjust, sem))
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return;
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} while (1);
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list_for_each_entry_safe(waiter, next, &sem->read_wait, list) {
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tsk = waiter->task;
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smp_store_release(&waiter->task, NULL);
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wake_up_process(tsk);
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put_task_struct(tsk);
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}
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INIT_LIST_HEAD(&sem->read_wait);
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sem->wait_readers = 0;
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}
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static inline int writer_trylock(struct ld_semaphore *sem)
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{
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/* only wake this writer if the active part of the count can be
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* transitioned from 0 -> 1
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*/
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long count = ldsem_atomic_update(LDSEM_ACTIVE_BIAS, sem);
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do {
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if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS)
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return 1;
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if (ldsem_cmpxchg(&count, count - LDSEM_ACTIVE_BIAS, sem))
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return 0;
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} while (1);
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}
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static void __ldsem_wake_writer(struct ld_semaphore *sem)
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{
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struct ldsem_waiter *waiter;
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waiter = list_entry(sem->write_wait.next, struct ldsem_waiter, list);
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wake_up_process(waiter->task);
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}
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here from up_xxxx(), then:
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* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
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* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
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* - the spinlock must be held by the caller
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* - woken process blocks are discarded from the list after having task zeroed
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*/
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static void __ldsem_wake(struct ld_semaphore *sem)
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{
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if (!list_empty(&sem->write_wait))
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__ldsem_wake_writer(sem);
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else if (!list_empty(&sem->read_wait))
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__ldsem_wake_readers(sem);
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}
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static void ldsem_wake(struct ld_semaphore *sem)
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{
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unsigned long flags;
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raw_spin_lock_irqsave(&sem->wait_lock, flags);
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__ldsem_wake(sem);
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raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
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}
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/*
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* wait for the read lock to be granted
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*/
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static struct ld_semaphore __sched *
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down_read_failed(struct ld_semaphore *sem, long count, long timeout)
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{
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struct ldsem_waiter waiter;
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struct task_struct *tsk = current;
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long adjust = -LDSEM_ACTIVE_BIAS + LDSEM_WAIT_BIAS;
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/* set up my own style of waitqueue */
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raw_spin_lock_irq(&sem->wait_lock);
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/* Try to reverse the lock attempt but if the count has changed
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* so that reversing fails, check if there are are no waiters,
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* and early-out if not */
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do {
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if (ldsem_cmpxchg(&count, count + adjust, sem))
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break;
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if (count > 0) {
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raw_spin_unlock_irq(&sem->wait_lock);
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return sem;
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}
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} while (1);
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list_add_tail(&waiter.list, &sem->read_wait);
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sem->wait_readers++;
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waiter.task = tsk;
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get_task_struct(tsk);
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/* if there are no active locks, wake the new lock owner(s) */
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if ((count & LDSEM_ACTIVE_MASK) == 0)
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__ldsem_wake(sem);
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raw_spin_unlock_irq(&sem->wait_lock);
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/* wait to be given the lock */
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for (;;) {
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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if (!smp_load_acquire(&waiter.task))
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break;
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if (!timeout)
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break;
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timeout = schedule_timeout(timeout);
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}
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__set_task_state(tsk, TASK_RUNNING);
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if (!timeout) {
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/* lock timed out but check if this task was just
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* granted lock ownership - if so, pretend there
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* was no timeout; otherwise, cleanup lock wait */
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raw_spin_lock_irq(&sem->wait_lock);
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if (waiter.task) {
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ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
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list_del(&waiter.list);
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raw_spin_unlock_irq(&sem->wait_lock);
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put_task_struct(waiter.task);
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return NULL;
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}
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raw_spin_unlock_irq(&sem->wait_lock);
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}
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return sem;
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}
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/*
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* wait for the write lock to be granted
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*/
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static struct ld_semaphore __sched *
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down_write_failed(struct ld_semaphore *sem, long count, long timeout)
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{
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struct ldsem_waiter waiter;
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struct task_struct *tsk = current;
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long adjust = -LDSEM_ACTIVE_BIAS;
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int locked = 0;
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/* set up my own style of waitqueue */
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raw_spin_lock_irq(&sem->wait_lock);
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/* Try to reverse the lock attempt but if the count has changed
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* so that reversing fails, check if the lock is now owned,
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* and early-out if so */
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do {
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if (ldsem_cmpxchg(&count, count + adjust, sem))
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break;
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if ((count & LDSEM_ACTIVE_MASK) == LDSEM_ACTIVE_BIAS) {
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raw_spin_unlock_irq(&sem->wait_lock);
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return sem;
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}
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} while (1);
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list_add_tail(&waiter.list, &sem->write_wait);
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waiter.task = tsk;
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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for (;;) {
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if (!timeout)
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break;
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raw_spin_unlock_irq(&sem->wait_lock);
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timeout = schedule_timeout(timeout);
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raw_spin_lock_irq(&sem->wait_lock);
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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locked = writer_trylock(sem);
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if (locked)
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break;
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}
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if (!locked)
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ldsem_atomic_update(-LDSEM_WAIT_BIAS, sem);
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list_del(&waiter.list);
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/*
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* In case of timeout, wake up every reader who gave the right of way
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* to writer. Prevent separation readers into two groups:
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* one that helds semaphore and another that sleeps.
