// SPDX-License-Identifier: GPL-2.0-only
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#include <linux/rwsem.h>
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#include <linux/sched/debug.h>
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#include <linux/sched/signal.h>
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#include <linux/export.h>
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#include <linux/blkdev.h>
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#include "rtmutex_common.h"
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/*
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* RT-specific reader/writer semaphores
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*
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* down_write()
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* 1) Lock sem->rtmutex
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* 2) Remove the reader BIAS to force readers into the slow path
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* 3) Wait until all readers have left the critical region
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* 4) Mark it write locked
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*
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* up_write()
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* 1) Remove the write locked marker
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* 2) Set the reader BIAS so readers can use the fast path again
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* 3) Unlock sem->rtmutex to release blocked readers
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*
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* down_read()
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* 1) Try fast path acquisition (reader BIAS is set)
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* 2) Take sem->rtmutex.wait_lock which protects the writelocked flag
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* 3) If !writelocked, acquire it for read
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* 4) If writelocked, block on sem->rtmutex
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* 5) unlock sem->rtmutex, goto 1)
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*
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* up_read()
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* 1) Try fast path release (reader count != 1)
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* 2) Wake the writer waiting in down_write()#3
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*
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* down_read()#3 has the consequence, that rw semaphores on RT are not writer
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* fair, but writers, which should be avoided in RT tasks (think mmap_sem),
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* are subject to the rtmutex priority/DL inheritance mechanism.
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*
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* It's possible to make the rw semaphores writer fair by keeping a list of
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* active readers. A blocked writer would force all newly incoming readers to
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* block on the rtmutex, but the rtmutex would have to be proxy locked for one
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* reader after the other. We can't use multi-reader inheritance because there
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* is no way to support that with SCHED_DEADLINE. Implementing the one by one
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* reader boosting/handover mechanism is a major surgery for a very dubious
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* value.
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*
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* The risk of writer starvation is there, but the pathological use cases
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* which trigger it are not necessarily the typical RT workloads.
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*/
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void __rwsem_init(struct rw_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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atomic_set(&sem->readers, READER_BIAS);
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}
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EXPORT_SYMBOL(__rwsem_init);
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int __down_read_trylock(struct rw_semaphore *sem)
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{
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int r, old;
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/*
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* Increment reader count, if sem->readers < 0, i.e. READER_BIAS is
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* set.
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*/
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for (r = atomic_read(&sem->readers); r < 0;) {
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old = atomic_cmpxchg(&sem->readers, r, r + 1);
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if (likely(old == r))
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return 1;
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r = old;
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}
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return 0;
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}
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static int __sched __down_read_common(struct rw_semaphore *sem, int state)
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{
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struct rt_mutex *m = &sem->rtmutex;
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struct rt_mutex_waiter waiter;
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int ret;
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if (__down_read_trylock(sem))
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return 0;
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/*
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* Flush blk before ->pi_blocked_on is set. At schedule() time it is too
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* late if one of the callbacks needs to acquire a sleeping lock.
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*/
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if (blk_needs_flush_plug(current))
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blk_schedule_flush_plug(current);
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might_sleep();
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raw_spin_lock_irq(&m->wait_lock);
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/*
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* Allow readers as long as the writer has not completely
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* acquired the semaphore for write.
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*/
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if (atomic_read(&sem->readers) != WRITER_BIAS) {
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atomic_inc(&sem->readers);
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raw_spin_unlock_irq(&m->wait_lock);
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return 0;
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}
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/*
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* Call into the slow lock path with the rtmutex->wait_lock
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* held, so this can't result in the following race:
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*
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* Reader1 Reader2 Writer
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* down_read()
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* down_write()
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* rtmutex_lock(m)
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* swait()
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* down_read()
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* unlock(m->wait_lock)
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* up_read()
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* swake()
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* lock(m->wait_lock)
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* sem->writelocked=true
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* unlock(m->wait_lock)
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*
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* up_write()
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* sem->writelocked=false
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* rtmutex_unlock(m)
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* down_read()
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* down_write()
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* rtmutex_lock(m)
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* swait()
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* rtmutex_lock(m)
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*
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* That would put Reader1 behind the writer waiting on
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* Reader2 to call up_read() which might be unbound.
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*/
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rt_mutex_init_waiter(&waiter, false);
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ret = rt_mutex_slowlock_locked(m, state, NULL, RT_MUTEX_MIN_CHAINWALK,
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NULL, &waiter);
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/*
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* The slowlock() above is guaranteed to return with the rtmutex (for
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* ret = 0) is now held, so there can't be a writer active. Increment
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* the reader count and immediately drop the rtmutex again.
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* For ret != 0 we don't hold the rtmutex and need unlock the wait_lock.
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* We don't own the lock then.
