-/* Copyright (C) 1995,1996, 1997, 2000, 2001, 2004 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
-/* ENHANCE-ME: If the cpu has an atomic test-and-set instruction it could be
- used instead of a mutex in try-arbiter and release-arbiter.
+/* 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
- For the i386 family, cmpxchg would suit but it's only available on 80486
- and higher so that would have to be checked, perhaps at run-time when
- setting up the definitions of the scheme procedures, or at compile time
- if we interpret a host cpu type like "i686" to mean not less than that
- chip. */
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)
#undef FUNC_NAME
-/* The mutex here is so two threads can't both see the arbiter unlocked and
- both proceed to lock and return #t. The arbiter itself wouldn't be
- corrupted by this, but two threads both getting #t would be entirely
- contrary to the intended semantics. */
+/* 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),
"@code{#f}.")
#define FUNC_NAME s_scm_try_arbiter
{
+ scm_t_bits old;
SCM_VALIDATE_SMOB (1, arb, arbiter);
-
- scm_mutex_lock (&scm_i_misc_mutex);
- if (SCM_ARB_LOCKED(arb))
- arb = SCM_BOOL_F;
- else
- {
- SCM_LOCK_ARB(arb);
- arb = SCM_BOOL_T;
- }
- scm_mutex_unlock (&scm_i_misc_mutex);
- 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
-/* The mutex here is so two threads can't both see the arbiter locked and
- both proceed to unlock 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. */
+/* 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),
"release it.")
#define FUNC_NAME s_scm_release_arbiter
{
- SCM ret;
+ scm_t_bits old;
SCM_VALIDATE_SMOB (1, arb, arbiter);
-
- scm_mutex_lock (&scm_i_misc_mutex);
- if (!SCM_ARB_LOCKED(arb))
- ret = SCM_BOOL_F;
- else
- {
- SCM_UNLOCK_ARB (arb);
- ret = SCM_BOOL_T;
- }
- scm_mutex_unlock (&scm_i_misc_mutex);
- return ret;
+ 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