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* (in case of no contention with a writer)
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*/
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if (!locked && list_empty(&sem->write_wait))
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__ldsem_wake_readers(sem);
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raw_spin_unlock_irq(&sem->wait_lock);
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__set_task_state(tsk, TASK_RUNNING);
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/* lock wait may have timed out */
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if (!locked)
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return NULL;
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return sem;
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}
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static int __ldsem_down_read_nested(struct ld_semaphore *sem,
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int subclass, long timeout)
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{
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long count;
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lockdep_acquire_read(sem, subclass, 0, _RET_IP_);
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count = ldsem_atomic_update(LDSEM_READ_BIAS, sem);
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if (count <= 0) {
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lock_stat(sem, contended);
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if (!down_read_failed(sem, count, timeout)) {
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lockdep_release(sem, 1, _RET_IP_);
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return 0;
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}
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}
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lock_stat(sem, acquired);
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return 1;
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}
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static int __ldsem_down_write_nested(struct ld_semaphore *sem,
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int subclass, long timeout)
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{
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long count;
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lockdep_acquire(sem, subclass, 0, _RET_IP_);
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count = ldsem_atomic_update(LDSEM_WRITE_BIAS, sem);
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if ((count & LDSEM_ACTIVE_MASK) != LDSEM_ACTIVE_BIAS) {
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lock_stat(sem, contended);
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if (!down_write_failed(sem, count, timeout)) {
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lockdep_release(sem, 1, _RET_IP_);
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return 0;
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}
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}
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lock_stat(sem, acquired);
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return 1;
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}
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/*
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* lock for reading -- returns 1 if successful, 0 if timed out
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*/
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int __sched ldsem_down_read(struct ld_semaphore *sem, long timeout)
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{
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might_sleep();
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return __ldsem_down_read_nested(sem, 0, timeout);
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}
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/*
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* trylock for reading -- returns 1 if successful, 0 if contention
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*/
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int ldsem_down_read_trylock(struct ld_semaphore *sem)
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{
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long count = sem->count;
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while (count >= 0) {
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if (ldsem_cmpxchg(&count, count + LDSEM_READ_BIAS, sem)) {
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lockdep_acquire_read(sem, 0, 1, _RET_IP_);
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lock_stat(sem, acquired);
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return 1;
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}
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}
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return 0;
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}
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/*
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* lock for writing -- returns 1 if successful, 0 if timed out
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*/
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int __sched ldsem_down_write(struct ld_semaphore *sem, long timeout)
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{
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might_sleep();
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return __ldsem_down_write_nested(sem, 0, timeout);
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}
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/*
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* trylock for writing -- returns 1 if successful, 0 if contention
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*/
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int ldsem_down_write_trylock(struct ld_semaphore *sem)
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{
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long count = sem->count;
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while ((count & LDSEM_ACTIVE_MASK) == 0) {
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if (ldsem_cmpxchg(&count, count + LDSEM_WRITE_BIAS, sem)) {
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lockdep_acquire(sem, 0, 1, _RET_IP_);
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lock_stat(sem, acquired);
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return 1;
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}
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}
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return 0;
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}
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/*
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* release a read lock
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*/
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void ldsem_up_read(struct ld_semaphore *sem)
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{
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long count;
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lockdep_release(sem, 1, _RET_IP_);
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count = ldsem_atomic_update(-LDSEM_READ_BIAS, sem);
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if (count < 0 && (count & LDSEM_ACTIVE_MASK) == 0)
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ldsem_wake(sem);
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}
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/*
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* release a write lock
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*/
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void ldsem_up_write(struct ld_semaphore *sem)
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{
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long count;
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lockdep_release(sem, 1, _RET_IP_);
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count = ldsem_atomic_update(-LDSEM_WRITE_BIAS, sem);
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if (count < 0)
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ldsem_wake(sem);
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}
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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int ldsem_down_read_nested(struct ld_semaphore *sem, int subclass, long timeout)
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{
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might_sleep();
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return __ldsem_down_read_nested(sem, subclass, timeout);
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}
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int ldsem_down_write_nested(struct ld_semaphore *sem, int subclass,
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long timeout)
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{
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might_sleep();
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return __ldsem_down_write_nested(sem, subclass, timeout);
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}
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#endif
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