Support WSTOPPED and WCONTINUED

kernel/src/process/clone.rs

@@ -543,11 +543,12 @@ fn create_child_process(
 }
 
 fn set_parent_and_group(parent: &Process, child: &Arc<Process>) {
-    // Lock order: process table -> children -> group of process
+    // Lock order: children of process -> process table -> group of process
     // -> group inner -> session inner
+    let mut children_mut = parent.children().lock();
+
     let mut process_table_mut = process_table::process_table_mut();
 
-    let mut children_mut = parent.children().lock();
     let process_group_mut = parent.process_group.lock();
 
     let process_group = process_group_mut.upgrade().unwrap();

kernel/src/process/mod.rs

@@ -33,7 +33,7 @@ pub use process_vm::{
 pub use program_loader::{check_executable_file, ProgramToLoad};
 pub use rlimit::ResourceType;
 pub use term_status::TermStatus;
-pub use wait::{wait_child_exit, WaitOptions};
+pub use wait::{do_wait, WaitOptions};
 
 pub(super) fn init() {
     process::init();

kernel/src/process/posix_thread/mod.rs

@@ -199,6 +199,13 @@ impl PosixThread {
         *self.signalled_waker.lock() = None;
     }
 
+    /// Wakes up the signalled waker.
+    pub fn wake_signalled_waker(&self) {
+        if let Some(waker) = &*self.signalled_waker.lock() {
+            waker.wake_up();
+        }
+    }
+
     /// Enqueues a thread-directed signal.
     ///
     /// This method does not perform permission checks on user signals. Therefore, unless the
@@ -233,9 +240,7 @@ impl PosixThread {
         _sig_dispositions: MutexGuard<SigDispositions>,
     ) {
         self.sig_queues.enqueue(signal);
-        if let Some(waker) = &*self.signalled_waker.lock() {
+        self.wake_signalled_waker();
-            waker.wake_up();
-        }
     }
 
     /// Returns a reference to the profiling clock of the current thread.

kernel/src/process/process/mod.rs

@@ -18,6 +18,7 @@ use super::{
 };
 use crate::{
     prelude::*,
+    process::{status::StopWaitStatus, WaitOptions},
     sched::{AtomicNice, Nice},
     thread::{AsThread, Thread},
     time::clocks::ProfClock,
@@ -672,6 +673,47 @@ impl Process {
         &self.status
     }
 
+    /// Stops the process.
+    //
+    // FIXME: `ptrace` is another reason that can cause a process to stop.
+    // Consider extending the method signature to support `ptrace` if necessary.
+    pub fn stop(&self, sig_num: SigNum) {
+        if self.status.stop_status().stop(sig_num) {
+            self.wake_up_parent();
+        }
+    }
+
+    /// Resumes the stopped process.
+    pub fn resume(&self) {
+        if self.status.stop_status().resume() {
+            self.wake_up_parent();
+
+            // Note that the resume function is called by the thread which deals with SIGCONT,
+            // since SIGCONT is handled by any thread in this process, we need to wake
+            // up other stopped threads in the same process.
+            for task in self.tasks.lock().as_slice() {
+                let posix_thread = task.as_posix_thread().unwrap();
+                posix_thread.wake_signalled_waker();
+            }
+        }
+    }
+
+    /// Returns whether the process is stopped.
+    pub fn is_stopped(&self) -> bool {
+        self.status.stop_status().is_stopped()
+    }
+
+    /// Gets and clears the stop status changes for the `wait` syscall.
+    pub(super) fn wait_stopped_or_continued(&self, options: WaitOptions) -> Option<StopWaitStatus> {
+        self.status.stop_status().wait(options)
+    }
+
+    fn wake_up_parent(&self) {
+        let parent_guard = self.parent.lock();
+        let parent = parent_guard.process().upgrade().unwrap();
+        parent.children_wait_queue.wake_all();
+    }
+
     // ******************* Subreaper ********************
 
     /// Sets the child subreaper attribute of the current process.

kernel/src/process/signal/mod.rs

@@ -140,12 +140,8 @@ pub fn handle_pending_signal(
                     do_exit_group(TermStatus::Killed(sig_num));
                 }
                 SigDefaultAction::Ign => {}
-                SigDefaultAction::Stop => {
+                SigDefaultAction::Stop => ctx.process.stop(sig_num),
-                    let _ = ctx.thread.stop();
+                SigDefaultAction::Cont => ctx.process.resume(),
-                }
-                SigDefaultAction::Cont => {
-                    let _ = ctx.thread.resume();
-                }
             }
         }
     }

kernel/src/process/status.rs

@@ -4,7 +4,10 @@
 
 use core::sync::atomic::{AtomicBool, AtomicU32, Ordering};
 
