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Recycle fluid numbers.

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* libguile/fluids.c: Remove outdated comment on the complexity of fluid GC.
  (FLUID_GROW): Increase to 128.
  (allocated_fluids): Change to `void **'.
  (allocated_fluids_num): Remove.
  (grow_dynamic_state): Remove race-condition checker.  Grow to
  ALLOCATED_FLUIDS_LEN.
  (next_fluid_num): Rename to...
  (new_fluid): ... this.  Return a fluid instead of a fluid number.
  Assume ALLOCATED_FLUIDS is sparse and search for a free number in it.
  Make ALLOCATED_FLUIDS point to pointerless memory and initialize
  individual elements with a pointer to the new fluid.  Register a
  disappearing link from there to the fluid.
  (scm_make_fluid): Use `new_fluid ()'.
  (scm_fluid_ref, scm_fluid_set_x, scm_i_swap_with_fluids): Remove
  assertion that wasn't checked in a thread-safe way.
This commit is contained in:
Ludovic Courtès 2010-03-05 13:38:28 +01:00
parent c02924d02b
commit bd5a75dcd8
1 changed files with 58 additions and 90 deletions
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142
libguile/fluids.c
View file

@ -22,7 +22,6 @@
#include <stdio.h>
#include <string.h>
#include <assert.h>
#include "libguile/_scm.h"
#include "libguile/print.h"
@ -34,37 +33,18 @@
#include "libguile/deprecation.h"
#include "libguile/lang.h"
#include "libguile/validate.h"
#include "libguile/bdw-gc.h"
#define FLUID_GROW 20
/* Number of additional slots to allocate when ALLOCATED_FLUIDS is full. */
#define FLUID_GROW 128
/* A lot of the complexity below stems from the desire to reuse fluid
slots. Normally, fluids should be pretty global and long-lived
things, so that reusing their slots should not be overly critical,
but it is the right thing to do nevertheless. The code therefore
puts the burdon on allocating and collection fluids and keeps
accessing fluids lock free. This is achieved by manipulating the
global state of the fluid machinery mostly in single threaded
sections.
Reusing a fluid slot means that it must be reset to #f in all
dynamic states. We do this by maintaining a weak list of all
dynamic states, which is used after a GC to do the resetting.
Also, the fluid vectors in the dynamic states need to grow from
time to time when more fluids are created. We do this in a single
threaded section so that threads do not need to lock when accessing
a fluid in the normal way.
*/
/* Vector of allocated fluids indexed by fluid numbers. Access is protected by
FLUID_ADMIN_MUTEX. */
static void **allocated_fluids = NULL;
static size_t allocated_fluids_len = 0;
static scm_i_pthread_mutex_t fluid_admin_mutex = SCM_I_PTHREAD_MUTEX_INITIALIZER;
/* Protected by fluid_admin_mutex, but also accessed during GC. See
next_fluid_num for a discussion of this.
*/
static size_t allocated_fluids_len = 0;
static size_t allocated_fluids_num = 0;
static char *allocated_fluids = NULL;
#define IS_FLUID(x) SCM_I_FLUID_P (x)
#define FLUID_NUM(x) SCM_I_FLUID_NUM (x)
@ -74,34 +54,26 @@ static char *allocated_fluids = NULL;
/* Grow STATE so that it can hold up to ALLOCATED_FLUIDS_NUM fluids. */
/* Grow STATE so that it can hold up to ALLOCATED_FLUIDS_LEN fluids. This may
be more than necessary since ALLOCATED_FLUIDS is sparse and the current
thread may not access all the fluids anyway. Memory usage could be improved
by using a 2-level array as is done in glibc for pthread keys (TODO). */
static void
grow_dynamic_state (SCM state)
{
SCM new_fluids;
SCM old_fluids = DYNAMIC_STATE_FLUIDS (state);
size_t i, new_len, old_len = SCM_SIMPLE_VECTOR_LENGTH (old_fluids);
size_t i, len, old_len = SCM_SIMPLE_VECTOR_LENGTH (old_fluids);
retry:
new_len = allocated_fluids_num;
new_fluids = scm_c_make_vector (new_len, SCM_BOOL_F);
/* Assume the assignment below is atomic. */
len = allocated_fluids_len;
scm_i_pthread_mutex_lock (&fluid_admin_mutex);
if (new_len != allocated_fluids_num)
{
/* We lost the race. */
scm_i_pthread_mutex_unlock (&fluid_admin_mutex);
goto retry;
}
assert (allocated_fluids_num > old_len);
new_fluids = scm_c_make_vector (len, SCM_BOOL_F);
for (i = 0; i < old_len; i++)
SCM_SIMPLE_VECTOR_SET (new_fluids, i,
SCM_SIMPLE_VECTOR_REF (old_fluids, i));
SET_DYNAMIC_STATE_FLUIDS (state, new_fluids);
scm_i_pthread_mutex_unlock (&fluid_admin_mutex);
}
void
@ -128,45 +100,53 @@ scm_i_with_fluids_print (SCM exp, SCM port, scm_print_state *pstate SCM_UNUSED)
scm_putc ('>', port);
}
static size_t
next_fluid_num ()
/* Return a new fluid. */
static SCM
new_fluid ()
{
size_t n;
SCM fluid;
size_t trial, n;
/* Fluids are pointerless cells: the first word is the type tag; the second
word is the fluid number. */
fluid = PTR2SCM (scm_gc_malloc_pointerless (sizeof (scm_t_cell), "fluid"));
SCM_SET_CELL_TYPE (fluid, scm_tc7_fluid);
scm_dynwind_begin (0);
scm_i_dynwind_pthread_mutex_lock (&fluid_admin_mutex);
if ((allocated_fluids_len > 0) &&
(allocated_fluids_num == allocated_fluids_len))
for (trial = 0; trial < 2; trial++)
{
/* All fluid numbers are in use. Run a GC to try to free some
up.
