* libguile/bdw-gc.h: Remove a needless compatibility hack.
* libguile/gc-inline.h: New file, implementing thread-local freelists
providing faster allocation if we already have a scm_i_thread*
pointer. Based on gc_inline.h from libgc.
* libguile/threads.h (scm_i_thread): Add freelists here.
* libguile/threads.c (guilify_self_1, guilify_self_2): Initialize
freelists.
* libguile/vm.c: Include gc-inline.h.
* libguile/vm-engine.c: Rename current_thread to thread. Use
scm_inline_cons instead of scm_cons, scm_inline_cell instead of
scm_cell, and scm_inline_words instead of words.
Fixes <http://bugs.gnu.org/13611>.
Reported by Mike Gran <spk121@yahoo.com>.
* libguile/smob.c (current_mark_stack_pointer,
current_mark_stack_limit): New variables.
(smob_mark): Use CURRENT_MARK_STACK_POINTER and
CURRENT_MARK_STACK_LIMIT instead of the same-named fields of
`SCM_I_CURRENT_THREAD'.
(scm_gc_mark): Likewise.
(scm_smob_prehistory): Initialize CURRENT_MARK_STACK_LIMIT and
CURRENT_MARK_STACK_POINTER.
* libguile/threads.h (scm_i_thread): Add comment that
`current_mark_stack_ptr' and `current_mark_stack_limit' are no longer
used.
* libguile/dynstack.h:
* libguile/dynstack.c: New files, implementing the dynamic stack as a
true stack instead of a linked list. This lowers the cost of
dynwinds: frames, winders, prompts, with-fluids, and dynamic-wind.
For the most part, we allocate these items directly on the stack.
* libguile/dynwinds.h:
* libguile/dynwinds.c: Adapt all manipulators of the wind stack to use
interfaces from dynstack.c. Remove heap-allocated winder and frame
object types.
(scm_dowinds, scm_i_dowinds): Remove these. The first was exported,
but it was not a public interface.
* libguile/continuations.c:
* libguile/continuations.h (scm_t_contregs): Continuation objects
reference scm_t_dynstack* values now. Adapt to the new interfaces.
* libguile/control.c:
* libguile/control.h: There is no longer a scm_tc7_prompt kind of object
that can be allocated on the heap. Instead, the prompt flags, key,
and registers are pushed on the dynwind stack. (The registers are
still on the heap.) Also, since the vm_cont will reference the
dynwinds, make the partial continuation stub take just one extra arg,
instead of storing the intwinds separately in the object table.
* libguile/fluids.c:
* libguile/fluids.h: No more with-fluids objects; instead, the fluids go
on the dynstack. The values still have to be on the heap, though.
(scm_prepare_fluids, scm_swap_fluids): New internal functions,
replacing scm_i_make_with_fluids and scm_i_swap_with_fluids.
* libguile/print.c: Remove prompt and with-fluids printers.
* libguile/tags.h: Revert prompt and with-fluids tc7 values to what they
were before they were allocated.
* libguile/vm-i-system.c (partial_cont_call): Just pop the vmcont, the
intwinds will not be passed as a second arg. Rewind the dynamic stack
from within the VM, so that any rewinder sees valid prompt entries.
(call_cc, tail_call_cc): Adapt to pass the dynstack to
scm_i_vm_capture_stack.
(prompt, wind, unwind, wind_fluids, unwind_fluids): Adapt to the new
interfaces.
* libguile/vm.h (scm_i_capture_current_stack): Rename from
scm_i_vm_capture_continuation.
(scm_i_vm_capture_stack): Take a dynstack as an argument.
* libguile/vm.c (vm_reinstate_partial_continuation): Don't wind here, as
that could result in winders seeing invalid prompts.
* libguile/eval.c:
* libguile/root.c:
* libguile/stacks.c:
* libguile/threads.c:
* libguile/threads.h:
* libguile/throw.c: Adapt other users of dynwinds to use the dynstack.
* libguile/null-threads.h:
* libguile/pthread-threads.h (SCM_DEFINE_ATFORK_HANDLERS_FOR_MUTEX)
(scm_i_pthread_atfork): Define a pthread_atfork shim, and a helper to
declare procedures to lock and unlock a mutex.
