urcu.h

   1 #ifndef _URCU_H
   2 #define _URCU_H
   3
   4 /*
   5  * urcu.h
   6  *
   7  * Userspace RCU header
   8  *
   9  * Copyright February 2009 - Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
  10  *
  11  * Credits for Paul e. McKenney <paulmck@linux.vnet.ibm.com>
  12  * for inspiration coming from the Linux kernel RCU and rcu-preempt.
  13  *
  14  * The barrier, mb, rmb, wmb, atomic_inc, smp_read_barrier_depends, ACCESS_ONCE
  15  * and rcu_dereference primitives come from the Linux kernel.
  16  *
  17  * Distributed under GPLv2
  18  */
  19
  20 /* The "volatile" is due to gcc bugs */
  21 #define barrier() __asm__ __volatile__("": : :"memory")
  22
  23 /* x86 32/64 specific */
  24 #define mb()    asm volatile("mfence":::"memory")
  25 #define rmb()   asm volatile("lfence":::"memory")
  26 #define wmb()   asm volatile("sfence" ::: "memory")
  27
  28 static inline void atomic_inc(int *v)
  29 {
  30         asm volatile("lock; incl %0"
  31                      : "+m" (*v));
  32 }
  33
  34 /* Nop everywhere except on alpha. */
  35 #define smp_read_barrier_depends()
  36
  37 /*
  38  * Prevent the compiler from merging or refetching accesses.  The compiler
  39  * is also forbidden from reordering successive instances of ACCESS_ONCE(),
  40  * but only when the compiler is aware of some particular ordering.  One way
  41  * to make the compiler aware of ordering is to put the two invocations of
  42  * ACCESS_ONCE() in different C statements.
  43  *
  44  * This macro does absolutely -nothing- to prevent the CPU from reordering,
  45  * merging, or refetching absolutely anything at any time.  Its main intended
  46  * use is to mediate communication between process-level code and irq/NMI
  47  * handlers, all running on the same CPU.
  48  */
  49 #define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
  50
  51 /**
  52  * rcu_dereference - fetch an RCU-protected pointer in an
  53  * RCU read-side critical section.  This pointer may later
  54  * be safely dereferenced.
  55  *
  56  * Inserts memory barriers on architectures that require them
  57  * (currently only the Alpha), and, more importantly, documents
  58  * exactly which pointers are protected by RCU.
  59  */
  60
  61 #define rcu_dereference(p)     ({ \
  62                                 typeof(p) _________p1 = ACCESS_ONCE(p); \
  63                                 smp_read_barrier_depends(); \
  64                                 (_________p1); \
  65                                 })
  66
  67 #define SIGURCU SIGUSR1
  68
  69 #ifdef DEBUG_YIELD
  70 #include <sched.h>
  71
  72 #define YIELD_READ      (1 << 0)
  73 #define YIELD_WRITE     (1 << 1)
  74
  75 extern int yield_active;
  76
  77 static inline void debug_yield_read(void)
  78 {
  79         if (yield_active & YIELD_READ)
  80                 sched_yield();
  81 }
  82
  83 static inline void debug_yield_write(void)
  84 {
  85         if (yield_active & YIELD_WRITE)
  86                 sched_yield();
  87 }
  88 #else
  89 static inline void debug_yield_read(void)
  90 {
  91 }
  92
  93 static inline void debug_yield_write(void)
  94 {
  95 }
  96 #endif
  97
  98 /* Global quiescent period parity */
  99 extern int urcu_qparity;
 100
 101 extern int __thread urcu_active_readers[2];
 102
 103 static inline int get_urcu_qparity(void)
 104 {
 105         return urcu_qparity;
 106 }
 107
 108 /*
 109  * urcu_parity should be declared on the caller's stack.
 110  */
 111 static inline void rcu_read_lock(int *urcu_parity)
 112 {
 113         debug_yield_read();
 114         *urcu_parity = get_urcu_qparity();
 115         debug_yield_read();
 116         urcu_active_readers[*urcu_parity]++;
 117         debug_yield_read();
 118         /*
 119          * Increment active readers count before accessing the pointer.
 120          * See force_mb_all_threads().
 121          */
 122         barrier();
 123         debug_yield_read();
 124 }
 125
 126 static inline void rcu_read_unlock(int *urcu_parity)
 127 {
 128         debug_yield_read();
 129         barrier();
 130         debug_yield_read();
 131         /*
 132          * Finish using rcu before decrementing the pointer.
 133          * See force_mb_all_threads().
 134          */
 135         urcu_active_readers[*urcu_parity]--;
 136         debug_yield_read();
 137 }
 138
 139 extern void *urcu_publish_content(void **ptr, void *new);
 140
 141 /*
 142  * Reader thread registration.
 143  */
 144 extern void urcu_register_thread(void);
 145 extern void urcu_unregister_thread(void);
 146
 147 #endif /* _URCU_H */