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 /* Global quiescent period parity */
  70 extern int urcu_qparity;
  71
  72 extern int __thread urcu_active_readers[2];
  73
  74 static inline int get_urcu_qparity(void)
  75 {
  76         return urcu_qparity;
  77 }
  78
  79 /*
  80  * urcu_parity should be declared on the caller's stack.
  81  */
  82 static inline void rcu_read_lock(int *urcu_parity)
  83 {
  84         *urcu_parity = get_urcu_qparity();
  85         urcu_active_readers[*urcu_parity]++;
  86         /*
  87          * Increment active readers count before accessing the pointer.
  88          * See force_mb_all_threads().
  89          */
  90         barrier();
  91 }
  92
  93 static inline void rcu_read_unlock(int *urcu_parity)
  94 {
  95         barrier();
  96         /*
  97          * Finish using rcu before decrementing the pointer.
  98          * See force_mb_all_threads().
  99          */
 100         urcu_active_readers[*urcu_parity]--;
 101 }
 102
 103 extern void *urcu_publish_content(void **ptr, void *new);
 104
 105 /*
 106  * Reader thread registration.
 107  */
 108 extern void urcu_register_thread(void);
 109 extern void urcu_unregister_thread(void);
 110
 111 #endif /* _URCU_H */