* libguile/threads.h (SCM_PTHREAD_ATFORK_LOCK_STATIC_MUTEX): New macro,
causes the snarfer to emit a pthread_atfork call to lock a mutex, if
threads are enabled.
This reverts commit ad432bc831.
Not all gensyms need to be universally-unique: most of them just need to
be unique within some portion of a Guile session. We'll take a
different tack on this problem in the next commit.
* libguile/symbols.c (scm_gensym): Make the gensym counter a 128-bit
thread-local, initialized to a random number upon the first call to
`gensym' within a given thread. This counter is rendered as a 22 byte
suffix of mostly base64 digits.
* libguile/threads.h (scm_i_thread): Add a thread-local gensym_counter.
* libguile/threads.c (guilify_self_1): Initialize gensym_counter to NULL.
* libguile/fluids.c (grow_dynamic_state, new_fluid): Arrange for the
default value in the dynamic-state vector to be SCM_UNDEFINED instead
of SCM_BOOL_F. If the value in the dynamic-state is #f, default to a
value attached to the fluid instead. This allows useful default
values.
(scm_make_fluid_with_default): New function, allows the user to
specify a default value for the fluid. Defaults to #f. Bound to
`make-fluid' on the Scheme side.
(scm_make_unbound_fluid): Use SCM_UNDEFINED as the default in all
threads.
(scm_fluid_unset_x): Also unset the default value. Not sure if this
is the right thing.
(fluid_ref): Update to the new default-value strategy.
* libguile/threads.c (scm_i_reset_fluid): Reset to SCM_UNDEFINED.
* libguile/threads.h: Remove extra arg to scm_i_reset_fluid.
* libguile/vm-i-system.c (fluid-ref): Update to new default-value
strategy.
* module/ice-9/vlist.scm (block-growth-factor): Default to 2 in all
threads. Fixes http://debbugs.gnu.org/10093.
* libguile/threads.h:
* libguile/threads.c (scm_i_reset_fluid): New internal function, resets
the binding of a fluid for all threads. Needed for fluid GC.
* libguile/fluids.c (new_fluid): Call scm_i_reset_fluid here.
* libguile/init.h:
* libguile/init.c (scm_i_init_guile): Change arg to this internal
function from SCM_STACKITEM* to void*. Actually it's a
struct GC_stack_base*.
* libguile/bdw-gc.h: Don't do pthread redirects, because we don't want
to affect applications' pthread_* bindings.
* libguile/pthread-threads.h (scm_i_pthread_create)
(scm_i_pthread_detach, scm_i_pthread_exit, scm_i_pthread_cancel)
(scm_i_pthread_sigmask): Do pthread redirects here, in this internal
header.
* libguile/threads.h: Remove declaration of internal
scm_i_with_guile_and_parent. Remove declaration of undefined
scm_threads_init_first_thread. Make declaration of internal
scm_threads_prehistory actually internal, and take a void* (actually a
struct GC_stack_base*).
* libguile/threads.c (GC_get_stack_base): Implement a shim if this
function is unavailable, and fold in the implementations of
get_thread_stack_base.
(GC_call_with_stack_base): Actually implement.
(guilify_self_1): Take a GC_stack_base* as an arg.
(scm_i_init_thread_for_guile): Likewise, and set up libgc for
registration of other threads.
(scm_init_guile): Use GC_get_stack_base instead of our own guesswork.
(with_guile_and_parent, scm_i_with_guile_and_parent): Rework to
trampoline through a GC_call_with_stack_base.
(scm_threads_prehistory): Pass the "base" arg on to guilify_self_1.
* libguile/threads.h: Always declare a scm_i_thread_key, for cleanup
purposes, in the BUILDING_LIBGUILE case.
* libguile/threads.c (scm_i_thread_key): Init with a cleanup handler, so
any guile-specific info for a thread can be cleaned up reliably.
(guilify_self_1): Always set the thread key.
(do_thread_exit_trampoline, on_thread_exit): Enter guile-mode for the
guile-mode cleanup handler, and trampoline through a
gc_call_with_stack_base for reasons explained in the code.
(init_thread_key, scm_i_init_thread_for_guile): Always init the key.
(scm_i_with_guile_and_parent): No need for pthread_cancel cleanup
handlers, as the pthread key destructor will take care of that for
us.
(really_launch): Remove needless pthread_exit call with incorrect
comment.
* configure.ac: Check for GC_call_with_gc_active.
