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(scm_try_arbiter): Use scm_i_misc_mutex instead of

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SCM_DEFER_INTS.
(scm_release_arbiter): Use scm_i_misc_mutex so return value can be
guaranteed if multiple threads compete to unlock.
This commit is contained in:
Kevin Ryde 2004-08-05 00:20:13 +00:00
parent fcc3adc2f9
commit 9bebea882a
1 changed files with 37 additions and 13 deletions
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48
libguile/arbiters.c
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@ -1,4 +1,4 @@
/* Copyright (C) 1995,1996, 1997, 2000, 2001 Free Software Foundation, Inc. /* Copyright (C) 1995,1996, 1997, 2000, 2001, 2004 Free Software Foundation, Inc.
* *
* This library is free software; you can redistribute it and/or * This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public * modify it under the terms of the GNU Lesser General Public
@ -26,12 +26,14 @@
#include "libguile/arbiters.h" #include "libguile/arbiters.h"
/* {Arbiters} /* 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.
* These procedures implement synchronization primitives. Processors
* with an atomic test-and-set instruction can use it here (and not For the i386 family, cmpxchg would suit but it's only available on 80486
* SCM_DEFER_INTS). 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; static scm_t_bits scm_tc16_arbiter;
@ -62,6 +64,12 @@ SCM_DEFINE (scm_make_arbiter, "make-arbiter", 1, 0, 0,
} }
#undef FUNC_NAME #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. */
SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0, SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0,
(SCM arb), (SCM arb),
"Return @code{#t} and lock the arbiter @var{arb} if the arbiter\n" "Return @code{#t} and lock the arbiter @var{arb} if the arbiter\n"
@ -69,7 +77,8 @@ SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0,
#define FUNC_NAME s_scm_try_arbiter #define FUNC_NAME s_scm_try_arbiter
{ {
SCM_VALIDATE_SMOB (1, arb, arbiter); SCM_VALIDATE_SMOB (1, arb, arbiter);
SCM_DEFER_INTS;
scm_mutex_lock (&scm_i_misc_mutex);
if (SCM_ARB_LOCKED(arb)) if (SCM_ARB_LOCKED(arb))
arb = SCM_BOOL_F; arb = SCM_BOOL_F;
else else
@ -77,23 +86,38 @@ SCM_DEFINE (scm_try_arbiter, "try-arbiter", 1, 0, 0,
SCM_LOCK_ARB(arb); SCM_LOCK_ARB(arb);
arb = SCM_BOOL_T; arb = SCM_BOOL_T;
} }
SCM_ALLOW_INTS; scm_mutex_unlock (&scm_i_misc_mutex);
return arb; return arb;
} }
#undef FUNC_NAME #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. */
SCM_DEFINE (scm_release_arbiter, "release-arbiter", 1, 0, 0, SCM_DEFINE (scm_release_arbiter, "release-arbiter", 1, 0, 0,
(SCM arb), (SCM arb),
"Return @code{#t} and unlock the arbiter @var{arb} if the\n" "Return @code{#t} and unlock the arbiter @var{arb} if the\n"
"arbiter was locked. Otherwise, return @code{#f}.") "arbiter was locked. Otherwise, return @code{#f}.")
#define FUNC_NAME s_scm_release_arbiter #define FUNC_NAME s_scm_release_arbiter
{ {
SCM ret;
SCM_VALIDATE_SMOB (1, arb, arbiter); SCM_VALIDATE_SMOB (1, arb, arbiter);
scm_mutex_lock (&scm_i_misc_mutex);
if (!SCM_ARB_LOCKED(arb)) if (!SCM_ARB_LOCKED(arb))
return SCM_BOOL_F; ret = SCM_BOOL_F;
SCM_UNLOCK_ARB (arb); else
return SCM_BOOL_T; {
SCM_UNLOCK_ARB (arb);
ret = SCM_BOOL_T;
}
scm_mutex_unlock (&scm_i_misc_mutex);
return ret;
} }
#undef FUNC_NAME #undef FUNC_NAME