*/
scm_gc ();
/* Look for a free fluid number. */
for (n = 0; n < allocated_fluids_len; n++)
/* TODO: Use `__sync_bool_compare_and_swap' where available. */
if (allocated_fluids[n] == NULL)
break;
if (trial == 0 && n >= allocated_fluids_len)
/* All fluid numbers are in use. Run a GC and retry. Explicitly
running the GC is costly and bad-style. We only do this because
dynamic state fluid vectors would grow unreasonably if fluid numbers
weren't reused. */
scm_i_gc ("fluids");
}
if (allocated_fluids_num < allocated_fluids_len)
{
for (n = 0; n < allocated_fluids_len; n++)
if (allocated_fluids[n] == 0)
break;
}
else
if (n >= allocated_fluids_len)
{
/* Grow the vector of allocated fluids. */
/* FIXME: Since we use `scm_malloc ()', ALLOCATED_FLUIDS is scanned by
the GC; therefore, all fluids remain reachable for the entire
program lifetime. Hopefully this is not a problem in practice. */
char *new_allocated_fluids =
scm_gc_malloc (allocated_fluids_len + FLUID_GROW,
void **new_allocated_fluids =
scm_gc_malloc_pointerless ((allocated_fluids_len + FLUID_GROW)
* sizeof (*allocated_fluids),
"allocated fluids");
/* Copy over old values and initialize rest. GC can not run
during these two operations since there is no safe point in
them.
*/
memcpy (new_allocated_fluids, allocated_fluids, allocated_fluids_len);
memset (new_allocated_fluids + allocated_fluids_len, 0, FLUID_GROW);
them. */
memcpy (new_allocated_fluids, allocated_fluids,
allocated_fluids_len * sizeof (*allocated_fluids));
memset (new_allocated_fluids + allocated_fluids_len, 0,
FLUID_GROW * sizeof (*allocated_fluids));
n = allocated_fluids_len;
/* Update the vector of allocated fluids. Dynamic states will
@ -176,11 +156,14 @@ next_fluid_num ()
allocated_fluids_len += FLUID_GROW;
}
allocated_fluids_num += 1;
allocated_fluids[n] = 1;
allocated_fluids[n] = SCM2PTR (fluid);
SCM_SET_CELL_WORD_1 (fluid, (scm_t_bits) n);
GC_GENERAL_REGISTER_DISAPPEARING_LINK (&allocated_fluids[n],
SCM2PTR (fluid));
scm_dynwind_end ();
return n;
return fluid;
}
SCM_DEFINE (scm_make_fluid, "make-fluid", 0, 0, 0,
@ -194,7 +177,7 @@ SCM_DEFINE (scm_make_fluid, "make-fluid", 0, 0, 0,
"with its own dynamic state, you can use fluids for thread local storage.")
#define FUNC_NAME s_scm_make_fluid
{
return scm_cell (scm_tc7_fluid, (scm_t_bits) next_fluid_num ());
return new_fluid ();
}
#undef FUNC_NAME
@ -229,11 +212,6 @@ SCM_DEFINE (scm_fluid_ref, "fluid-ref", 1, 0, 0,
if (SCM_UNLIKELY (FLUID_NUM (fluid) >= SCM_SIMPLE_VECTOR_LENGTH (fluids)))
{
/* We should only get there when the current thread's dynamic state
turns out to be too small compared to the set of currently allocated
fluids. */
assert (SCM_SIMPLE_VECTOR_LENGTH (fluids) < allocated_fluids_num);
/* Lazily grow the current thread's dynamic state. */
grow_dynamic_state (SCM_I_CURRENT_THREAD->dynamic_state);
@ -255,11 +233,6 @@ SCM_DEFINE (scm_fluid_set_x, "fluid-set!", 2, 0, 0,
if (SCM_UNLIKELY (FLUID_NUM (fluid) >= SCM_SIMPLE_VECTOR_LENGTH (fluids)))
{
/* We should only get there when the current thread's dynamic state
turns out to be too small compared to the set of currently allocated
fluids. */
assert (SCM_SIMPLE_VECTOR_LENGTH (fluids) < allocated_fluids_num);
/* Lazily grow the current thread's dynamic state. */
grow_dynamic_state (SCM_I_CURRENT_THREAD->dynamic_state);
@ -330,11 +303,6 @@ scm_i_swap_with_fluids (SCM wf, SCM dynstate)
if (SCM_UNLIKELY (max >= SCM_SIMPLE_VECTOR_LENGTH (fluids)))
{
/* We should only get there when the current thread's dynamic state turns
out to be too small compared to the set of currently allocated
fluids. */
assert (SCM_SIMPLE_VECTOR_LENGTH (fluids) < allocated_fluids_num);
/* Lazily grow the current thread's dynamic state. */
grow_dynamic_state (dynstate);