+use ostd::sync::SpinLock;
+
 use super::ExitCode;
+use crate::process::{signal::sig_num::SigNum, WaitOptions};
 
 /// The status of a process.
 ///
@@ -12,12 +15,14 @@ use super::ExitCode;
 /// 1. Whether the process is a zombie (i.e., all its threads have exited);
 /// 2. Whether the process is the vfork child, which shares the user-space virtual memory
 ///    with its parent process;
-/// 3. The exit code of the process.
+/// 3. The exit code of the process;
+/// 4. Whether the process is stopped (by a signal or ptrace).
 #[derive(Debug)]
 pub struct ProcessStatus {
     is_zombie: AtomicBool,
     is_vfork_child: AtomicBool,
     exit_code: AtomicU32,
+    stop_status: StopStatus,
 }
 
 impl Default for ProcessStatus {
@@ -26,6 +31,7 @@ impl Default for ProcessStatus {
             is_zombie: AtomicBool::new(false),
             is_vfork_child: AtomicBool::new(false),
             exit_code: AtomicU32::new(0),
+            stop_status: StopStatus::new(),
         }
     }
 }
@@ -71,3 +77,96 @@ impl ProcessStatus {
         self.exit_code.store(exit_code, Ordering::Relaxed);
     }
 }
+
+impl ProcessStatus {
+    pub(super) fn stop_status(&self) -> &StopStatus {
+        &self.stop_status
+    }
+}
+
+#[derive(Debug)]
+pub(super) struct StopStatus {
+    /// Indicates whether the process is stopped.
+    is_stopped: AtomicBool,
+
+    /// Indicates whether the process's status has changed and has not yet been waited on.
+    ///
+    /// User programs may use the wait* syscalls to check for changes in
+    /// the process's status. This field will be set to `Some(_)` once the
+    /// process's status changes and will be set to `None` if the process
+    /// has already been waited on.
+    wait_status: SpinLock<Option<StopWaitStatus>>,
+}
+
+impl StopStatus {
+    pub(self) const fn new() -> Self {
+        Self {
+            is_stopped: AtomicBool::new(false),
+            wait_status: SpinLock::new(None),
+        }
+    }
+
+    /// Stops the process by some signal.
+    ///
+    /// The return value indicates whether the stop status has changed.
+    pub(super) fn stop(&self, signum: SigNum) -> bool {
+        // Hold the lock first to avoid race conditions
+        let mut wait_status = self.wait_status.lock();
+
+        if self.is_stopped.load(Ordering::Relaxed) {
+            false
+        } else {
+            self.is_stopped.store(true, Ordering::Relaxed);
+            *wait_status = Some(StopWaitStatus::Stopped(signum));
+            true
+        }
+    }
+
+    /// Resumes the process.
+    ///
+    /// The return value indicates whether the stop status has changed.
+    pub(super) fn resume(&self) -> bool {
+        // Hold the lock first to avoid race conditions
+        let mut wait_status = self.wait_status.lock();
+
+        if self.is_stopped.load(Ordering::Relaxed) {
+            self.is_stopped.store(false, Ordering::Relaxed);
+            *wait_status = Some(StopWaitStatus::Continue);
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Returns whether the process is stopped.
+    pub(super) fn is_stopped(&self) -> bool {
+        self.is_stopped.load(Ordering::Relaxed)
+    }
+
+    /// Gets and clears the stop status changes for the `wait` syscall.
+    pub(super) fn wait(&self, options: WaitOptions) -> Option<StopWaitStatus> {
+        let mut wait_status = self.wait_status.lock();
+
+        if options.contains(WaitOptions::WSTOPPED) {
+            if let Some(StopWaitStatus::Stopped(_)) = wait_status.as_ref() {
+                return wait_status.take();
+            }
+        }
+
+        if options.contains(WaitOptions::WCONTINUED) {
+            if let Some(StopWaitStatus::Continue) = wait_status.as_ref() {
+                return wait_status.take();
+            }
+        }
+
+        None
+    }
+}
+
+#[derive(Debug)]
+pub(super) enum StopWaitStatus {
+    // FIXME: A process can also be stopped by ptrace.
+    // Extend this enum to support ptrace.
+    Stopped(SigNum),
+    Continue,
+}

kernel/src/process/wait.rs

@@ -1,7 +1,5 @@
 // SPDX-License-Identifier: MPL-2.0
 
-#![expect(dead_code)]
-
 use super::{
     process_filter::ProcessFilter,
     signal::{constants::SIGCHLD, with_sigmask_changed},
@@ -12,14 +10,16 @@ use crate::{
     process::{
         posix_thread::{thread_table, AsPosixThread},
         process_table,
+        signal::sig_num::SigNum,
+        status::StopWaitStatus,
     },
+    time::clocks::ProfClock,
 };
 