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*/
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if (!ret)
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atomic_inc(&sem->readers);
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raw_spin_unlock_irq(&m->wait_lock);
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if (!ret)
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__rt_mutex_unlock(m);
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debug_rt_mutex_free_waiter(&waiter);
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return ret;
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}
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void __down_read(struct rw_semaphore *sem)
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{
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int ret;
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ret = __down_read_common(sem, TASK_UNINTERRUPTIBLE);
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WARN_ON_ONCE(ret);
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}
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int __down_read_interruptible(struct rw_semaphore *sem)
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{
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int ret;
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ret = __down_read_common(sem, TASK_INTERRUPTIBLE);
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if (likely(!ret))
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return ret;
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WARN_ONCE(ret != -EINTR, "Unexpected state: %d\n", ret);
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return -EINTR;
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}
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int __down_read_killable(struct rw_semaphore *sem)
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{
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int ret;
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ret = __down_read_common(sem, TASK_KILLABLE);
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if (likely(!ret))
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return ret;
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WARN_ONCE(ret != -EINTR, "Unexpected state: %d\n", ret);
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return -EINTR;
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}
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void __up_read(struct rw_semaphore *sem)
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{
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struct rt_mutex *m = &sem->rtmutex;
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struct task_struct *tsk;
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/*
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* sem->readers can only hit 0 when a writer is waiting for the
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* active readers to leave the critical region.
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*/
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if (!atomic_dec_and_test(&sem->readers))
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return;
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raw_spin_lock_irq(&m->wait_lock);
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/*
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* Wake the writer, i.e. the rtmutex owner. It might release the
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* rtmutex concurrently in the fast path (due to a signal), but to
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* clean up the rwsem it needs to acquire m->wait_lock. The worst
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* case which can happen is a spurious wakeup.
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*/
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tsk = rt_mutex_owner(m);
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if (tsk)
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wake_up_process(tsk);
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raw_spin_unlock_irq(&m->wait_lock);
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}
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static void __up_write_unlock(struct rw_semaphore *sem, int bias,
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unsigned long flags)
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{
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struct rt_mutex *m = &sem->rtmutex;
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atomic_add(READER_BIAS - bias, &sem->readers);
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raw_spin_unlock_irqrestore(&m->wait_lock, flags);
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__rt_mutex_unlock(m);
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}
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static int __sched __down_write_common(struct rw_semaphore *sem, int state)
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{
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struct rt_mutex *m = &sem->rtmutex;
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unsigned long flags;
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/*
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* Flush blk before ->pi_blocked_on is set. At schedule() time it is too
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* late if one of the callbacks needs to acquire a sleeping lock.
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*/
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if (blk_needs_flush_plug(current))
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blk_schedule_flush_plug(current);
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/* Take the rtmutex as a first step */
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if (__rt_mutex_lock_state(m, state))
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return -EINTR;
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/* Force readers into slow path */
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atomic_sub(READER_BIAS, &sem->readers);
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might_sleep();
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set_current_state(state);
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for (;;) {
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raw_spin_lock_irqsave(&m->wait_lock, flags);
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/* Have all readers left the critical region? */
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if (!atomic_read(&sem->readers)) {
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atomic_set(&sem->readers, WRITER_BIAS);
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__set_current_state(TASK_RUNNING);
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raw_spin_unlock_irqrestore(&m->wait_lock, flags);
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return 0;
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}
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if (signal_pending_state(state, current)) {
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__set_current_state(TASK_RUNNING);
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__up_write_unlock(sem, 0, flags);
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return -EINTR;
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}
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raw_spin_unlock_irqrestore(&m->wait_lock, flags);
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if (atomic_read(&sem->readers) != 0) {
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schedule();
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set_current_state(state);
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}
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}
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}
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void __sched __down_write(struct rw_semaphore *sem)
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{
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__down_write_common(sem, TASK_UNINTERRUPTIBLE);
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}
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int __sched __down_write_killable(struct rw_semaphore *sem)
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{
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return __down_write_common(sem, TASK_KILLABLE);
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}
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int __down_write_trylock(struct rw_semaphore *sem)
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{
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struct rt_mutex *m = &sem->rtmutex;
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unsigned long flags;
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if (!__rt_mutex_trylock(m))
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return 0;
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atomic_sub(READER_BIAS, &sem->readers);
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raw_spin_lock_irqsave(&m->wait_lock, flags);
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if (!atomic_read(&sem->readers)) {
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atomic_set(&sem->readers, WRITER_BIAS);
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raw_spin_unlock_irqrestore(&m->wait_lock, flags);
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return 1;
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}
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__up_write_unlock(sem, 0, flags);
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return 0;
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}
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void __up_write(struct rw_semaphore *sem)
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{
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struct rt_mutex *m = &sem->rtmutex;
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unsigned long flags;
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raw_spin_lock_irqsave(&m->wait_lock, flags);
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__up_write_unlock(sem, WRITER_BIAS, flags);
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}
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void __downgrade_write(struct rw_semaphore *sem)
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{
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struct rt_mutex *m = &sem->rtmutex;
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unsigned long flags;
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raw_spin_lock_irqsave(&m->wait_lock, flags);
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/* Release it and account current as reader */
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__up_write_unlock(sem, WRITER_BIAS - 1, flags);
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}
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