| 1 | /* Copyright (C) 1995,1996, 1997, 2000, 2001, 2004, 2005, 2006, 2008, 2011 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 License |
| 5 | * as published by the Free Software Foundation; either version 3 of |
| 6 | * the License, or (at your option) any later version. |
| 7 | * |
| 8 | * This library is distributed in the hope that it will be useful, but |
| 9 | * 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 |
| 16 | * 02110-1301 USA |
| 17 | */ |
| 18 | |
| 19 | |
| 20 | \f |
| 21 | #ifdef HAVE_CONFIG_H |
| 22 | # include <config.h> |
| 23 | #endif |
| 24 | |
| 25 | #include "libguile/_scm.h" |
| 26 | #include "libguile/ports.h" |
| 27 | #include "libguile/smob.h" |
| 28 | |
| 29 | #include "libguile/validate.h" |
| 30 | #include "libguile/arbiters.h" |
| 31 | |
| 32 | \f |
| 33 | /* FETCH_STORE sets "fet" to the value fetched from "mem" and then stores |
| 34 | "sto" there. The fetch and store are done atomically, so once the fetch |
| 35 | has been done no other thread or processor can fetch from there before |
| 36 | the store is done. |
| 37 | |
| 38 | The operands are scm_t_bits, fet and sto are plain variables, mem is a |
| 39 | memory location (ie. an lvalue). |
| 40 | |
| 41 | ENHANCE-ME: Add more cpu-specifics. glibc atomicity.h has some of the |
| 42 | sort of thing required. FETCH_STORE could become some sort of |
| 43 | compare-and-store if that better suited what various cpus do. */ |
| 44 | |
| 45 | #if defined (__GNUC__) && defined (i386) && SIZEOF_SCM_T_BITS == 4 |
| 46 | /* This is for i386 with the normal 32-bit scm_t_bits. The xchg instruction |
| 47 | is atomic on a single processor, and it automatically asserts the "lock" |
| 48 | bus signal so it's atomic on a multi-processor (no need for the lock |
| 49 | prefix on the instruction). |
| 50 | |
| 51 | The mem operand is read-write but "+" is not used since old gcc |
| 52 | (eg. 2.7.2) doesn't support that. "1" for the mem input doesn't work |
| 53 | (eg. gcc 3.3) when mem is a pointer dereference like current usage below. |
| 54 | Having mem as a plain input should be ok though. It tells gcc the value |
| 55 | is live, but as an "m" gcc won't fetch it itself (though that would be |
| 56 | harmless). */ |
| 57 | |
| 58 | #define FETCH_STORE(fet,mem,sto) \ |
| 59 | do { \ |
| 60 | asm ("xchg %0, %1" \ |
| 61 | : "=r" (fet), "=m" (mem) \ |
| 62 | : "0" (sto), "m" (mem)); \ |
| 63 | } while (0) |
| 64 | #endif |
| 65 | |
| 66 | #ifndef FETCH_STORE |
| 67 | /* This is a generic version, with a mutex to ensure the operation is |
| 68 | atomic. Unfortunately this approach probably makes arbiters no faster |
| 69 | than mutexes (though still using less memory of course), so some |
| 70 | CPU-specifics are highly desirable. */ |
| 71 | #define FETCH_STORE(fet,mem,sto) \ |
| 72 | do { \ |
| 73 | scm_i_scm_pthread_mutex_lock (&scm_i_misc_mutex); \ |
| 74 | (fet) = (mem); \ |
| 75 | (mem) = (sto); \ |
| 76 | scm_i_pthread_mutex_unlock (&scm_i_misc_mutex); \ |
| 77 | } while (0) |
| 78 | #endif |
| 79 | |
| 80 | |
| 81 | static scm_t_bits scm_tc16_arbiter; |
| 82 | |
| 83 | |
| 84 | #define SCM_LOCK_VAL (scm_tc16_arbiter | (1L << 16)) |
| 85 | #define SCM_UNLOCK_VAL scm_tc16_arbiter |
| 86 | #define SCM_ARB_LOCKED(arb) ((SCM_CELL_WORD_0 (arb)) & (1L << 16)) |
| 87 | |
| 88 | |
