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