LXR linux/kernel/locking/qrwlock.c

   1// SPDX-License-Identifier: GPL-2.0-or-later
   2/*
   3 * Queued read/write locks
   4 *
   5 * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
   6 *
   7 * Authors: Waiman Long <waiman.long@hp.com>
   8 */
   9#include <linux/smp.h>
  10#include <linux/bug.h>
  11#include <linux/cpumask.h>
  12#include <linux/percpu.h>
  13#include <linux/hardirq.h>
  14#include <linux/spinlock.h>
  15#include <asm/qrwlock.h>
  16
  17/**
  18 * queued_read_lock_slowpath - acquire read lock of a queue rwlock
  19 * @lock: Pointer to queue rwlock structure
  20 */
  21void queued_read_lock_slowpath(struct qrwlock *lock)
  22{
  23        /*
  24         * Readers come here when they cannot get the lock without waiting
  25         */
  26        if (unlikely(in_interrupt())) {
  27                /*
  28                 * Readers in interrupt context will get the lock immediately
  29                 * if the writer is just waiting (not holding the lock yet),
  30                 * so spin with ACQUIRE semantics until the lock is available
  31                 * without waiting in the queue.
  32                 */
  33                atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
  34                return;
  35        }
  36        atomic_sub(_QR_BIAS, &lock->cnts);
  37
  38        /*
  39         * Put the reader into the wait queue
  40         */
  41        arch_spin_lock(&lock->wait_lock);
  42        atomic_add(_QR_BIAS, &lock->cnts);
  43
  44        /*
  45         * The ACQUIRE semantics of the following spinning code ensure
  46         * that accesses can't leak upwards out of our subsequent critical
  47         * section in the case that the lock is currently held for write.
  48         */
  49        atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
  50
  51        /*
  52         * Signal the next one in queue to become queue head
  53         */
  54        arch_spin_unlock(&lock->wait_lock);
  55}
  56EXPORT_SYMBOL(queued_read_lock_slowpath);
  57
  58/**
  59 * queued_write_lock_slowpath - acquire write lock of a queue rwlock
  60 * @lock : Pointer to queue rwlock structure
  61 */
  62void queued_write_lock_slowpath(struct qrwlock *lock)
  63{
  64        /* Put the writer into the wait queue */
  65        arch_spin_lock(&lock->wait_lock);
  66
  67        /* Try to acquire the lock directly if no reader is present */
  68        if (!atomic_read(&lock->cnts) &&
  69            (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
  70                goto unlock;
  71
  72        /* Set the waiting flag to notify readers that a writer is pending */
  73        atomic_add(_QW_WAITING, &lock->cnts);
  74
  75        /* When no more readers or writers, set the locked flag */
  76        do {
  77                atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
  78        } while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
  79                                        _QW_LOCKED) != _QW_WAITING);
  80unlock:
  81        arch_spin_unlock(&lock->wait_lock);
  82}
  83EXPORT_SYMBOL(queued_write_lock_slowpath);
  84