 // The definition of WaitOptions is from Occlum
 bitflags! {
     pub struct WaitOptions: u32 {
         const WNOHANG = 0x1;
-        //Note: Below flags are not supported yet
         const WSTOPPED = 0x2; // Same as WUNTRACED
         const WEXITED = 0x4;
         const WCONTINUED = 0x8;
@@ -31,34 +31,56 @@ bitflags! {
 }
 
 impl WaitOptions {
-    pub fn supported(&self) -> bool {
+    pub fn check(&self) -> Result<()> {
-        let unsupported_flags = WaitOptions::all() - WaitOptions::WNOHANG;
+        if self.intersects(WaitOptions::WSTOPPED | WaitOptions::WCONTINUED)
-        !self.intersects(unsupported_flags)
+            && self.contains(WaitOptions::WNOWAIT)
+        {
+            return_errno_with_message!(
+                Errno::EINVAL,
+                "WNOWAIT cannot be used toghther with WSTOPPED or WCONTINUED"
+            );
+        }
+
+        let supported_args = WaitOptions::WNOHANG
+            | WaitOptions::WSTOPPED
+            | WaitOptions::WCONTINUED
+            | WaitOptions::WNOWAIT;
+        if !supported_args.contains(*self) {
+            warn!(
+                "unsupported wait options are found: {:?}",
+                *self - supported_args
+            );
+        }
+
+        Ok(())
     }
 }
 
-pub fn wait_child_exit(
+pub fn do_wait(
     child_filter: ProcessFilter,
     wait_options: WaitOptions,
     ctx: &Context,
-) -> Result<Option<Arc<Process>>> {
+) -> Result<Option<WaitStatus>> {
-    let current = ctx.process;
+    wait_options.check()?;
+
     let zombie_child = with_sigmask_changed(
         ctx,
         |sigmask| sigmask + SIGCHLD,
         || {
-            current.children_wait_queue().pause_until(|| {
+            ctx.process.children_wait_queue().pause_until(|| {
-                let unwaited_children = current
+                // Acquire the children lock at first to prevent race conditions.
-                    .children()
+                // We want to ensure that multiple waiting threads
-                    .lock()
+                // do not return the same waited process status.
+                let mut children_lock = ctx.process.children().lock();
+
+                let unwaited_children = children_lock
                     .values()
                     .filter(|child| match child_filter {
                         ProcessFilter::Any => true,
                         ProcessFilter::WithPid(pid) => child.pid() == pid,
                         ProcessFilter::WithPgid(pgid) => child.pgid() == pgid,
                     })
-                    .cloned()
+                    .collect::<Box<_>>();
-                    .collect::<Vec<_>>();
 
                 if unwaited_children.is_empty() {
                     return Some(Err(Error::with_message(
@@ -67,20 +89,15 @@ pub fn wait_child_exit(
                     )));
                 }
 
-                // return immediately if we find a zombie child
+                if let Some(status) = wait_zombie(&unwaited_children) {
-                let zombie_child = unwaited_children
+                    if !wait_options.contains(WaitOptions::WNOWAIT) {
-                    .iter()
+                        reap_zombie_child(status.pid(), &mut children_lock);
-                    .find(|child| child.status().is_zombie());
-
-                if let Some(zombie_child) = zombie_child {
-                    let zombie_pid = zombie_child.pid();
-                    if wait_options.contains(WaitOptions::WNOWAIT) {
-                        // does not reap child, directly return
-                        return Some(Ok(Some(zombie_child.clone())));
-                    } else {
-                        reap_zombie_child(current, zombie_pid);
-                        return Some(Ok(Some(zombie_child.clone())));
                     }
+                    return Some(Ok(Some(status)));
+                }
+
+                if let Some(status) = wait_stopped_or_continued(&unwaited_children, wait_options) {
+                    return Some(Ok(Some(status)));
                 }
 
                 if wait_options.contains(WaitOptions::WNOHANG) {
@@ -96,17 +113,80 @@ pub fn wait_child_exit(
     Ok(zombie_child)
 }
 
