kernel/locking/qrwlock.c

   1 /*
   2  * Queued read/write locks
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License as published by
   6  * the Free Software Foundation; either version 2 of the License, or
   7  * (at your option) any later version.
   8  *
   9  * This program is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  * GNU General Public License for more details.
  13  *
  14  * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
  15  *
  16  * Authors: Waiman Long <waiman.long@hp.com>
  17  */
  18 #include <linux/smp.h>
  19 #include <linux/bug.h>
  20 #include <linux/cpumask.h>
  21 #include <linux/percpu.h>
  22 #include <linux/hardirq.h>
  23 #include <linux/spinlock.h>
  24 #include <asm/qrwlock.h>
  25
  26 /**
  27  * queued_read_lock_slowpath - acquire read lock of a queue rwlock
  28  * @lock: Pointer to queue rwlock structure
  29  */
  30 void queued_read_lock_slowpath(struct qrwlock *lock)
  31 {
  32         /*
  33          * Readers come here when they cannot get the lock without waiting
  34          */
  35         if (unlikely(in_interrupt())) {
  36                 /*
  37                  * Readers in interrupt context will get the lock immediately
  38                  * if the writer is just waiting (not holding the lock yet),
  39                  * so spin with ACQUIRE semantics until the lock is available
  40                  * without waiting in the queue.
  41                  */
  42                 atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
  43                 return;
  44         }
  45         atomic_sub(_QR_BIAS, &lock->cnts);
  46
  47         /*
  48          * Put the reader into the wait queue
  49          */
  50         arch_spin_lock(&lock->wait_lock);
  51         atomic_add(_QR_BIAS, &lock->cnts);
  52
  53         /*
  54          * The ACQUIRE semantics of the following spinning code ensure
  55          * that accesses can't leak upwards out of our subsequent critical
  56          * section in the case that the lock is currently held for write.
  57          */
  58         atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
  59
  60         /*
  61          * Signal the next one in queue to become queue head
  62          */
  63         arch_spin_unlock(&lock->wait_lock);
  64 }
  65 EXPORT_SYMBOL(queued_read_lock_slowpath);
  66
  67 /**
  68  * queued_write_lock_slowpath - acquire write lock of a queue rwlock
  69  * @lock : Pointer to queue rwlock structure
  70  */
  71 void queued_write_lock_slowpath(struct qrwlock *lock)
  72 {
  73         /* Put the writer into the wait queue */
  74         arch_spin_lock(&lock->wait_lock);
  75
  76         /* Try to acquire the lock directly if no reader is present */
  77         if (!atomic_read(&lock->cnts) &&
  78             (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
  79                 goto unlock;
  80
  81         /* Set the waiting flag to notify readers that a writer is pending */
  82         atomic_add(_QW_WAITING, &lock->cnts);
  83
  84         /* When no more readers or writers, set the locked flag */
  85         do {
  86                 atomic_cond_read_acquire(&lock->cnts, VAL == _QW_WAITING);
  87         } while (atomic_cmpxchg_relaxed(&lock->cnts, _QW_WAITING,
  88                                         _QW_LOCKED) != _QW_WAITING);
  89 unlock:
  90         arch_spin_unlock(&lock->wait_lock);
  91 }
  92 EXPORT_SYMBOL(queued_write_lock_slowpath);