* libguile/threads.h (scm_i_thread): Remove "top", as it's not used.
* libguile/threads.c (with_gc_inactive, with_gc_active): Define shims to
GC_do_blocking and GC_call_with_gc_active.
(scm_i_init_thread_for_guile): Don't do thread base adjustment here,
do it in scm_i_with_guile_and_parent. The previous logic would never
be run.
(scm_i_with_guile_and_parent): If we enter Guile mode, leave it too.
Take care of adjusting the thread stack base here too. Also, call
with_gc_active.
(scm_without_guile): Refactor.
* libguile/debug.h (scm_t_debug_frame): Remove this type, as it was
internal to the old evaluator.
(SCM_EVALFRAME, SCM_APPLYFRAME, SCM_VOIDFRAME, SCM_MACROEXPF)
(SCM_TAILREC, SCM_TRACED_FRAME, SCM_ARGS_READY, SCM_DOVERFLOW)
(SCM_MAX_FRAME_SIZE, SCM_FRAMETYPE)
(SCM_EVALFRAMEP, SCM_APPLYFRAMEP, SCM_VOIDFRAMEP, SCM_MACROEXPFP)
(SCM_TAILRECP, SCM_TRACED_FRAME_P, SCM_ARGS_READY_P, SCM_OVERFLOWP)
(SCM_SET_MACROEXP, SCM_SET_TAILREC, SCM_SET_TRACED_FRAME)
(SCM_SET_ARGSREADY, SCM_SET_OVERFLOW)
(SCM_CLEAR_MACROEXP, SCM_CLEAR_TRACED_FRAME, SCM_CLEAR_ARGSREADY):
Remove macro accessors to scm_t_debug_frame.
(SCM_DEBUGOBJP, SCM_DEBUGOBJ_FRAME, SCM_SET_DEBUGOBJ_FRAME):
(scm_debug_object_p, scm_make_debugobj): Remove debugobj accessors.
(scm_i_unmemoize_expr): Remove unused declaration.
* libguile/debug.c (scm_debug_options): No more max limit on frame
sizes.
(scm_start_stack): Just call out to scm_vm_call_with_new_stack.
(scm_debug_object_p, scm_make_debugobj, scm_init_debug): No more
debugobj smob type.
* libguile/deprecated.h:
* libguile/deprecated.c (scm_i_deprecated_last_debug_frame)
(scm_last_debug_frame): Remove deprecated debug-frame bits.
* libguile/stacks.c (scm_make_stack): Rework this function and its
dependents to only walk VM frames.
(scm_stack_id): Call out to the holder of the VM frame in question,
which should be a VM or a VM continuation, for the stack ID. Currently
this bit is stubbed out.
(scm_last_stack_frame): Removed. It seems this is mainly useful for a
debugger, and we need to rewrite the debugger to work on the Scheme
level.
* test-suite/tests/continuations.test ("continuations"): Remove test for
last-stack-frame.
* libguile/continuations.h (struct scm_t_contregs):
* libguile/continuations.c (scm_make_continuation):
(copy_stack_and_call, scm_i_with_continuation_barrier): No need to
save and restore debug frames.
* libguile/threads.h (scm_i_thread): Don't track debug frames.
(scm_i_last_debug_frame, scm_i_set_last_debug_frame): Remove macro
accessors.
* libguile/threads.c (guilify_self_1): Don't track debug frames.
* libguile/throw.c: No need to track debug frames in a jmpbuf.
* libguile/vm-engine.c (vm_engine, VM_PUSH_DEBUG_FRAMES): Don't push
debug frames.
* libguile/vm.h:
* libguile/vm.c (scm_vm_call_with_new_stack): New function. Currently
stubbed out though.
* libguile/__scm.h (scm_async_tick): New declaration.
(SCM_ASYNC_TICK)[!BUILDING_LIBGUILE]: Use `scm_async_tick ()'.
* libguile/async.c (scm_critical_section_start,
scm_critical_section_end, scm_async_tick): New functions.
* libguile/async.h (scm_i_critical_section_mutex): Made internal.
(scm_critical_section_start, scm_critical_section_end): New
declarations.
(SCM_CRITICAL_SECTION_START,
SCM_CRITICAL_SECTION_END)[!BUILDING_LIBGUILE]: Use the same-named
function (lower-case).