+pub enum WaitStatus {
+    Zombie(Arc<Process>),
+    Stop(Arc<Process>, SigNum),
+    Continue(Arc<Process>),
+}
+
+impl WaitStatus {
+    pub fn pid(&self) -> u32 {
+        self.process().pid()
+    }
+
+    pub fn status_code(&self) -> u32 {
+        match self {
+            Self::Zombie(process) => process.status().exit_code(),
+            Self::Stop(_, sig_num) => ((sig_num.as_u8() as u32) << 8) | 0x7f,
+            Self::Continue(_) => 0xffff,
+        }
+    }
+
+    pub fn prof_clock(&self) -> &Arc<ProfClock> {
+        self.process().prof_clock()
+    }
+
+    fn process(&self) -> &Arc<Process> {
+        match self {
+            WaitStatus::Zombie(process)
+            | WaitStatus::Stop(process, _)
+            | WaitStatus::Continue(process) => process,
+        }
+    }
+}
+
+fn wait_zombie(unwaited_children: &[&Arc<Process>]) -> Option<WaitStatus> {
+    unwaited_children
+        .iter()
+        .find(|child| child.status().is_zombie())
+        .map(|child| WaitStatus::Zombie((*child).clone()))
+}
+
+fn wait_stopped_or_continued(
+    unwaited_children: &[&Arc<Process>],
+    wait_options: WaitOptions,
+) -> Option<WaitStatus> {
+    if !wait_options.intersects(WaitOptions::WSTOPPED | WaitOptions::WCONTINUED) {
+        return None;
+    }
+
+    // Lock order: children of process -> tasks of process
+    for process in unwaited_children.iter() {
+        let Some(stop_wait_status) = process.wait_stopped_or_continued(wait_options) else {
+            continue;
+        };
+
+        let wait_status = match stop_wait_status {
+            StopWaitStatus::Stopped(sig_num) => WaitStatus::Stop((*process).clone(), sig_num),
+            StopWaitStatus::Continue => WaitStatus::Continue((*process).clone()),
+        };
+        return Some(wait_status);
+    }
+
+    None
+}
+
 /// Free zombie child with pid, returns the exit code of child process.
-fn reap_zombie_child(process: &Process, pid: Pid) -> ExitCode {
+fn reap_zombie_child(pid: Pid, children_lock: &mut BTreeMap<Pid, Arc<Process>>) -> ExitCode {
-    let child_process = process.children().lock().remove(&pid).unwrap();
+    let child_process = children_lock.remove(&pid).unwrap();
     assert!(child_process.status().is_zombie());
 
     for task in child_process.tasks().lock().as_slice() {
         thread_table::remove_thread(task.as_posix_thread().unwrap().tid());
     }
 
-    // Lock order: session table -> group table -> process table -> group of process
+    // Lock order: children of process -> session table -> group table
-    // -> group inner -> session inner
+    // -> process table -> group of process -> group inner -> session inner
     let mut session_table_mut = process_table::session_table_mut();
     let mut group_table_mut = process_table::group_table_mut();
 

kernel/src/syscall/wait4.rs

@@ -3,12 +3,12 @@
 use super::{getrusage::rusage_t, SyscallReturn};
 use crate::{
     prelude::*,
-    process::{wait_child_exit, ProcessFilter, WaitOptions},
+    process::{do_wait, ProcessFilter, WaitOptions},
 };
 
 pub fn sys_wait4(
     wait_pid: u64,
-    exit_status_ptr: u64,
+    status_ptr: u64,
     wait_options: u32,
     rusage_addr: Vaddr,
     ctx: &Context,
@@ -16,31 +16,30 @@ pub fn sys_wait4(
     let wait_options = WaitOptions::from_bits(wait_options)
         .ok_or_else(|| Error::with_message(Errno::EINVAL, "unknown wait option"))?;
     debug!(
-        "pid = {}, exit_status_ptr = {}, wait_options: {:?}",
+        "pid = {}, status_ptr = {}, wait_options: {:?}",
-        wait_pid as i32, exit_status_ptr, wait_options
+        wait_pid as i32, status_ptr, wait_options
     );
     debug!("wait4 current pid = {}", ctx.process.pid());
     let process_filter = ProcessFilter::from_id(wait_pid as _);
 
-    let waited_process =
+    let wait_status =
-        wait_child_exit(process_filter, wait_options, ctx).map_err(|err| match err.error() {
+        do_wait(process_filter, wait_options, ctx).map_err(|err| match err.error() {
             Errno::EINTR => Error::new(Errno::ERESTARTSYS),
             _ => err,
         })?;
-    let Some(process) = waited_process else {
+    let Some(wait_status) = wait_status else {
         return Ok(SyscallReturn::Return(0 as _));
     };
 
