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70c2ef5911
NuttX implemented fork() and vfork() as the same function. Both were libc
wrappers around a single up_fork() syscall; vfork() differed only by a
trailing waitpid(). Underneath, the child joined the parent's address
environment -- the same addrenv_join() that pthread_create() uses -- and got
a private copy of the stack. So the child shared .data, .bss and the heap
with its parent and ran concurrently with it.
That is not fork(). It is vfork()-with-a-private-stack under fork()'s name,
and the history says so: today's fork() is NuttX's old vfork(), renamed in
c33d1c9c97 (2023) without any change of behaviour. The failure was silent --
a program written against POSIX fork() compiled, ran, and had its child's
writes land in the parent's variables.
Separate them into two primitives, chosen by which function the caller
called rather than by what the hardware happens to be:
fork() child gets its own copy of the parent's memory at the same
virtual addresses; runs concurrently. Only where an address
environment can be duplicated -- elsewhere it is not declared at
all, so calling it is a build error naming the function.
vfork() child shares the parent's memory; parent suspended until the
child _exit()s or exec()s. Implementable everywhere.
Below libc there is still one syscall. up_fork() gains a bool saying which
primitive the caller used, since the per-architecture register snapshot is
the same for both, and passes it to nxtask_setup_fork(), which is the single
place the memory semantics are decided. The argument arrives in the first
argument register and is never touched: each architecture's snapshot takes
some other call-clobbered register for its scratch, so the flag is simply
still there when the C worker is called.
The vfork() parent suspension moves out of libc into nxtask_start_fork(),
released from nxsched_release_tcb() by nxtask_resume_vfork(). Two things
follow: the parent is resumed at exec(), since exec_swap() has already handed
the child's pid to the loaded program by the time the vfork stub exits, and
vfork() no longer depends on CONFIG_SCHED_WAITPID.
Releasing there requires one fix in nxtask_exit(). It raises rtcb->lockcount
directly rather than through sched_lock() while it tears the TCB down, so the
nxsem_post() that wakes the vfork() parent leaves it queued where a blocked
task collects while pre-emption is off -- g_pendingtasks, or g_readytorun on
SMP -- and the matching raw lockcount-- does not publish it the way
sched_unlock() would, leaving the parent stranded with nothing to move it on.
The fix mirrors sched_unlock() for each case: nxsched_merge_pending(), or
nxsched_deliver_task() under CONFIG_SMP. Both are no-ops while pre-emption is
still disabled, and up_exit() re-reads this_task() afterwards, so a change of
the ready-to-run head is honoured. Without it vfork() deadlocks wherever no
other task happens to call sched_unlock() afterwards -- rv-virt:nsh64 and
rv-virt:pnsh64, where NSH is blocked in waitpid() holding the lock, and
qemu-armv8a:citest_smp, which hangs the moment the vfork() test runs.
fork() is built on a new addrenv_fork(), backed by an up_addrenv_fork() hook
that duplicates an address environment into freshly allocated pages mapped at
the same virtual addresses -- unlike up_addrenv_clone(), which copies only
the representation and leaves both pointing at the same page tables. The
child then adopts the parent's stack geometry rather than being given a
relocated copy: a pointer to a stack local taken before fork() must name the
same object in the child that it named in the parent, and the parent's stack
is already in the duplicate, with its contents, at the parent's address.
No architecture implements up_addrenv_fork() yet, so this commit leaves
fork() unavailable everywhere. That is the intended state. It withdraws
fork() from ARCH_ARM, flat ARCH_ARM64, ARCH_RISCV, ARCH_SIM and ARCH_X86_64,
where until now it named the sharing primitive; per-architecture patches
restore it, with POSIX semantics, as up_addrenv_fork() lands. In the
meantime the sharing primitive is still there under the name that describes
it: vfork() for a child that runs a program, pthread_create() for a second
flow of control that shares memory, posix_spawn() for both at once.
Kconfig: ARCH_HAVE_VFORK inherits ARCH_HAVE_FORK's select lines, conditions
included, so no configuration gains machinery; ARCH_HAVE_FORK is redefined to
mean "can provide POSIX fork() semantics" and now depends on ARCH_ADDRENV.
There is one deliberate departure from "verbatim". ARCH_ARM selected the
fork family unconditionally, BUILD_KERNEL included, and that has never
worked: on a kernel build the architecture's fork entry point sees the
kernel's return address and stack pointer rather than the caller's, so the
child resumes at a kernel address. On qemu-armv7a:knsh master faults in
ostest's fork case with "Child did not run" and then a data abort; without
the condition this change faults the same way through vfork(). ARCH_ARM64
and ARCH_X86_64 already carried "if !BUILD_KERNEL" for exactly this reason --
ARM was the outlier. Conditioning it turns a runtime fault into an honest
absence, which is the whole point of the change; arch/arm takes the condition
off again in the patch that adds its saved-syscall-frame path. Only the
MMU-capable ARM ports are affected, since Cortex-M cannot build BUILD_KERNEL
at all.
Also fixes two latent syntax errors found on the way: a missing comma in
riscv_fork.c and mips_fork.c, both in *_FRAMEPOINTER && !SAVE_GP branches
that are never compiled today.
BREAKING CHANGE: fork() is withdrawn from every architecture. It is no
longer declared in unistd.h, so code that calls it fails to build with an error
naming the function, and the sharing behaviour it used to have is gone rather
than renamed. CONFIG_ARCH_HAVE_FORK no longer means "fork() exists"; it means
"this configuration can provide POSIX fork() semantics", and no architecture
selects it yet.
