Add support for robust futex syscalls

src/libos/src/lib.rs

@@ -20,6 +20,7 @@
 #![feature(get_mut_unchecked)]
 // for std::hint::black_box
 #![feature(test)]
+#![feature(atomic_from_mut)]
 
 #[macro_use]
 extern crate alloc;

src/libos/src/process/do_exit.rs

@@ -44,6 +44,9 @@ fn exit_thread(term_status: TermStatus) {
         futex_wake(ctid_ptr.as_ptr() as *const i32, 1);
     }
 
+    // Notify waiters that the owner of robust futex has died.
+    thread.wake_robust_list();
+
     // Keep the main thread's tid available as long as the process is not destroyed.
     // This is important as the user space may still attempt to access the main
     // thread's ThreadRef through the process's pid after the process has become

src/libos/src/process/do_robust_list.rs

@@ -0,0 +1,195 @@
+/// Robust futexes provide a mechanism that is used in addition to normal futex,
+/// for kernel assist of cleanup of held locks on thread exit.
+///
+/// Actual locking and unlocking is handled entirely by user level code with the
+/// existing futex mechanism to wait or wakeup locks.
+/// The kernels only essential involvement in robust futex is to remember where
+/// the list head is, and to walk the list on thread exit, handling locks still
+/// held by the departing thread.
+/// Ref: https://www.kernel.org/doc/html/latest/locking/robust-futex-ABI.html
+///
+use std::ptr::NonNull;
+use std::sync::atomic::{AtomicU32, Ordering};
+
+use crate::prelude::*;
+use crate::util::mem_util::from_user::*;
+
+pub fn do_set_robust_list(list_head_ptr: *mut RobustListHead, len: usize) -> Result<()> {
+    debug!(
+        "set_robust_list: list_head_ptr: {:?}, len: {}",
+        list_head_ptr, len
+    );
+    if std::mem::size_of::<RobustListHead>() != len {
+        return_errno!(EINVAL, "invalid size for RobustListHead");
+    }
+    // We do not check if the pointer is a valid user space pointer, deferring
+    // it in waking the robust list. If the pointer is invalid, we just stop
+    // waking the robust list.
+    let robust_list = NonNull::new(list_head_ptr);
+    let current = current!();
+    current.set_robust_list(robust_list);
+    Ok(())
+}
+
+pub fn do_get_robust_list(tid: pid_t) -> Result<*mut RobustListHead> {
+    debug!("get_robust_list: tid: {}", tid);
+    let thread = if tid == 0 {
+        current!()
+    } else {
+        super::table::get_thread(tid)?
+    };
+    let robust_list_ptr = thread
+        .robust_list()
+        .map(|robust_list| robust_list.as_ptr())
+        .unwrap_or(std::ptr::null_mut());
+    Ok(robust_list_ptr)
+}
+
+/// This struct is same as Linux's robust_list
+#[repr(C)]
+struct RobustList {
+    next: *const RobustList,
+}
+
+/// This struct is same as Linux's robust_list_head
+#[repr(C)]
+pub struct RobustListHead {
+    /// Linked list of lock entries
+    ///
+    /// If it points to the head of the list, then it is the end of the list.
+    /// If it is an invalid user space pointer or a null pointer, stop iterating
+    /// the list.
+    list: RobustList,
+    /// Specifies the offset from the address of the lock entry to the address
+    /// of the futex.
+    futex_offset: isize,
+    /// Contains transient copy of the address of the lock entry, during list
+    /// insertion and removal.
+    list_op_pending: *const RobustList,
+}
+
+impl RobustListHead {
+    /// Return an iterator for all futexes in the robust list.
+    ///
+    /// The futex refered to by `list_op_pending`, if any, will be returned as
+    /// the last item.
+    pub fn futexes<'a>(&'a self) -> FutexIter<'a> {
+        FutexIter::new(self)
+    }
+
+    /// Return the pending futex address if exist
+    fn pending_futex_addr(&self) -> Option<*const i32> {