-    let (return_pid, exit_code) = (process.pid(), process.status().exit_code());
+    let (return_pid, status_code) = (wait_status.pid(), wait_status.status_code());
-    if exit_status_ptr != 0 {
+    if status_ptr != 0 {
-        ctx.user_space()
+        ctx.user_space().write_val(status_ptr as _, &status_code)?;
-            .write_val(exit_status_ptr as _, &exit_code)?;
     }
 
     if rusage_addr != 0 {
         let rusage = rusage_t {
-            ru_utime: process.prof_clock().user_clock().read_time().into(),
+            ru_utime: wait_status.prof_clock().user_clock().read_time().into(),
-            ru_stime: process.prof_clock().kernel_clock().read_time().into(),
+            ru_stime: wait_status.prof_clock().kernel_clock().read_time().into(),
             ..Default::default()
         };
 

kernel/src/syscall/waitid.rs

@@ -3,7 +3,7 @@
 use super::SyscallReturn;
 use crate::{
     prelude::*,
-    process::{wait_child_exit, ProcessFilter, WaitOptions},
+    process::{do_wait, ProcessFilter, WaitOptions},
 };
 
 pub fn sys_waitid(
@@ -19,12 +19,12 @@ pub fn sys_waitid(
     let wait_options = WaitOptions::from_bits(options as u32)
         .ok_or(Error::with_message(Errno::EINVAL, "invalid options"))?;
 
-    let waited_process =
+    let wait_status =
-        wait_child_exit(process_filter, wait_options, ctx).map_err(|err| match err.error() {
+        do_wait(process_filter, wait_options, ctx).map_err(|err| match err.error() {
             Errno::EINTR => Error::new(Errno::ERESTARTSYS),
             _ => err,
         })?;
 
-    let pid = waited_process.map_or(0, |process| process.pid());
+    let pid = wait_status.map_or(0, |wait_status| wait_status.pid());
     Ok(SyscallReturn::Return(pid as _))
 }

kernel/src/thread/mod.rs

@@ -2,14 +2,13 @@
 
 //! Posix thread implementation
 
-use core::sync::atomic::Ordering;
+use core::sync::atomic::{AtomicBool, Ordering};
 
 use ostd::{
     cpu::{AtomicCpuSet, CpuSet},
     task::Task,
 };
 
-use self::status::{AtomicThreadStatus, ThreadStatus};
 use crate::{
     prelude::*,
     sched::{SchedAttr, SchedPolicy},
@@ -18,7 +17,6 @@ use crate::{
 pub mod exception;
 pub mod kernel_thread;
 pub mod oops;
-pub mod status;
 pub mod task;
 pub mod work_queue;
 
@@ -52,7 +50,7 @@ pub struct Thread {
 
     // mutable part
     /// Thread status
-    status: AtomicThreadStatus,
+    is_exited: AtomicBool,
     /// Thread CPU affinity
     cpu_affinity: AtomicCpuSet,
     sched_attr: SchedAttr,
@@ -69,7 +67,7 @@ impl Thread {
         Thread {
             task,
             data: Box::new(data),
-            status: AtomicThreadStatus::new(ThreadStatus::Init),
+            is_exited: AtomicBool::new(false),
             cpu_affinity: AtomicCpuSet::new(cpu_affinity),
             sched_attr: SchedAttr::new(sched_policy),
         }
@@ -91,59 +89,16 @@ impl Thread {
     /// Runs this thread at once.
     #[track_caller]
     pub fn run(&self) {
-        self.status.store(ThreadStatus::Running, Ordering::Release);
         self.task.upgrade().unwrap().run();
     }
 
     /// Returns whether the thread is exited.
     pub fn is_exited(&self) -> bool {
-        self.status.load(Ordering::Acquire).is_exited()
+        self.is_exited.load(Ordering::Acquire)
-    }
-
-    /// Returns whether the thread is stopped.
-    pub fn is_stopped(&self) -> bool {
-        self.status.load(Ordering::Acquire).is_stopped()
-    }
-
-    /// Stops the thread if it is running.
-    ///
-    /// If the previous status is not [`ThreadStatus::Running`], this function
-    /// returns [`Err`] with the previous state. Otherwise, it sets the status
-    /// to [`ThreadStatus::Stopped`] and returns [`Ok`] with the previous state.
-    ///
-    /// This function only sets the status to [`ThreadStatus::Stopped`],
-    /// without initiating a reschedule.
-    pub fn stop(&self) -> core::result::Result<ThreadStatus, ThreadStatus> {
-        self.status.compare_exchange(
-            ThreadStatus::Running,
-            ThreadStatus::Stopped,
-            Ordering::AcqRel,
-            Ordering::Acquire,
-        )
-    }
-
-    /// Resumes running the thread if it is stopped.
-    ///
-    /// If the previous status is not [`ThreadStatus::Stopped`], this function
-    /// returns [`None`]. Otherwise, it sets the status to
-    /// [`ThreadStatus::Running`] and returns [`Some(())`].
-    ///
-    /// This function only sets the status to [`ThreadStatus::Running`],
-    /// without initiating a reschedule.
-    pub fn resume(&self) -> Option<()> {
-        self.status
-            .compare_exchange(
-                ThreadStatus::Stopped,
-                ThreadStatus::Running,
-                Ordering::AcqRel,
-                Ordering::Acquire,
-            )
-            .ok()
-            .map(|_| ())
     }
 
     pub(super) fn exit(&self) {
-        self.status.store(ThreadStatus::Exited, Ordering::Release);
+        self.is_exited.store(true, Ordering::Release);
     }
 
