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 <asm/qrwlock.h>
  24
  25 /*
  26  * This internal data structure is used for optimizing access to some of
  27  * the subfields within the atomic_t cnts.
  28  */
  29 struct __qrwlock {
  30         union {
  31                 atomic_t cnts;
  32                 struct {
  33 #ifdef __LITTLE_ENDIAN
  34                         u8 wmode;       /* Writer mode   */
  35                         u8 rcnts[3];    /* Reader counts */
  36 #else
  37                         u8 rcnts[3];    /* Reader counts */
  38                         u8 wmode;       /* Writer mode   */
  39 #endif
  40                 };
  41         };
  42         arch_spinlock_t lock;
  43 };
  44
  45 /**
  46  * rspin_until_writer_unlock - inc reader count & spin until writer is gone
  47  * @lock  : Pointer to queue rwlock structure
  48  * @writer: Current queue rwlock writer status byte
  49  *
  50  * In interrupt context or at the head of the queue, the reader will just
  51  * increment the reader count & wait until the writer releases the lock.
  52  */
  53 static __always_inline void
  54 rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
  55 {
  56         while ((cnts & _QW_WMASK) == _QW_LOCKED) {
  57                 cpu_relax_lowlatency();
  58                 cnts = atomic_read_acquire(&lock->cnts);
  59         }
  60 }
  61
  62 /**
  63  * queued_read_lock_slowpath - acquire read lock of a queue rwlock
  64  * @lock: Pointer to queue rwlock structure
  65  * @cnts: Current qrwlock lock value
  66  */
  67 void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
  68 {
  69         /*
  70          * Readers come here when they cannot get the lock without waiting
  71          */
  72         if (unlikely(in_interrupt())) {
  73                 /*
  74                  * Readers in interrupt context will get the lock immediately
  75                  * if the writer is just waiting (not holding the lock yet).
  76                  * The rspin_until_writer_unlock() function returns immediately
  77                  * in this case. Otherwise, they will spin (with ACQUIRE
  78                  * semantics) until the lock is available without waiting in
  79                  * the queue.
  80                  */
  81                 rspin_until_writer_unlock(lock, cnts);
  82                 return;
  83         }
  84         atomic_sub(_QR_BIAS, &lock->cnts);
  85
  86         /*
  87          * Put the reader into the wait queue
  88          */
  89         arch_spin_lock(&lock->lock);
  90
  91         /*
  92          * The ACQUIRE semantics of the following spinning code ensure
  93          * that accesses can't leak upwards out of our subsequent critical
  94          * section in the case that the lock is currently held for write.
  95          */
  96         cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS;
  97         rspin_until_writer_unlock(lock, cnts);
  98
  99         /*
 100          * Signal the next one in queue to become queue head
 101          */
 102         arch_spin_unlock(&lock->lock);
 103 }
 104 EXPORT_SYMBOL(queued_read_lock_slowpath);
 105
 106 /**
 107  * queued_write_lock_slowpath - acquire write lock of a queue rwlock
 108  * @lock : Pointer to queue rwlock structure
 109  */
 110 void queued_write_lock_slowpath(struct qrwlock *lock)
 111 {
 112         u32 cnts;
 113
 114         /* Put the writer into the wait queue */
 115         arch_spin_lock(&lock->lock);
 116
 117         /* Try to acquire the lock directly if no reader is present */
 118         if (!atomic_read(&lock->cnts) &&
 119             (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
 120                 goto unlock;
 121
 122         /*
 123          * Set the waiting flag to notify readers that a writer is pending,
 124          * or wait for a previous writer to go away.
 125          */
 126         for (;;) {
 127                 struct __qrwlock *l = (struct __qrwlock *)lock;
 128
 129                 if (!READ_ONCE(l->wmode) &&
 130                    (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
 131                         break;
 132
 133                 cpu_relax_lowlatency();
 134         }
 135
 136         /* When no more readers, set the locked flag */
 137         for (;;) {
 138                 cnts = atomic_read(&lock->cnts);
 139                 if ((cnts == _QW_WAITING) &&
 140                     (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
 141                                             _QW_LOCKED) == _QW_WAITING))
 142                         break;
 143
 144                 cpu_relax_lowlatency();
 145         }
 146 unlock:
 147         arch_spin_unlock(&lock->lock);
 148 }
 149 EXPORT_SYMBOL(queued_write_lock_slowpath);