+        if self.list_op_pending.is_null() {
+            None
+        } else {
+            Some(unsafe { self.futex_addr(self.list_op_pending) })
+        }
+    }
+
+    /// Get the futex address
+    unsafe fn futex_addr(&self, entry_ptr: *const RobustList) -> *const i32 {
+        (entry_ptr as *const u8).offset(self.futex_offset) as *const i32
+    }
+}
+
+const ROBUST_LIST_LIMIT: isize = 2048;
+
+pub struct FutexIter<'a> {
+    robust_list: &'a RobustListHead,
+    entry_ptr: *const RobustList,
+    count: isize,
+}
+
+impl<'a> FutexIter<'a> {
+    fn new(robust_list: &'a RobustListHead) -> Self {
+        Self {
+            robust_list,
+            entry_ptr: robust_list.list.next,
+            count: 0,
+        }
+    }
+
+    // The `self.count` is normally a positive value used to iterate the list
+    // to avoid excessively long or circular list, we use a special value -1
+    // to represent the end of the Iterator.
+    fn set_end(&mut self) {
+        self.count = -1;
+    }
+
+    fn is_end(&self) -> bool {
+        self.count < 0
+    }
+}
+
+impl<'a> Iterator for FutexIter<'a> {
+    type Item = *const i32;
+
+    /// Returns the futex address.
+    fn next(&mut self) -> Option<*const i32> {
+        if self.is_end() {
+            return None;
+        }
+
+        // Iterate the linked list
+        while self.entry_ptr != &self.robust_list.list {
+            // Avoid excessively long or circular list
+            if self.count == ROBUST_LIST_LIMIT {
+                break;
+            }
+            // Invalid pointer, stop iterating the robust list
+            if check_ptr(self.entry_ptr).is_err() {
+                return None;
+            }
+            // A pending lock might already be on the list
+            let futex_addr = if self.entry_ptr != self.robust_list.list_op_pending {
+                Some(unsafe { self.robust_list.futex_addr(self.entry_ptr) })
+            } else {
+                None
+            };
+            self.entry_ptr = unsafe { (*self.entry_ptr).next };
+            self.count += 1;
+            if futex_addr.is_some() {
+                return futex_addr;
+            }
+        }
+
+        // End of iterating the linked list
+        // If the pending futex exists, return it as the last one
+        self.set_end();
+        self.robust_list.pending_futex_addr()
+    }
+}
+
+const FUTEX_WAITERS: u32 = 0x8000_0000;
+const FUTEX_OWNER_DIED: u32 = 0x4000_0000;
+const FUTEX_TID_MASK: u32 = 0x3FFF_FFFF;
+
+/// Wakeup one robust futex owned by the thread
+pub fn wake_robust_futex(futex_addr: *const i32, tid: pid_t) -> Result<()> {
+    let futex_val = {
+        check_ptr(futex_addr)?;
+        unsafe { AtomicU32::from_mut(&mut *(futex_addr as *mut u32)) }
+    };
+    let mut old_val = futex_val.load(Ordering::SeqCst);
+    loop {
+        // This futex may held by another thread, do nothing
+        if old_val & FUTEX_TID_MASK != tid {
+            break;
+        }
+        let new_val = (old_val & FUTEX_WAITERS) | FUTEX_OWNER_DIED;
+        if let Err(cur_val) =
+            futex_val.compare_exchange(old_val, new_val, Ordering::SeqCst, Ordering::SeqCst)
+        {
+            // The futex value has changed, let's retry with current value
+            old_val = cur_val;
+            continue;
+        }
+        // Wakeup one waiter
+        if futex_val.load(Ordering::SeqCst) & FUTEX_WAITERS != 0 {
+            debug!("wake robust futex addr: {:?}", futex_addr);
+            super::do_futex::futex_wake(futex_addr, 1)?;
+        }
+        break;
+    }
+    Ok(())
+}

src/libos/src/process/mod.rs

@@ -22,6 +22,7 @@ use self::wait::{WaitQueue, Waiter};
 