     /// Returns the reference to the atomic CPU affinity.

kernel/src/thread/status.rs

@@ -1,40 +0,0 @@
-// SPDX-License-Identifier: MPL-2.0
-
-use core::sync::atomic::AtomicU8;
-
-use atomic_integer_wrapper::define_atomic_version_of_integer_like_type;
-use int_to_c_enum::TryFromInt;
-
-define_atomic_version_of_integer_like_type!(ThreadStatus, try_from = true, {
-    #[derive(Debug)]
-    pub struct AtomicThreadStatus(AtomicU8);
-});
-
-#[derive(Clone, Copy, PartialEq, Eq, Debug, TryFromInt)]
-#[repr(u8)]
-pub enum ThreadStatus {
-    Init = 0,
-    Running = 1,
-    Exited = 2,
-    Stopped = 3,
-}
-
-impl ThreadStatus {
-    pub fn is_running(&self) -> bool {
-        *self == ThreadStatus::Running
-    }
-
-    pub fn is_exited(&self) -> bool {
-        *self == ThreadStatus::Exited
-    }
-
-    pub fn is_stopped(&self) -> bool {
-        *self == ThreadStatus::Stopped
-    }
-}
-
-impl From<ThreadStatus> for u8 {
-    fn from(value: ThreadStatus) -> Self {
-        value as u8
-    }
-}

kernel/src/thread/task.rs

@@ -2,6 +2,7 @@
 
 use ostd::{
     cpu::context::UserContext,
+    sync::Waiter,
     task::{Task, TaskOptions},
     user::{ReturnReason, UserContextApi, UserMode},
 };
@@ -32,6 +33,7 @@ pub fn create_new_user_task(
         let current_posix_thread = current_thread.as_posix_thread().unwrap();
         let current_thread_local = current_task.as_thread_local().unwrap();
         let current_process = current_posix_thread.process();
+        let (stop_waiter, _) = Waiter::new_pair();
 
         let user_ctx = current_task
             .user_ctx()
@@ -71,11 +73,13 @@ pub fn create_new_user_task(
             task: &current_task,
         };
 
-        loop {
+        while !current_thread.is_exited() {
+            // Execute the user code
             let return_reason = user_mode.execute(has_kernel_event_fn);
+
+            // Handle user events
             let user_ctx = user_mode.context_mut();
             let mut syscall_number = None;
-            // handle user event:
             match return_reason {
                 ReturnReason::UserException => handle_exception(&ctx, user_ctx),
                 ReturnReason::UserSyscall => {
@@ -85,22 +89,26 @@ pub fn create_new_user_task(
                 ReturnReason::KernelEvent => {}
             };
 
+            // Exit if the thread terminates
             if current_thread.is_exited() {
                 break;
             }
+
+            // Handle signals
             handle_pending_signal(user_ctx, &ctx, syscall_number);
-            // If current is suspended, wait for a signal to wake up self
+
-            while current_thread.is_stopped() {
+            // Handle signals while the thread is stopped
-                Thread::yield_now();
+            // FIXME: Currently, we handle all signals when the process is stopped.
-                debug!("{} is suspended.", current_posix_thread.tid());
+            // However, when the process is stopped, at least signals with user-provided handlers
+            // should not be handled; these signals should only be handled when the process is continued.
+            // Certain signals, such as SIGKILL, should be handled even if the process is stopped.
+            // We need to further investigate Linux behavior regarding which signals should be handled
+            // when the thread is stopped.
+            while !current_thread.is_exited() && current_process.is_stopped() {
+                let _ = stop_waiter.pause_until(|| (!current_process.is_stopped()).then_some(()));
                 handle_pending_signal(user_ctx, &ctx, None);
             }
-            if current_thread.is_exited() {
-                debug!("exit due to signal");
-                break;
-            }
         }
-        debug!("exit user loop");
     }
 