* libguile/stackchk.h (SCM_STACK_OVERFLOW_P): Conditionalize on
`BUILDING_LIBGUILE'.
* libguile/threads.h (SCM_I_CURRENT_THREAD, scm_i_dynwinds,
scm_i_set_dynwinds, scm_i_last_debug_frame,
scm_i_set_last_debug_frame): Conditionalize on `BUILDING_LIBGUILE'.
The crux of this problem was that the thread doing a throw, and so
checking scm_i_critical_section_level, was different from the thread
that was in a critical section.
* libguile/async.h (scm_i_critical_section_level): Removed, replaced
by per-thread critical_section_level.
(SCM_CRITICAL_SECTION_START, SCM_CRITICAL_SECTION_END): Use
per-thread critical_section_level.
* libguile/continuations.c (scm_dynthrow): Check per-thread
critical_section_level.
* libguile/threads.c (guilify_self_1): Init per-thread
critical_section_level.
(scm_i_critical_section_level): Removed.
* libguile/threads.h (scm_i_thread): New critical_section_level field.
* libguile/throw.c (scm_ithrow): Check per-thread critical_section_level.
Problem was that if an application includes both libguile.h and the
system's setjmp.h, and is compiled on IA64, it gets compile errors
because of jmp_buf, setjmp and longjmp being multiply defined.
* libguile/__scm.h (__ia64__): Define scm_i_jmp_buf, SCM_I_SETJMP and
SCM_I_LONGJMP instead of jmp_buf, setjmp and longjmp.
(all other platforms): Map scm_i_jmp_buf, SCM_I_SETJMP and
SCM_I_LONGJMP to jmp_buf, setjmp and longjmp.
* libguile/continuations.c (scm_make_continuation): Use `SCM_I_SETJMP'
instead of `setjmp'.
(copy_stack_and_call): Use `SCM_I_LONJMP' instead of `longjmp'.
(scm_ia64_longjmp): Use type `scm_i_jmp_buf' instead of `jmp_buf'.
* libguile/continuations.h (scm_t_contregs): Use type `scm_i_jmp_buf'
instead of `jmp_buf'.
* libguile/threads.c (suspend): Use `SCM_I_SETJMP' instead of
`setjmp'.
* libguile/threads.h (scm_i_thread): Use type `scm_i_jmp_buf' instead
of `jmp_buf'.
* libguile/throw.c (JBJMPBUF, make_jmpbuf, jmp_buf_and_retval): Use
type `scm_i_jmp_buf' instead of `jmp_buf'.
(scm_c_catch): Use `SCM_I_SETJMP' instead of `setjmp'.
(scm_ithrow): Use `SCM_I_LONGJMP' instead of `longjmp'.
Reported by Roland Haeder. The declaration and definition of
scm_pthread_cond_timedwait were using possibly different types for the
third arg.
* THANKS: Added Roland Haeder.
* libguile/threads.h (scm_pthread_cond_timedwait): Use scm_t_timespec
for third arg rather than struct timespec, for consistency with the
function implementation.
For each thread that goes into Guile mode, Guile pushes a cleanup
function, scm_leave_guile_cleanup, whose purpose is to execute
`scm_leave_guile ()' if the thread is terminated while in Guile mode.
The problem is that there are various places - like
scm_pthread_cond_wait, scm_without_guile and scm_std_select - where
the thread temporarily leaves Guile mode (which means unlocking the
heap mutex), and the cleanup function is still in place. Therefore if
the thread is terminated at these places, the cleanup function ends up
trying to unlock a mutex (the heap mutex) which isn't actually locked.
* libguile/threads.h (scm_i_thread): New heap_mutex_locked_by_self field.
* libguile/threads.c (scm_enter_guile): Set heap_mutex_locked_by_self.
(scm_leave_guile): Only unlock if heap_mutex_locked_by_self is 1.
(guilify_self_1): Initialize heap_mutex_locked_by_self.
(scm_i_thread_sleep_for_gc): Remove incorrect use of t->held_mutex
here.
* libguile/threads.h (held_mutex): New field.
* libguile/threads.c (enqueue, remqueue, dequeue): Use critical
section to protect access to the queue.
(guilify_self_1): Initialize held_mutex field.
(on_thread_exit): If held_mutex non-null, unlock it.