 pub use self::do_exit::handle_force_exit;
 pub use self::do_futex::{futex_wait, futex_wake};
+pub use self::do_robust_list::RobustListHead;
 pub use self::do_spawn::do_spawn_without_exec;
 pub use self::process::{Process, ProcessFilter, ProcessStatus, IDLE};
 pub use self::spawn_attribute::posix_spawnattr_t;
@@ -36,6 +37,7 @@ mod do_exec;
 mod do_exit;
 mod do_futex;
 mod do_getpid;
+mod do_robust_list;
 mod do_set_tid_address;
 mod do_spawn;
 mod do_wait4;

src/libos/src/process/syscalls.rs

@@ -2,6 +2,7 @@ use super::do_arch_prctl::ArchPrctlCode;
 use super::do_clone::CloneFlags;
 use super::do_exec::do_exec;
 use super::do_futex::{FutexFlags, FutexOp, FutexTimeout};
+use super::do_robust_list::RobustListHead;
 use super::do_spawn::FileAction;
 use super::do_wait4::WaitOptions;
 use super::prctl::PrctlCmd;
@@ -446,3 +447,26 @@ pub fn do_execve(path: *const i8, argv: *const *const i8, envp: *const *const i8
 
     do_exec(&path, &argv, &envp, &current)
 }
+
+pub fn do_set_robust_list(list_head_ptr: *mut RobustListHead, len: usize) -> Result<isize> {
+    if !list_head_ptr.is_null() {
+        check_mut_ptr(list_head_ptr)?;
+    }
+    super::do_robust_list::do_set_robust_list(list_head_ptr, len)?;
+    Ok(0)
+}
+
+pub fn do_get_robust_list(
+    tid: pid_t,
+    list_head_ptr_ptr: *mut *mut RobustListHead,
+    len_ptr: *mut usize,
+) -> Result<isize> {
+    check_mut_ptr(list_head_ptr_ptr)?;
+    check_mut_ptr(len_ptr)?;
+    let list_head_ptr = super::do_robust_list::do_get_robust_list(tid)?;
+    unsafe {
+        list_head_ptr_ptr.write(list_head_ptr);
+        len_ptr.write(std::mem::size_of::<RobustListHead>());
+    }
+    Ok(0)
+}

src/libos/src/process/thread/builder.rs

@@ -1,8 +1,9 @@
 use std::ptr::NonNull;
 
 use super::{
-    FileTableRef, FsViewRef, ProcessRef, ProcessVM, ProcessVMRef, ResourceLimitsRef, SchedAgentRef,
-    SigQueues, SigSet, Task, Thread, ThreadId, ThreadInner, ThreadName, ThreadRef,
+    FileTableRef, FsViewRef, ProcessRef, ProcessVM, ProcessVMRef, ResourceLimitsRef,
+    RobustListHead, SchedAgentRef, SigQueues, SigSet, Task, Thread, ThreadId, ThreadInner,
+    ThreadName, ThreadRef,
 };
 use crate::events::HostEventFd;
 use crate::prelude::*;
@@ -22,6 +23,7 @@ pub struct ThreadBuilder {
     rlimits: Option<ResourceLimitsRef>,
     sig_mask: Option<SigSet>,
     clear_ctid: Option<NonNull<pid_t>>,
+    robust_list: Option<NonNull<RobustListHead>>,
     name: Option<ThreadName>,
 }
 
@@ -38,6 +40,7 @@ impl ThreadBuilder {
             rlimits: None,
             sig_mask: None,
             clear_ctid: None,
+            robust_list: None,
             name: None,
         }
     }
@@ -92,6 +95,11 @@ impl ThreadBuilder {
         self
     }
 
+    pub fn robust_list(mut self, robust_list_addr: NonNull<RobustListHead>) -> Self {
+        self.robust_list = Some(robust_list_addr);
+        self
+    }
+
     pub fn name(mut self, name: ThreadName) -> Self {
         self.name = Some(name);
         self
@@ -103,6 +111,7 @@ impl ThreadBuilder {
             .ok_or_else(|| errno!(EINVAL, "task is mandatory"))?;
         let tid = self.tid.unwrap_or_else(|| ThreadId::new());
         let clear_ctid = RwLock::new(self.clear_ctid);
+        let robust_list = RwLock::new(self.robust_list);
         let inner = SgxMutex::new(ThreadInner::new());
         let process = self
             .process
@@ -130,6 +139,7 @@ impl ThreadBuilder {
             task,
             tid,
             clear_ctid,
+            robust_list,
             inner,
             process,
             vm,

