// SPDX-License-Identifier: GPL-2.0-only
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#include <linux/atomic.h>
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#include <linux/percpu.h>
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#include <linux/wait.h>
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#include <linux/lockdep.h>
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#include <linux/percpu-rwsem.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/sched/task.h>
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#include <linux/slab.h>
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#include <linux/errno.h>
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#include <trace/hooks/dtask.h>
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/*
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* trace_android_vh_record_pcpu_rwsem_starttime is called in
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* include/linux/percpu-rwsem.h by including include/hooks/dtask.h, which
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* will result to build-err. So we create
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* func:_trace_android_vh_record_pcpu_rwsem_starttime for percpu-rwsem.h to call.
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*/
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void _trace_android_vh_record_pcpu_rwsem_starttime(struct task_struct *tsk,
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unsigned long settime)
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{
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trace_android_vh_record_pcpu_rwsem_starttime(tsk, settime);
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}
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EXPORT_SYMBOL_GPL(_trace_android_vh_record_pcpu_rwsem_starttime);
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int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
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const char *name, struct lock_class_key *key)
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{
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sem->read_count = alloc_percpu(int);
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if (unlikely(!sem->read_count))
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return -ENOMEM;
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rcu_sync_init(&sem->rss);
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rcuwait_init(&sem->writer);
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init_waitqueue_head(&sem->waiters);
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atomic_set(&sem->block, 0);
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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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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return 0;
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}
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EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
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void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
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{
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/*
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* XXX: temporary kludge. The error path in alloc_super()
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* assumes that percpu_free_rwsem() is safe after kzalloc().
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*/
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if (!sem->read_count)
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return;
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rcu_sync_dtor(&sem->rss);
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free_percpu(sem->read_count);
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sem->read_count = NULL; /* catch use after free bugs */
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}
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EXPORT_SYMBOL_GPL(percpu_free_rwsem);
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static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
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{
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this_cpu_inc(*sem->read_count);
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/*
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* Due to having preemption disabled the decrement happens on
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* the same CPU as the increment, avoiding the
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* increment-on-one-CPU-and-decrement-on-another problem.
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*
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* If the reader misses the writer's assignment of sem->block, then the
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* writer is guaranteed to see the reader's increment.
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*
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* Conversely, any readers that increment their sem->read_count after
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* the writer looks are guaranteed to see the sem->block value, which
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* in turn means that they are guaranteed to immediately decrement
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* their sem->read_count, so that it doesn't matter that the writer
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* missed them.
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*/
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smp_mb(); /* A matches D */
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/*
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* If !sem->block the critical section starts here, matched by the
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* release in percpu_up_write().
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*/
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if (likely(!atomic_read_acquire(&sem->block)))
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return true;
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this_cpu_dec(*sem->read_count);
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/* Prod writer to re-evaluate readers_active_check() */
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rcuwait_wake_up(&sem->writer);
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return false;
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}
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static inline bool __percpu_down_write_trylock(struct percpu_rw_semaphore *sem)
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{
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if (atomic_read(&sem->block))
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return false;
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return atomic_xchg(&sem->block, 1) == 0;
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}
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static bool __percpu_rwsem_trylock(struct percpu_rw_semaphore *sem, bool reader)
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{
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if (reader) {
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bool ret;
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preempt_disable();
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ret = __percpu_down_read_trylock(sem);
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preempt_enable();
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return ret;
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}
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return __percpu_down_write_trylock(sem);
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}
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/*
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* The return value of wait_queue_entry::func means:
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*
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* <0 - error, wakeup is terminated and the error is returned
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* 0 - no wakeup, a next waiter is tried
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* >0 - woken, if EXCLUSIVE, counted towards @nr_exclusive.
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*
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* We use EXCLUSIVE for both readers and writers to preserve FIFO order,
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* and play games with the return value to allow waking multiple readers.
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*
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* Specifically, we wake readers until we've woken a single writer, or until a
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* trylock fails.
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*/
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static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry,
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unsigned int mode, int wake_flags,
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void *key)
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{
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bool reader = wq_entry->flags & WQ_FLAG_CUSTOM;
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struct percpu_rw_semaphore *sem = key;
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struct task_struct *p;
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/* concurrent against percpu_down_write(), can get stolen */
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if (!__percpu_rwsem_trylock(sem, reader))
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return 1;
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p = get_task_struct(wq_entry->private);
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list_del_init(&wq_entry->entry);
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smp_store_release(&wq_entry->private, NULL);
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wake_up_process(p);
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put_task_struct(p);
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return !reader; /* wake (readers until) 1 writer */
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}
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static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)
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{
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DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);
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bool wait;
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spin_lock_irq(&sem->waiters.lock);
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/*
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* Serialize against the wakeup in percpu_up_write(), if we fail
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* the trylock, the wakeup must see us on the list.