(fat_mutex_unlock, fat_cond_free, scm_make_condition_variable,
fat_cond_signal, fat_cond_broadcast): Delete now unnecessary uses
of c->lock.
(fat_mutex_unlock): Pass m->lock to block_self() instead of
c->lock; move scm_i_pthread_mutex_unlock(m->lock) call from before
block_self() to after.
(scm_pthread_cond_wait, scm_pthread_cond_timedwait,
scm_i_thread_sleep_for_gc): Set held_mutex before pthread call;
reset it afterwards.
I was seeing a hang in srfi-18.test, when running make check in master,
in the "exception handler installation is thread-safe" test. It wasn't
100% reproducible, so looked like a race.
The problem is that wait-condition-variable is not actually
atomic in the way that it is supposed to be. It unlocks the mutex,
then starts waiting on the cond var. So it is possible for another
thread to lock the same mutex, and signal the cond var, before the
wait-condition-variable thread starts waiting.
In order for wait-condition-variable to be atomic - e.g. in a race
where thread A holds (Scheme-level) mutex M, and calls
(wait-condition-variable C M), and thread B calls (begin (lock-mutex
M) (signal-condition-variable C)) - it needs to call pthread_cond_wait
with the same underlying mutex as is involved in the `lock-mutex'
call. In terms of the threads.c code, this means that it has to use
M->lock, not C->lock.
block_self() used its mutex arg for two purposes: for protecting
access and changes to the wait queue, and for the pthread_cond_wait
call. But it wouldn't work reliably to use M->lock to protect C's
wait queue, because in theory two threads can call
(wait-condition-variable C M1) and (wait-condition-variable C M2)
concurrently, with M1 and M2 different. So we either have to pass
both C->lock and M->lock into block_self(), or use some other mutex to
protect the wait queue. For this patch, I switched to using the
critical section mutex, because that is a global and so easily
available. (If that turns out to be a problem for performance, we
could make each queue structure have its own mutex, but there's no
reason to believe yet that it is a problem, because the critical
section mutex isn't used much overall.)
So then we call block_self() with M->lock, and move where M->lock is
unlocked to after the block_self() call, instead of before.
That solves the first hang, but introduces a new one, when a SRFI-18
thread is terminated (`thread-terminate!') between being launched
(`make-thread') and started (`thread-start!'). The problem now is
that pthread_cond_wait is a cancellation point (see man
pthread_cancel), so the pthread_cond_wait call is one of the few
places where a thread-terminate! call can take effect. If the thread
is cancelled at that point, M->lock ends up still being locked, and
then when do_thread_exit() tries to lock M->lock again, it hangs.
The fix for that is a new `held_mutex' field in scm_i_thread, which is
set to point to the mutex just before a pthread_cond_(timed)wait call,
and set to NULL again afterwards. If on_thread_exit() finds that
held_mutex is non-NULL, it unlocks that mutex.
A detail is that checking and unlocking held_mutex must be done before
on_thread_exit() calls scm_i_ensure_signal_delivery_thread(), because
the innards of scm_i_ensure_signal_delivery_thread() can do another
pthread_cond_wait() call and so overwrite held_mutex. But that's OK,
because it's fine for the mutex check and unlock to happen outside
Guile mode.
Lastly, C->lock is then not needed, so I've removed it.
* libguile/threads.h (held_mutex): New field.
* libguile/threads.c (enqueue, remqueue, dequeue): Use critical
section to protect access to the queue.
(guilify_self_1): Initialize held_mutex field.
(on_thread_exit): If held_mutex non-null, unlock it.
(fat_mutex_unlock, fat_cond_free, scm_make_condition_variable,
fat_cond_signal, fat_cond_broadcast): Delete now unnecessary uses
of c->lock.
(fat_mutex_unlock): Pass m->lock to block_self() instead of
c->lock; move scm_i_pthread_mutex_unlock(m->lock) call from before
block_self() to after.
(scm_pthread_cond_wait, scm_pthread_cond_timedwait,
scm_i_thread_sleep_for_gc): Set held_mutex before pthread call;
reset it afterwards.
I was seeing a hang in srfi-18.test, when running make check in master,
in the "exception handler installation is thread-safe" test. It wasn't
100% reproducible, so looked like a race.
