-/* Copyright (C) 1995,1996, 1997, 2000, 2001 Free Software Foundation, Inc.
+/* Copyright (C) 1995,1996, 1997, 2000, 2001, 2004, 2005, 2006, 2008 Free Software Foundation, Inc.
*
* This library is free software; you can redistribute it and/or
- * modify it under the terms of the GNU Lesser General Public
- * License as published by the Free Software Foundation; either
- * version 2.1 of the License, or (at your option) any later version.
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 3 of
+ * the License, or (at your option) any later version.
*
- * This library is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
- * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301 USA
*/
\f
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
#include "libguile/_scm.h"
#include "libguile/ports.h"
#include "libguile/arbiters.h"
\f
-/* {Arbiters}
- *
- * These procedures implement synchronization primitives. Processors
- * with an atomic test-and-set instruction can use it here (and not
- * SCM_DEFER_INTS).
- */
+/* FETCH_STORE sets "fet" to the value fetched from "mem" and then stores
+ "sto" there. The fetch and store are done atomically, so once the fetch
+ has been done no other thread or processor can fetch from there before
+ the store is done.
+
+ The operands are scm_t_bits, fet and sto are plain variables, mem is a
+ memory location (ie. an lvalue).
+
+ ENHANCE-ME: Add more cpu-specifics. glibc atomicity.h has some of the
+ sort of thing required. FETCH_STORE could become some sort of
+ compare-and-store if that better suited what various cpus do. */
+
+#if defined (__GNUC__) && defined (i386) && SIZEOF_SCM_T_BITS == 4
+/* This is for i386 with the normal 32-bit scm_t_bits. The xchg instruction
+ is atomic on a single processor, and it automatically asserts the "lock"
+ bus signal so it's atomic on a multi-processor (no need for the lock
+ prefix on the instruction).
+
+ The mem operand is read-write but "+" is not used since old gcc
+ (eg. 2.7.2) doesn't support that. "1" for the mem input doesn't work
+ (eg. gcc 3.3) when mem is a pointer dereference like current usage below.
+ Having mem as a plain input should be ok though. It tells gcc the value
+ is live, but as an "m" gcc won't fetch it itself (though that would be
+ harmless). */
+
+#define FETCH_STORE(fet,mem,sto) \
+ do { \
+ asm ("xchg %0, %1" \
+ : "=r" (fet), "=m" (mem) \
+ : "0" (sto), "m" (mem)); \
+ } while (0)
+#endif
+
+#ifndef FETCH_STORE
+/* This is a generic version, with a mutex to ensure the operation is
+ atomic. Unfortunately this approach probably makes arbiters no faster
+ than mutexes (though still using less memory of course), so some
+ CPU-specifics are highly desirable. */
+#define FETCH_STORE(fet,mem,sto) \
+ do { \
+ scm_i_scm_pthread_mutex_lock (&scm_i_misc_mutex); \
+ (fet) = (mem); \
+ (mem) = (sto); \
+ scm_i_pthread_mutex_unlock (&scm_i_misc_mutex); \
+ } while (0)
+#endif
+
static scm_t_bits scm_tc16_arbiter;
+#define SCM_LOCK_VAL (scm_tc16_arbiter | (1L << 16))
+#define SCM_UNLOCK_VAL scm_tc16_arbiter
#define SCM_ARB_LOCKED(arb) ((SCM_CELL_WORD_0 (arb)) & (1L << 16))
-#define SCM_LOCK_ARB(arb) (SCM_SET_CELL_WORD_0 ((arb), scm_tc16_arbiter | (1L << 16)));
-#define SCM_UNLOCK_ARB(arb) (SCM_SET_CELL_WORD_0 ((arb), scm_tc16_arbiter));
+
static int
arbiter_print (SCM exp, SCM port, scm_print_state *pstate)
SCM_DEFINE (scm_make_arbiter, "make-arbiter", 1, 0, 0,
(SCM name),
- "Return an object of type arbiter and name @var{name}. Its\n"
- "state is initially unlocked. Arbiters are a way to achieve\n"
- "process synchronization.")
+ "Return an arbiter object, initially unlocked. Currently\n"
+ "@var{name} is only used for diagnostic output.")
#define FUNC_NAME s_scm_make_arbiter
{
SCM_RETURN_NEWSMOB (scm_tc16_arbiter, SCM_UNPACK (name));
}
#undef FUNC_NAME
+
+/* The atomic FETCH_STORE here is so two threads can't both see the arbiter
+ unlocked and return #t. The arbiter itself wouldn't be corrupted by
+ this, but two threads both getting #t would be contrary to the intended
+ semantics. */
+
SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0,
(SCM arb),
- "Return @code{#t} and lock the arbiter @var{arb} if the arbiter\n"
- "was unlocked. Otherwise, return @code{#f}.")
+ "If @var{arb} is unlocked, then lock it and return @code{#t}.\n"
+ "If @var{arb} is already locked, then do nothing and return\n"
+ "@code{#f}.")
#define FUNC_NAME s_scm_try_arbiter
{
+ scm_t_bits old;
SCM_VALIDATE_SMOB (1, arb, arbiter);
- SCM_DEFER_INTS;
- if (SCM_ARB_LOCKED(arb))
- arb = SCM_BOOL_F;
- else
- {
- SCM_LOCK_ARB(arb);
- arb = SCM_BOOL_T;
- }
- SCM_ALLOW_INTS;
- return arb;
+ FETCH_STORE (old, * (scm_t_bits *) SCM_CELL_OBJECT_LOC(arb,0), SCM_LOCK_VAL);
+ return scm_from_bool (old == SCM_UNLOCK_VAL);
}
#undef FUNC_NAME
-SCM_DEFINE (scm_release_arbiter, "release-arbiter", 1, 0, 0,
+/* The atomic FETCH_STORE here is so two threads can't both see the arbiter
+ locked and return #t. The arbiter itself wouldn't be corrupted by this,
+ but we don't want two threads both thinking they were the unlocker. The
+ intended usage is for the code which locked to be responsible for
+ unlocking, but we guarantee the return value even if multiple threads
+ compete. */
+
+SCM_DEFINE (scm_release_arbiter, "release-arbiter", 1, 0, 0,
(SCM arb),
- "Return @code{#t} and unlock the arbiter @var{arb} if the\n"
- "arbiter was locked. Otherwise, return @code{#f}.")
+ "If @var{arb} is locked, then unlock it and return @code{#t}.\n"
+ "If @var{arb} is already unlocked, then do nothing and return\n"
+ "@code{#f}.\n"
+ "\n"
+ "Typical usage is for the thread which locked an arbiter to\n"
+ "later release it, but that's not required, any thread can\n"
+ "release it.")
#define FUNC_NAME s_scm_release_arbiter
{
+ scm_t_bits old;
SCM_VALIDATE_SMOB (1, arb, arbiter);
- if (!SCM_ARB_LOCKED(arb))
- return SCM_BOOL_F;
- SCM_UNLOCK_ARB (arb);
- return SCM_BOOL_T;
+ FETCH_STORE (old, *(scm_t_bits*)SCM_CELL_OBJECT_LOC(arb,0), SCM_UNLOCK_VAL);
+ return scm_from_bool (old == SCM_LOCK_VAL);
}
#undef FUNC_NAME
HCoop / bpt / guile.git / blobdiff