     TaskOptions::new(|| {

test/apps/process/wait4.c

@@ -0,0 +1,335 @@
+// SPDX-License-Identifier: MPL-2.0
+
+#include "../network/test.h"
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include <signal.h>
+#include <linux/wait.h>
+#include <pthread.h>
+#include <sys/mman.h>
+#include <fcntl.h>
+
+static pid_t pid;
+static int status;
+
+FN_SETUP(fork_child)
+{
+	pid = CHECK(fork());
+
+	if (pid == 0) {
+		// Child entering an infinite loop until killed by parent.
+		while (1) {
+			usleep(100);
+		}
+
+		exit(EXIT_SUCCESS);
+	}
+
+	// Parent process
+	sleep(1); // Ensure the child process is running
+}
+END_SETUP()
+
+FN_TEST(stop_child)
+{
+	// Stop the child process
+	TEST_SUCC(kill(pid, SIGSTOP));
+	TEST_RES(wait4(pid, &status, WSTOPPED, NULL),
+		 _ret == pid && WIFSTOPPED(status) &&
+			 WSTOPSIG(status) == SIGSTOP);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+FN_TEST(continue_child)
+{
+	TEST_SUCC(kill(pid, SIGCONT));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED, NULL),
+		 _ret == pid && WIFCONTINUED(status));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+FN_TEST(wait_nowait)
+{
+	TEST_SUCC(kill(pid, SIGSTOP));
+
+	status = 0;
+	TEST_ERRNO(wait4(pid, &status, WSTOPPED | WNOWAIT, NULL), EINVAL);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED, NULL),
+		 _ret == pid && WIFSTOPPED(status) &&
+			 WSTOPSIG(status) == SIGSTOP);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+
+	TEST_SUCC(kill(pid, SIGCONT));
+
+	status = 0;
+	TEST_ERRNO(wait4(pid, &status, WCONTINUED | WNOWAIT, NULL), EINVAL);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED, NULL),
+		 _ret == pid && WIFCONTINUED(status));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+FN_TEST(wait_stopped_and_continued)
+{
+	TEST_SUCC(kill(pid, SIGSTOP));
+	sleep(1);
+	TEST_SUCC(kill(pid, SIGCONT));
+	sleep(1);
+
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WCONTINUED, NULL),
+		 _ret == pid && WIFCONTINUED(status));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WCONTINUED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+FN_TEST(continue_not_stopped)
+{
+	TEST_SUCC(kill(pid, SIGCONT));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+FN_TEST(stop_continue_continue)
+{
+	TEST_SUCC(kill(pid, SIGSTOP));
+	sleep(1);
+	TEST_SUCC(kill(pid, SIGCONT));
+	sleep(1);
+	TEST_SUCC(kill(pid, SIGCONT));
+	sleep(1);
+
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WNOHANG, NULL),
+		 status == 0 && _ret == 0);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WCONTINUED, NULL),
+		 _ret == pid && WIFCONTINUED(status));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WCONTINUED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+FN_TEST(stop_continue_stop)
+{
+	TEST_SUCC(kill(pid, SIGSTOP));
+	sleep(1);
+	TEST_SUCC(kill(pid, SIGCONT));
+	sleep(1);
+	TEST_SUCC(kill(pid, SIGSTOP));
+	sleep(1);
+
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED | WNOHANG, NULL), status == 0);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED, NULL),
+		 _ret == pid && WIFSTOPPED(status) &&
+			 WSTOPSIG(status) == SIGSTOP);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+
+	// Restore the state
+	TEST_SUCC(kill(pid, SIGCONT));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED, NULL),
+		 _ret == pid && WIFCONTINUED(status));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WCONTINUED | WSTOPPED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+FN_TEST(stop_stopped)
+{
+	TEST_SUCC(kill(pid, SIGSTOP));
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED, NULL),
+		 _ret == pid && WIFSTOPPED(status) &&
+			 WSTOPSIG(status) == SIGSTOP);
+
+	TEST_SUCC(kill(pid, SIGSTOP));
+	sleep(1);
+	status = 0;
+	TEST_RES(wait4(pid, &status, WSTOPPED | WNOHANG, NULL),
+		 _ret == 0 && status == 0);
+}
+END_TEST()
+