src/libos/src/process/thread/mod.rs

@@ -4,7 +4,7 @@ use std::ptr::NonNull;
 use super::task::Task;
 use super::{
     FileTableRef, ForcedExitStatus, FsViewRef, ProcessRef, ProcessVM, ProcessVMRef,
-    ResourceLimitsRef, SchedAgentRef, TermStatus, ThreadRef,
+    ResourceLimitsRef, RobustListHead, SchedAgentRef, TermStatus, ThreadRef,
 };
 use crate::events::HostEventFd;
 use crate::fs::{EventCreationFlags, EventFile};
@@ -28,6 +28,7 @@ pub struct Thread {
     tid: ThreadId,
     // Mutable info
     clear_ctid: RwLock<Option<NonNull<pid_t>>>,
+    robust_list: RwLock<Option<NonNull<RobustListHead>>>,
     inner: SgxMutex<ThreadInner>,
     name: RwLock<ThreadName>,
     // Process
@@ -139,6 +140,37 @@ impl Thread {
         *self.clear_ctid.write().unwrap() = new_clear_ctid;
     }
 
+    pub fn robust_list(&self) -> Option<NonNull<RobustListHead>> {
+        *self.robust_list.read().unwrap()
+    }
+
+    pub fn set_robust_list(&self, new_robust_list: Option<NonNull<RobustListHead>>) {
+        *self.robust_list.write().unwrap() = new_robust_list;
+    }
+
+    /// Walks the robust futex list, marking futex dead and wake waiters.
+    /// It corresponds to Linux's exit_robust_list(), errors are silently ignored.
+    pub fn wake_robust_list(&self) {
+        let list_head_ptr = match self.robust_list() {
+            None => {
+                return;
+            }
+            Some(robust_list) => robust_list.as_ptr(),
+        };
+        debug!("wake the rubust_list: {:?}", list_head_ptr);
+        let robust_list = {
+            // Invalid pointer, stop scanning the list further
+            if crate::util::mem_util::from_user::check_ptr(list_head_ptr).is_err() {
+                return;
+            }
+            unsafe { &*list_head_ptr }
+        };
+        for futex_addr in robust_list.futexes() {
+            super::do_robust_list::wake_robust_futex(futex_addr, self.tid());
+        }
+        self.set_robust_list(None);
+    }
+
     pub fn name(&self) -> ThreadName {
         self.name.read().unwrap().clone()
     }

src/libos/src/syscall/mod.rs

@@ -41,10 +41,11 @@ use crate::net::{
     do_shutdown, do_socket, do_socketpair, mmsghdr, msghdr, msghdr_mut,
 };
 use crate::process::{
-    do_arch_prctl, do_clone, do_execve, do_exit, do_exit_group, do_futex, do_getegid, do_geteuid,
-    do_getgid, do_getgroups, do_getpgid, do_getpid, do_getppid, do_gettid, do_getuid, do_prctl,
-    do_set_tid_address, do_spawn_for_glibc, do_spawn_for_musl, do_wait4, pid_t, posix_spawnattr_t,
-    FdOp, SpawnFileActions, ThreadStatus,
+    do_arch_prctl, do_clone, do_execve, do_exit, do_exit_group, do_futex, do_get_robust_list,
+    do_getegid, do_geteuid, do_getgid, do_getgroups, do_getpgid, do_getpid, do_getppid, do_gettid,
+    do_getuid, do_prctl, do_set_robust_list, do_set_tid_address, do_spawn_for_glibc,
+    do_spawn_for_musl, do_wait4, pid_t, posix_spawnattr_t, FdOp, RobustListHead, SpawnFileActions,
+    ThreadStatus,
 };
 use crate::sched::{do_getcpu, do_sched_getaffinity, do_sched_setaffinity, do_sched_yield};
 use crate::signal::{
@@ -359,8 +360,8 @@ macro_rules! process_syscall_table_with_callback {
             (Pselect6 = 270) => handle_unsupported(),
             (Ppoll = 271) => handle_unsupported(),
             (Unshare = 272) => handle_unsupported(),
-            (SetRobustList = 273) => handle_unsupported(),
-            (GetRobustList = 274) => handle_unsupported(),
+            (SetRobustList = 273) => do_set_robust_list(list_head_ptr: *mut RobustListHead, len: usize),
+            (GetRobustList = 274) => do_get_robust_list(tid: pid_t, list_head_ptr_ptr: *mut *mut RobustListHead, len_ptr: *mut usize),
             (Splice = 275) => handle_unsupported(),
             (Tee = 276) => handle_unsupported(),
             (SyncFileRange = 277) => handle_unsupported(),