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*/
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wait = !__percpu_rwsem_trylock(sem, reader);
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if (wait) {
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wq_entry.flags |= WQ_FLAG_EXCLUSIVE | reader * WQ_FLAG_CUSTOM;
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__add_wait_queue_entry_tail(&sem->waiters, &wq_entry);
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}
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spin_unlock_irq(&sem->waiters.lock);
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while (wait) {
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set_current_state(TASK_UNINTERRUPTIBLE);
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if (!smp_load_acquire(&wq_entry.private))
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break;
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schedule();
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}
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__set_current_state(TASK_RUNNING);
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}
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bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
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{
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if (__percpu_down_read_trylock(sem))
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return true;
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if (try)
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return false;
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preempt_enable();
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percpu_rwsem_wait(sem, /* .reader = */ true);
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preempt_disable();
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return true;
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}
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EXPORT_SYMBOL_GPL(__percpu_down_read);
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#define per_cpu_sum(var) \
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({ \
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typeof(var) __sum = 0; \
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int cpu; \
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compiletime_assert_atomic_type(__sum); \
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for_each_possible_cpu(cpu) \
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__sum += per_cpu(var, cpu); \
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__sum; \
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})
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/*
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* Return true if the modular sum of the sem->read_count per-CPU variable is
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* zero. If this sum is zero, then it is stable due to the fact that if any
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* newly arriving readers increment a given counter, they will immediately
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* decrement that same counter.
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*
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* Assumes sem->block is set.
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*/
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static bool readers_active_check(struct percpu_rw_semaphore *sem)
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{
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if (per_cpu_sum(*sem->read_count) != 0)
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return false;
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/*
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* If we observed the decrement; ensure we see the entire critical
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* section.
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*/
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smp_mb(); /* C matches B */
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return true;
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}
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void percpu_down_write(struct percpu_rw_semaphore *sem)
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{
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might_sleep();
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rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
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/* Notify readers to take the slow path. */
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rcu_sync_enter(&sem->rss);
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/*
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* Try set sem->block; this provides writer-writer exclusion.
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* Having sem->block set makes new readers block.
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*/
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if (!__percpu_down_write_trylock(sem))
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percpu_rwsem_wait(sem, /* .reader = */ false);
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/* smp_mb() implied by __percpu_down_write_trylock() on success -- D matches A */
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/*
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* If they don't see our store of sem->block, then we are guaranteed to
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* see their sem->read_count increment, and therefore will wait for
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* them.
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*/
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/* Wait for all active readers to complete. */
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rcuwait_wait_event(&sem->writer, readers_active_check(sem), TASK_UNINTERRUPTIBLE);
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trace_android_vh_record_pcpu_rwsem_starttime(current, jiffies);
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}
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EXPORT_SYMBOL_GPL(percpu_down_write);
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void percpu_up_write(struct percpu_rw_semaphore *sem)
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{
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trace_android_vh_record_pcpu_rwsem_starttime(current, 0);
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rwsem_release(&sem->dep_map, _RET_IP_);
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/*
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* Signal the writer is done, no fast path yet.
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*
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* One reason that we cannot just immediately flip to readers_fast is
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* that new readers might fail to see the results of this writer's
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* critical section.
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*
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* Therefore we force it through the slow path which guarantees an
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* acquire and thereby guarantees the critical section's consistency.
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*/
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atomic_set_release(&sem->block, 0);
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/*
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* Prod any pending reader/writer to make progress.
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*/
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__wake_up(&sem->waiters, TASK_NORMAL, 1, sem);
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/*
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* Once this completes (at least one RCU-sched grace period hence) the
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* reader fast path will be available again. Safe to use outside the
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* exclusive write lock because its counting.
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*/
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rcu_sync_exit(&sem->rss);
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}
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EXPORT_SYMBOL_GPL(percpu_up_write);
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static LIST_HEAD(destroy_list);
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static DEFINE_SPINLOCK(destroy_list_lock);
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static void destroy_list_workfn(struct work_struct *work)
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{
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struct percpu_rw_semaphore_atomic *sem, *sem2;
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LIST_HEAD(to_destroy);
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spin_lock(&destroy_list_lock);
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list_splice_init(&destroy_list, &to_destroy);
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spin_unlock(&destroy_list_lock);
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if (list_empty(&to_destroy))
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return;
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list_for_each_entry_safe(sem, sem2, &to_destroy, destroy_list_entry) {
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percpu_free_rwsem(&sem->rw_sem);
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kfree(sem);
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}
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}
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static DECLARE_WORK(destroy_list_work, destroy_list_workfn);
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void percpu_rwsem_async_destroy(struct percpu_rw_semaphore_atomic *sem)
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{
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spin_lock(&destroy_list_lock);
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list_add_tail(&sem->destroy_list_entry, &destroy_list);
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spin_unlock(&destroy_list_lock);
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schedule_work(&destroy_list_work);
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}
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