Quick fix, chosen by why the call was made:
to run a program vfork() + exec*(), or better posix_spawn()
a second flow of control that pthread_create()
shares the caller's memory
a genuinely independent copy keep fork(), and wait for the per-arch patch
of the process that implements up_addrenv_fork() and selects
CONFIG_ARCH_HAVE_FORK
Out-of-tree code that tests CONFIG_ARCH_HAVE_FORK to decide whether a
fork-then-exec path is available wants CONFIG_ARCH_HAVE_VFORK instead, which is
selected in exactly the places CONFIG_ARCH_HAVE_FORK used to be. The full
migration guide is Documentation/guides/fork_vfork_migration.rst.
Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>
175 lines
5.3 KiB
C
175 lines
5.3 KiB
C
/****************************************************************************
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* sched/task/task_exit.c
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*
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* SPDX-License-Identifier: Apache-2.0
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*
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* Licensed to the Apache Software Foundation (ASF) under one or more
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* contributor license agreements. See the NOTICE file distributed with
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* this work for additional information regarding copyright ownership. The
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* ASF licenses this file to you under the Apache License, Version 2.0 (the
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* "License"); you may not use this file except in compliance with the
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* License. You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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* License for the specific language governing permissions and limitations
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* under the License.
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*
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****************************************************************************/
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/****************************************************************************
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* Included Files
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****************************************************************************/
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#include <nuttx/config.h>
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#include <sched.h>
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#include <nuttx/debug.h>
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#include <nuttx/sched_note.h>
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#include "sched/sched.h"
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#ifdef CONFIG_SMP
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# include "irq/irq.h"
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#endif
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#include "signal/signal.h"
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#include "task/task.h"
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/****************************************************************************
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* Public Functions
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****************************************************************************/
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/****************************************************************************
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* Name: nxtask_exit
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*
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* Description:
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* This is a part of the logic used to implement _exit(). The full
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* implementation of _exit() is architecture-dependent. The _exit()
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* function also implements the bottom half of exit() and pthread_exit().
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*
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* This function causes the currently running task (i.e., the task at the
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* head of the ready-to-run list) to cease to exist. This function should
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* never be called from normal user code, but only from the architecture-
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* specific implementation of exit.
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*
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* Threads/tasks could also be terminated via pthread_cancel,
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* task_delete(), and task_restart(). In the last two cases, the
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* task will be terminated as though exit() were called.
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*
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* Input Parameters:
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* None
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*
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* Returned Value:
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* OK on success; or ERROR on failure
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*
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* Assumptions:
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* Executing within a critical section established by the caller.
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*
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****************************************************************************/
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int nxtask_exit(void)
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{
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FAR struct tcb_s *dtcb;
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FAR struct tcb_s *rtcb;
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int ret;
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#ifdef CONFIG_SMP
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/* Avoid using this_task() because it may assume a state that is not
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* appropriate for an exiting task.
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*/
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dtcb = current_task(this_cpu());
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#else
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dtcb = this_task();
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#endif
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sinfo("%s pid=%d,TCB=%p\n", get_task_name(dtcb),
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dtcb->pid, dtcb);
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/* Remove the TCB of the current task from the ready-to-run list. A
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* context switch will definitely be necessary -- that must be done
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* by the architecture-specific logic.
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*
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* nxsched_remove_readytorun will mark the task at the head of the
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* ready-to-run with state == TSTATE_TASK_RUNNING
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*/
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nxsched_remove_self(dtcb);
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/* Get the new task at the head of the ready to run list */
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#ifdef CONFIG_SMP
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rtcb = current_task(this_cpu());
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#else
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rtcb = this_task();
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#endif
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/* We are now in a bad state -- the head of the ready to run task list
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* does not correspond to the thread that is running. Disabling pre-
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* emption on this TCB and marking the new ready-to-run task as not
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* running.
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*
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* We disable pre-emption here by directly incrementing the lockcount
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* (vs. calling sched_lock()).
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*/
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rtcb->lockcount++;
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rtcb->task_state = TSTATE_TASK_READYTORUN;
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#ifdef CONFIG_SMP
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/* NOTE:
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* During nxtask_terminate(), enter_critical_section() will be called
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* to deallocate tcb. However, this would acquire g_cpu_irqlock if
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* rtcb->irqcount = 0, event though we are in critical section.
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* To prevent from acquiring, increment rtcb->irqcount here.
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*/
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rtcb->irqcount++;
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#endif
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dtcb->task_state = TSTATE_TASK_INACTIVE;
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/* Update scheduler parameters.
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*
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* When the thread exits, SYS_restore_context is called to
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* restore the context, which does not update the scheduling
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* information.
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* We need to update the scheduling information before tcb is released.
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*/
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nxsched_switch_context(dtcb, rtcb);
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sched_note_stop(dtcb);
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ret = nxsched_release_tcb(dtcb, dtcb->flags & TCB_FLAG_TTYPE_MASK);
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#ifdef CONFIG_SMP
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rtcb->irqcount--;
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#endif
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rtcb->task_state = TSTATE_TASK_RUNNING;
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/* Decrement the lockcount on rctb. */
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rtcb->lockcount--;
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/* Publish anything woken while the TCB was being released. lockcount was
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* raised directly rather than through sched_lock(), so the matching
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* decrement above does not publish the way sched_unlock() would, and a
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* vfork() parent released by nxsched_release_tcb() would be stranded --
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* in g_pendingtasks, or in g_readytorun on SMP. This mirrors what
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* sched_unlock() does for each case.
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*/
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#ifdef CONFIG_SMP
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nxsched_deliver_task(this_cpu(), rtcb->cpu, SWITCH_HIGHER);
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#else
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nxsched_merge_pending();
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#endif
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return ret;
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}
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