+void *thread_func(void *arg)
+{
+	while (1) {
+		sleep(0.1);
+	}
+}
+
+void child_process()
+{
+	pthread_t t1, t2;
+
+	CHECK(pthread_create(&t1, NULL, thread_func, NULL));
+	CHECK(pthread_create(&t2, NULL, thread_func, NULL));
+
+	pthread_join(t1, NULL);
+	pthread_join(t2, NULL);
+
+	exit(EXIT_SUCCESS);
+}
+
+FN_TEST(multithread)
+{
+	pid_t child_pid = TEST_SUCC(fork());
+
+	if (child_pid == 0) {
+		child_process();
+	}
+
+	sleep(1);
+
+	TEST_SUCC(kill(child_pid, SIGSTOP));
+	int status = 0;
+	TEST_RES(wait4(child_pid, &status, WSTOPPED, NULL),
+		 _ret == child_pid && WIFSTOPPED(status) &&
+			 WSTOPSIG(status) == SIGSTOP);
+	TEST_RES(wait4(child_pid, &status, WSTOPPED | WNOHANG, NULL),
+		 _ret == 0);
+
+	TEST_SUCC(kill(child_pid, SIGKILL));
+	TEST_RES(wait4(child_pid, NULL, 0, NULL), _ret == child_pid);
+}
+END_TEST()
+
+volatile int sigint_counter = 0;
+volatile int sigtrap_counter = 0;
+
+void handle_sigint(int signum, siginfo_t *_info, void *_context)
+{
+	sigint_counter += 1;
+}
+
+void handle_sigtrap(int signum, siginfo_t *_info, void *_context)
+{
+	sigtrap_counter += 1;
+}
+
+void child_process2(int *pipe_fds)
+{
+	CHECK(close(pipe_fds[0]));
+
+	struct sigaction new_action = {};
+	new_action.sa_sigaction = handle_sigint;
+	CHECK(sigaction(SIGINT, &new_action, NULL));
+
+	new_action.sa_sigaction = handle_sigtrap;
+	CHECK(sigaction(SIGTRAP, &new_action, NULL));
+
+	while (1) {
+		usleep(100);
+		if (sigint_counter == 1) {
+			sigint_counter = 0;
+			CHECK(write(pipe_fds[1], "a", 1));
+		}
+		if (sigtrap_counter == 1) {
+			sigtrap_counter = 0;
+			CHECK(write(pipe_fds[1], "b", 1));
+		}
+	}
+
+	exit(EXIT_SUCCESS);
+}
+
+FN_TEST(nested_signals)
+{
+	int pipe_fds[2];
+	TEST_SUCC(pipe(pipe_fds));
+
+	for (int i = 0; i < 2; i++) {
+		int fd = pipe_fds[i];
+		int flags = TEST_SUCC(fcntl(fd, F_GETFL, 0));
+		TEST_SUCC(fcntl(fd, F_SETFL, flags | O_NONBLOCK));
+	}
+
+	int child_pid = TEST_SUCC(fork());
+
+	if (child_pid == 0) {
+		child_process2(pipe_fds);
+	}
+
+	TEST_SUCC(close(pipe_fds[1]));
+	sleep(1);
+
+	char buf[1] = { 0 };
+
+	// SIGINT -> SIGTRAP
+	TEST_SUCC(kill(child_pid, SIGINT));
+	sleep(1);
+	TEST_RES(read(pipe_fds[0], buf, 1), _ret == 1 && buf[0] == 'a');
+
+	TEST_SUCC(kill(child_pid, SIGTRAP));
+	sleep(1);
+	TEST_RES(read(pipe_fds[0], buf, 1), _ret == 1 && buf[0] == 'b');
+
+	// SIGSTOP -> SIGINT -> SIGTRAP -> SIGCONT
+	TEST_SUCC(kill(child_pid, SIGSTOP));
+	sleep(1);
+
+	// FIXME: The following two read pipe checks are commented out because
+	// Asterinas currently handles signals with user-provided handlers
+	// when the thread is stopped, while Linux does not.
+	TEST_SUCC(kill(child_pid, SIGINT));
+	sleep(1);
+	// TEST_ERRNO(read(pipe_fds[0], buf, 1), EAGAIN);
+
+	TEST_SUCC(kill(child_pid, SIGTRAP));
+	sleep(1);
+	// TEST_ERRNO(read(pipe_fds[0], buf, 1), EAGAIN);
+
+	TEST_RES(wait4(child_pid, &status, WSTOPPED, NULL),
+		 _ret == child_pid && WIFSTOPPED(status) &&
+			 WSTOPSIG(status) == SIGSTOP);
+
+	TEST_SUCC(kill(child_pid, SIGCONT));
+	sleep(1);
+	TEST_RES(read(pipe_fds[0], buf, 1), _ret == 1 && buf[0] == 'a');
+	TEST_RES(read(pipe_fds[0], buf, 1), _ret == 1 && buf[0] == 'b');
+
+	TEST_RES(wait4(child_pid, &status, WCONTINUED, NULL),
+		 _ret == child_pid && WIFCONTINUED(status));
+
+	// SIGKILL
+	TEST_SUCC(kill(child_pid, SIGKILL));
+	TEST_RES(wait4(child_pid, NULL, 0, NULL), _ret == child_pid);
+}
+END_TEST()
+
+FN_SETUP(kill_stopped)
+{
+	CHECK(kill(pid, SIGKILL));
+	sleep(1);
+	CHECK_WITH(wait4(pid, &status, WSTOPPED, NULL),
+		   _ret == pid && WIFSIGNALED(status) &&
+			   WTERMSIG(status) == SIGKILL);
+}
+END_SETUP()

test/apps/scripts/process.sh

@@ -33,6 +33,7 @@ mmap/mmap_readahead
 mmap/mmap_vmrss
 process/group_session
 process/job_control
+process/wait4
 pthread/pthread_test
 pty/open_pty
 pty/pty_blocking