| 89 | static int |
| 90 | arbiter_print (SCM exp, SCM port, scm_print_state *pstate) |
| 91 | { |
| 92 | scm_puts_unlocked ("#<arbiter ", port); |
| 93 | if (SCM_ARB_LOCKED (exp)) |
| 94 | scm_puts_unlocked ("locked ", port); |
| 95 | scm_iprin1 (SCM_PACK (SCM_SMOB_DATA (exp)), port, pstate); |
| 96 | scm_putc_unlocked ('>', port); |
| 97 | return !0; |
| 98 | } |
| 99 | |
| 100 | SCM_DEFINE (scm_make_arbiter, "make-arbiter", 1, 0, 0, |
| 101 | (SCM name), |
| 102 | "Return an arbiter object, initially unlocked. Currently\n" |
| 103 | "@var{name} is only used for diagnostic output.") |
| 104 | #define FUNC_NAME s_scm_make_arbiter |
| 105 | { |
| 106 | SCM_RETURN_NEWSMOB (scm_tc16_arbiter, SCM_UNPACK (name)); |
| 107 | } |
| 108 | #undef FUNC_NAME |
| 109 | |
| 110 | |
| 111 | /* The atomic FETCH_STORE here is so two threads can't both see the arbiter |
| 112 | unlocked and return #t. The arbiter itself wouldn't be corrupted by |
| 113 | this, but two threads both getting #t would be contrary to the intended |
| 114 | semantics. */ |
| 115 | |
| 116 | SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0, |
| 117 | (SCM arb), |
| 118 | "If @var{arb} is unlocked, then lock it and return @code{#t}.\n" |
| 119 | "If @var{arb} is already locked, then do nothing and return\n" |
| 120 | "@code{#f}.") |
| 121 | #define FUNC_NAME s_scm_try_arbiter |
| 122 | { |
| 123 | scm_t_bits old; |
| 124 | scm_t_bits *loc; |
| 125 | SCM_VALIDATE_SMOB (1, arb, arbiter); |
| 126 | loc = (scm_t_bits*)SCM_SMOB_OBJECT_N_LOC (arb, 0); |
| 127 | FETCH_STORE (old, *loc, SCM_LOCK_VAL); |
| 128 | return scm_from_bool (old == SCM_UNLOCK_VAL); |
| 129 | } |
| 130 | #undef FUNC_NAME |
| 131 | |
| 132 | |
| 133 | /* The atomic FETCH_STORE here is so two threads can't both see the arbiter |
| 134 | locked and return #t. The arbiter itself wouldn't be corrupted by this, |
| 135 | but we don't want two threads both thinking they were the unlocker. The |
| 136 | intended usage is for the code which locked to be responsible for |
| 137 | unlocking, but we guarantee the return value even if multiple threads |
| 138 | compete. */ |
| 139 | |
| 140 | SCM_DEFINE (scm_release_arbiter, "release-arbiter", 1, 0, 0, |
| 141 | (SCM arb), |
| 142 | "If @var{arb} is locked, then unlock it and return @code{#t}.\n" |
| 143 | "If @var{arb} is already unlocked, then do nothing and return\n" |
| 144 | "@code{#f}.\n" |
| 145 | "\n" |
| 146 | "Typical usage is for the thread which locked an arbiter to\n" |
| 147 | "later release it, but that's not required, any thread can\n" |
| 148 | "release it.") |
| 149 | #define FUNC_NAME s_scm_release_arbiter |
| 150 | { |
| 151 | scm_t_bits old; |
| 152 | scm_t_bits *loc; |
| 153 | SCM_VALIDATE_SMOB (1, arb, arbiter); |
| 154 | loc = (scm_t_bits*)SCM_SMOB_OBJECT_N_LOC (arb, 0); |
| 155 | FETCH_STORE (old, *loc, SCM_UNLOCK_VAL); |
| 156 | return scm_from_bool (old == SCM_LOCK_VAL); |
| 157 | } |
| 158 | #undef FUNC_NAME |
| 159 | |
| 160 | |
| 161 | |
| 162 | void |
| 163 | scm_init_arbiters () |
| 164 | { |
| 165 | scm_tc16_arbiter = scm_make_smob_type ("arbiter", 0); |
| 166 | scm_set_smob_print (scm_tc16_arbiter, arbiter_print); |
| 167 | #include "libguile/arbiters.x" |
| 168 | } |
| 169 | |
| 170 | /* |
| 171 | Local Variables: |
| 172 | c-file-style: "gnu" |
| 173 | End: |
| 174 | */ |