libguile/arbiters.c

   1 /*      Copyright (C) 1995,1996, 1997, 2000, 2001, 2004, 2005 Free Software Foundation, Inc.
   2  *
   3  * This library is free software; you can redistribute it and/or
   4  * modify it under the terms of the GNU Lesser General Public
   5  * License as published by the Free Software Foundation; either
   6  * version 2.1 of the License, or (at your option) any later version.
   7  *
   8  * This library is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  11  * Lesser General Public License for more details.
  12  *
  13  * You should have received a copy of the GNU Lesser General Public
  14  * License along with this library; if not, write to the Free Software
  15  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  16  */
  17
  18
  19 \f
  20
  21 #include "libguile/_scm.h"
  22 #include "libguile/ports.h"
  23 #include "libguile/smob.h"
  24
  25 #include "libguile/validate.h"
  26 #include "libguile/arbiters.h"
  27
  28 \f
  29 /* FETCH_STORE sets "fet" to the value fetched from "mem" and then stores
  30    "sto" there.  The fetch and store are done atomically, so once the fetch
  31    has been done no other thread or processor can fetch from there before
  32    the store is done.
  33
  34    The operands are scm_t_bits, fet and sto are plain variables, mem is a
  35    memory location (ie. an lvalue).
  36
  37    ENHANCE-ME: Add more cpu-specifics.  glibc atomicity.h has some of the
  38    sort of thing required.  FETCH_STORE could become some sort of
  39    compare-and-store if that better suited what various cpus do.  */
  40
  41 #if defined (__GNUC__) && defined (i386) && SIZEOF_SCM_T_BITS == 4
  42 /* This is for i386 with the normal 32-bit scm_t_bits.  The xchg instruction
  43    is atomic on a single processor, and it automatically asserts the "lock"
  44    bus signal so it's atomic on a multi-processor (no need for the lock
  45    prefix on the instruction).
  46
  47    The mem operand is read-write but "+" is not used since old gcc
  48    (eg. 2.7.2) doesn't support that.  "1" for the mem input doesn't work
  49    (eg. gcc 3.3) when mem is a pointer dereference like current usage below.
  50    Having mem as a plain input should be ok though.  It tells gcc the value
  51    is live, but as an "m" gcc won't fetch it itself (though that would be
  52    harmless).  */
  53
  54 #define FETCH_STORE(fet,mem,sto)                \
  55   do {                                          \
  56     asm ("xchg %0, %1"                          \
  57          : "=r" (fet), "=m" (mem)               \
  58          : "0"  (sto), "m"  (mem));             \
  59   } while (0)
  60 #endif
  61
  62 #ifndef FETCH_STORE
  63 /* This is a generic version, with a mutex to ensure the operation is
  64    atomic.  Unfortunately this approach probably makes arbiters no faster
  65    than mutexes (though still using less memory of course), so some
  66    CPU-specifics are highly desirable.  */
  67 #define FETCH_STORE(fet,mem,sto)                        \
  68   do {                                                  \
  69     scm_i_scm_pthread_mutex_lock (&scm_i_misc_mutex);   \
  70     (fet) = (mem);                                      \
  71     (mem) = (sto);                                      \
  72     scm_i_pthread_mutex_unlock (&scm_i_misc_mutex);     \
  73   } while (0)
  74 #endif
  75
  76
  77 static scm_t_bits scm_tc16_arbiter;
  78
  79
  80 #define SCM_LOCK_VAL         (scm_tc16_arbiter | (1L << 16))
  81 #define SCM_UNLOCK_VAL       scm_tc16_arbiter
  82 #define SCM_ARB_LOCKED(arb)  ((SCM_CELL_WORD_0 (arb)) & (1L << 16))
  83
  84
  85 static int
  86 arbiter_print (SCM exp, SCM port, scm_print_state *pstate)
  87 {
  88   scm_puts ("#<arbiter ", port);
  89   if (SCM_ARB_LOCKED (exp))
  90     scm_puts ("locked ", port);
  91   scm_iprin1 (SCM_PACK (SCM_SMOB_DATA (exp)), port, pstate);
  92   scm_putc ('>', port);
  93   return !0;
  94 }
  95
  96 SCM_DEFINE (scm_make_arbiter, "make-arbiter", 1, 0, 0,
  97             (SCM name),
  98             "Return an arbiter object, initially unlocked.  Currently\n"
  99             "@var{name} is only used for diagnostic output.")
 100 #define FUNC_NAME s_scm_make_arbiter
 101 {
 102   SCM_RETURN_NEWSMOB (scm_tc16_arbiter, SCM_UNPACK (name));
 103 }
 104 #undef FUNC_NAME
 105
 106
 107 /* The atomic FETCH_STORE here is so two threads can't both see the arbiter
 108    unlocked and return #t.  The arbiter itself wouldn't be corrupted by
 109    this, but two threads both getting #t would be contrary to the intended
 110    semantics.  */
 111
 112 SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0,
 113             (SCM arb),
 114             "If @var{arb} is unlocked, then lock it and return @code{#t}.\n"
 115             "If @var{arb} is already locked, then do nothing and return\n"
 116             "@code{#f}.")
 117 #define FUNC_NAME s_scm_try_arbiter
 118 {
 119   scm_t_bits old;
 120   SCM_VALIDATE_SMOB (1, arb, arbiter);
 121   FETCH_STORE (old, * (scm_t_bits *) SCM_CELL_OBJECT_LOC(arb,0), SCM_LOCK_VAL);
 122   return scm_from_bool (old == SCM_UNLOCK_VAL);
 123 }
 124 #undef FUNC_NAME
 125
 126
 127 /* The atomic FETCH_STORE here is so two threads can't both see the arbiter
 128    locked and return #t.  The arbiter itself wouldn't be corrupted by this,
 129    but we don't want two threads both thinking they were the unlocker.  The
 130    intended usage is for the code which locked to be responsible for
 131    unlocking, but we guarantee the return value even if multiple threads
 132    compete.  */
 133
 134 SCM_DEFINE (scm_release_arbiter, "release-arbiter", 1, 0, 0,
 135             (SCM arb),
 136             "If @var{arb} is locked, then unlock it and return @code{#t}.\n"
 137             "If @var{arb} is already unlocked, then do nothing and return\n"
 138             "@code{#f}.\n"
 139             "\n"
 140             "Typical usage is for the thread which locked an arbiter to\n"
 141             "later release it, but that's not required, any thread can\n"
 142             "release it.")
 143 #define FUNC_NAME s_scm_release_arbiter
 144 {
 145   scm_t_bits old;
 146   SCM_VALIDATE_SMOB (1, arb, arbiter);
 147   FETCH_STORE (old, *(scm_t_bits*)SCM_CELL_OBJECT_LOC(arb,0), SCM_UNLOCK_VAL);
 148   return scm_from_bool (old == SCM_LOCK_VAL);
 149 }
 150 #undef FUNC_NAME
 151
 152
 153
 154 void
 155 scm_init_arbiters ()
 156 {
 157   scm_tc16_arbiter = scm_make_smob_type ("arbiter", 0);
 158   scm_set_smob_mark (scm_tc16_arbiter, scm_markcdr);
 159   scm_set_smob_print (scm_tc16_arbiter, arbiter_print);
 160 #include "libguile/arbiters.x"
 161 }
 162
 163 /*
 164   Local Variables:
 165   c-file-style: "gnu"
 166   End:
 167 */