include/asm-generic/qrwlock.h

   1 /* SPDX-License-Identifier: GPL-2.0-or-later */
   2 /*
   3  * Queue read/write lock
   4  *
   5  * These use generic atomic and locking routines, but depend on a fair spinlock
   6  * implementation in order to be fair themselves.  The implementation in
   7  * asm-generic/spinlock.h meets these requirements.
   8  *
   9  * (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
  10  *
  11  * Authors: Waiman Long <waiman.long@hp.com>
  12  */
  13 #ifndef __ASM_GENERIC_QRWLOCK_H
  14 #define __ASM_GENERIC_QRWLOCK_H
  15
  16 #include <linux/atomic.h>
  17 #include <asm/barrier.h>
  18 #include <asm/processor.h>
  19
  20 #include <asm-generic/qrwlock_types.h>
  21
  22 /* Must be included from asm/spinlock.h after defining arch_spin_is_locked.  */
  23
  24 /*
  25  * Writer states & reader shift and bias.
  26  */
  27 #define _QW_WAITING     0x100           /* A writer is waiting     */
  28 #define _QW_LOCKED      0x0ff           /* A writer holds the lock */
  29 #define _QW_WMASK       0x1ff           /* Writer mask             */
  30 #define _QR_SHIFT       9               /* Reader count shift      */
  31 #define _QR_BIAS        (1U << _QR_SHIFT)
  32
  33 /*
  34  * External function declarations
  35  */
  36 extern void queued_read_lock_slowpath(struct qrwlock *lock);
  37 extern void queued_write_lock_slowpath(struct qrwlock *lock);
  38
  39 /**
  40  * queued_read_trylock - try to acquire read lock of a queued rwlock
  41  * @lock : Pointer to queued rwlock structure
  42  * Return: 1 if lock acquired, 0 if failed
  43  */
  44 static inline int queued_read_trylock(struct qrwlock *lock)
  45 {
  46         int cnts;
  47
  48         cnts = atomic_read(&lock->cnts);
  49         if (likely(!(cnts & _QW_WMASK))) {
  50                 cnts = (u32)atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
  51                 if (likely(!(cnts & _QW_WMASK)))
  52                         return 1;
  53                 atomic_sub(_QR_BIAS, &lock->cnts);
  54         }
  55         return 0;
  56 }
  57
  58 /**
  59  * queued_write_trylock - try to acquire write lock of a queued rwlock
  60  * @lock : Pointer to queued rwlock structure
  61  * Return: 1 if lock acquired, 0 if failed
  62  */
  63 static inline int queued_write_trylock(struct qrwlock *lock)
  64 {
  65         int cnts;
  66
  67         cnts = atomic_read(&lock->cnts);
  68         if (unlikely(cnts))
  69                 return 0;
  70
  71         return likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts,
  72                                 _QW_LOCKED));
  73 }
  74 /**
  75  * queued_read_lock - acquire read lock of a queued rwlock
  76  * @lock: Pointer to queued rwlock structure
  77  */
  78 static inline void queued_read_lock(struct qrwlock *lock)
  79 {
  80         int cnts;
  81
  82         cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
  83         if (likely(!(cnts & _QW_WMASK)))
  84                 return;
  85
  86         /* The slowpath will decrement the reader count, if necessary. */
  87         queued_read_lock_slowpath(lock);
  88 }
  89
  90 /**
  91  * queued_write_lock - acquire write lock of a queued rwlock
  92  * @lock : Pointer to queued rwlock structure
  93  */
  94 static inline void queued_write_lock(struct qrwlock *lock)
  95 {
  96         int cnts = 0;
  97         /* Optimize for the unfair lock case where the fair flag is 0. */
  98         if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
  99                 return;
 100
 101         queued_write_lock_slowpath(lock);
 102 }
 103
 104 /**
 105  * queued_read_unlock - release read lock of a queued rwlock
 106  * @lock : Pointer to queued rwlock structure
 107  */
 108 static inline void queued_read_unlock(struct qrwlock *lock)
 109 {
 110         /*
 111          * Atomically decrement the reader count
 112          */
 113         (void)atomic_sub_return_release(_QR_BIAS, &lock->cnts);
 114 }
 115
 116 /**
 117  * queued_write_unlock - release write lock of a queued rwlock
 118  * @lock : Pointer to queued rwlock structure
 119  */
 120 static inline void queued_write_unlock(struct qrwlock *lock)
 121 {
 122         smp_store_release(&lock->wlocked, 0);
 123 }
 124
 125 /**
 126  * queued_rwlock_is_contended - check if the lock is contended
 127  * @lock : Pointer to queued rwlock structure
 128  * Return: 1 if lock contended, 0 otherwise
 129  */
 130 static inline int queued_rwlock_is_contended(struct qrwlock *lock)
 131 {
 132         return arch_spin_is_locked(&lock->wait_lock);
 133 }
 134
 135 /*
 136  * Remapping rwlock architecture specific functions to the corresponding
 137  * queued rwlock functions.
 138  */
 139 #define arch_read_lock(l)               queued_read_lock(l)
 140 #define arch_write_lock(l)              queued_write_lock(l)
 141 #define arch_read_trylock(l)            queued_read_trylock(l)
 142 #define arch_write_trylock(l)           queued_write_trylock(l)
 143 #define arch_read_unlock(l)             queued_read_unlock(l)
 144 #define arch_write_unlock(l)            queued_write_unlock(l)
 145 #define arch_rwlock_is_contended(l)     queued_rwlock_is_contended(l)
 146
 147 #endif /* __ASM_GENERIC_QRWLOCK_H */