test/pthread/main.c

@@ -84,6 +84,83 @@ static int test_mutex_with_concurrent_counter(void) {
     return 0;
 }
 
+// ============================================================================
+// The test case of robust mutex
+// ============================================================================
+
+struct thread_robust_arg {
+    int                         ti;
+    volatile int                *global_count;
+    pthread_mutex_t             *mutex;
+};
+
+int ret_err = -1;
+
+static void *thread_worker(void *_arg) {
+    struct thread_robust_arg *arg = _arg;
+    int err = pthread_mutex_lock(arg->mutex);
+    if (err == EOWNERDEAD) {
+        // The mutex is locked by the thread here, but the state is marked as
+        // inconsistent, the thread should call 'pthread_mutex_consistent' to
+        // make the mutex consistent again.
+        if (pthread_mutex_consistent(arg->mutex) != 0) {
+            printf("ERROR: failed to recover the mutex\n");
+            return &ret_err;
+        }
+    } else if (err != 0) {
+        printf("ERROR: failed to lock the mutex with error: %d\n", err);
+        return &ret_err;
+    }
+    // Mutex is locked
+    (*arg->global_count)++;
+    // Wait for other threads to acquire the lock
+    sleep(1);
+    // Exit without unlocking the mutex, this will makes the mutex in an
+    // inconsistent state.
+    return NULL;
+}
+
+static int test_robust_mutex_with_concurrent_counter(void) {
+    volatile int global_count = 0;
+    pthread_t threads[NTHREADS];
+    struct thread_robust_arg thread_args[NTHREADS];
+    // Init robust mutex
+    pthread_mutex_t mutex;
+    pthread_mutexattr_t attr;
+    pthread_mutexattr_init(&attr);
+    pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST);
+    pthread_mutex_init(&mutex, &attr);
+    // Start the threads
+    for (int ti = 0; ti < NTHREADS; ti++) {
+        struct thread_robust_arg *thread_arg = &thread_args[ti];
+        thread_arg->ti = ti;
+        thread_arg->global_count = &global_count;
+        thread_arg->mutex = &mutex;
+
+        if (pthread_create(&threads[ti], NULL, thread_worker, thread_arg) < 0) {
+            THROW_ERROR("pthread_create failed (ti = %d)", ti);
+        }
+    }
+    // Wait for the threads to finish
+    for (int ti = 0; ti < NTHREADS; ti++) {
+        int *ret_val;
+        if (pthread_join(threads[ti], (void **)&ret_val) < 0) {
+            THROW_ERROR("pthread_join failed (ti = %d)", ti);
+        }
+        if (ret_val && *ret_val != 0) {
+            THROW_ERROR("run thread failed (ti = %d) with return val: %d", ti, *ret_val);
+        }
+    }
+    // Check the result
+    if (global_count != NTHREADS) {
+        THROW_ERROR("incorrect global_count (actual = %d, expected = %d)", global_count,
+                    NTHREADS);
+    }
+
+    pthread_mutex_destroy(&mutex);
+    return 0;
+}
+
 // ============================================================================
 // The test case of waiting condition variable
 // ============================================================================
@@ -189,6 +266,7 @@ static int test_mutex_timedlock() {
 
 static test_case_t test_cases[] = {
     TEST_CASE(test_mutex_with_concurrent_counter),
+    TEST_CASE(test_robust_mutex_with_concurrent_counter),
     TEST_CASE(test_mutex_with_cond_wait),
     TEST_CASE(test_mutex_timedlock),
 };