The problem is that wait-condition-variable is not actually
atomic in the way that it is supposed to be. It unlocks the mutex,
then starts waiting on the cond var. So it is possible for another
thread to lock the same mutex, and signal the cond var, before the
wait-condition-variable thread starts waiting.
In order for wait-condition-variable to be atomic - e.g. in a race
where thread A holds (Scheme-level) mutex M, and calls
(wait-condition-variable C M), and thread B calls (begin (lock-mutex
M) (signal-condition-variable C)) - it needs to call pthread_cond_wait
with the same underlying mutex as is involved in the `lock-mutex'
call. In terms of the threads.c code, this means that it has to use
M->lock, not C->lock.
block_self() used its mutex arg for two purposes: for protecting
access and changes to the wait queue, and for the pthread_cond_wait
call. But it wouldn't work reliably to use M->lock to protect C's
wait queue, because in theory two threads can call
(wait-condition-variable C M1) and (wait-condition-variable C M2)
concurrently, with M1 and M2 different. So we either have to pass
both C->lock and M->lock into block_self(), or use some other mutex to
protect the wait queue. For this patch, I switched to using the
critical section mutex, because that is a global and so easily
available. (If that turns out to be a problem for performance, we
could make each queue structure have its own mutex, but there's no
reason to believe yet that it is a problem, because the critical
section mutex isn't used much overall.)
So then we call block_self() with M->lock, and move where M->lock is
unlocked to after the block_self() call, instead of before.
That solves the first hang, but introduces a new one, when a SRFI-18
thread is terminated (`thread-terminate!') between being launched
(`make-thread') and started (`thread-start!'). The problem now is
that pthread_cond_wait is a cancellation point (see man
pthread_cancel), so the pthread_cond_wait call is one of the few
places where a thread-terminate! call can take effect. If the thread
is cancelled at that point, M->lock ends up still being locked, and
then when do_thread_exit() tries to lock M->lock again, it hangs.
The fix for that is a new `held_mutex' field in scm_i_thread, which is
set to point to the mutex just before a pthread_cond_(timed)wait call,
and set to NULL again afterwards. If on_thread_exit() finds that
held_mutex is non-NULL, it unlocks that mutex.
A detail is that checking and unlocking held_mutex must be done before
on_thread_exit() calls scm_i_ensure_signal_delivery_thread(), because
the innards of scm_i_ensure_signal_delivery_thread() can do another
pthread_cond_wait() call and so overwrite held_mutex. But that's OK,
because it's fine for the mutex check and unlock to happen outside
Guile mode.
Lastly, C->lock is then not needed, so I've removed it.
* ice-9/Makefile.am: Don't compile popen.scm, its behaviour at runtime
is not consistent -- seems to miss some GC references? I suspect a bug
in the compiler. In any case without popen.scm being compiled,
continuations.test, r4rs.tes, and r5rs_pitfall.test do pass.
* libguile/threads.h (scm_i_thread):
* libguile/threads.c (thread_mark, guilify_self_2): Add a field for the
thread's vm. Previously I had this as a fluid, but it seems that newly
created threads share their fluid values from the creator thread; as
expected, I guess. In any case one VM should not be active in two
threads.
* libguile/vm.c (scm_the_vm): Change to access the thread-local vm,
instead of accessing a fluid.
(scm_the_vm_fluid): Removed.
* module/system/vm/vm.scm: Removed *the-vm*.
* libguile/gc.c (scm_gc): Don't use `scm_gc_running_p' as
an lvalue.
* libguile/gc.h (scm_gc_running_p): Define to 0.
* libguile/threads.h (scm_i_thread)[gc_running_p]: Remove.
Actually, threads would "go to sleep" either by blocking on a heap
allocation, or by noticing `scm_i_thread_go_to_sleep' is set when
running `SCM_TICK', which limits the applicability of this technique
(e.g., it was not appropriate for the shared string code).
* libguile/threads.c (scm_i_thread_go_to_sleep, scm_i_thread_put_to_sleep,
scm_i_thread_invalidate_freelists, scm_i_thread_wake_up,
scm_i_thread_sleep_for_gc): Remove.
* libguile/threads.h (scm_i_thread_go_to_sleep, scm_i_thread_put_to_sleep,
scm_i_thread_invalidate_freelists, scm_i_thread_wake_up,
scm_i_thread_sleep_for_gc): Remove declarations.
(SCM_THREAD_SWITCHING_CODE): Do nothing.
