std: xous: add thread_local_key

Add an implementation of thread local storage. This uses a container
that is pointed to by the otherwise-unsed `$tp` register. This container
is allocated on-demand, so threads that use no TLS will not allocate
this extra memory.

Signed-off-by: Sean Cross <sean@xobs.io>

library/std/src/sys/xous/mod.rs

@@ -28,7 +28,6 @@ pub mod pipe;
 pub mod process;
 pub mod stdio;
 pub mod thread;
-#[path = "../unsupported/thread_local_key.rs"]
 pub mod thread_local_key;
 #[path = "../unsupported/thread_parking.rs"]
 pub mod thread_parking;

library/std/src/sys/xous/thread_local_key.rs

@@ -0,0 +1,190 @@
+use crate::mem::ManuallyDrop;
+use crate::ptr;
+use crate::sync::atomic::AtomicPtr;
+use crate::sync::atomic::AtomicUsize;
+use crate::sync::atomic::Ordering::SeqCst;
+use core::arch::asm;
+
+use crate::os::xous::ffi::{map_memory, unmap_memory, MemoryFlags};
+
+/// Thread Local Storage
+///
+/// Currently, we are limited to 1023 TLS entries. The entries
+/// live in a page of memory that's unique per-process, and is
+/// stored in the `$tp` register. If this register is 0, then
+/// TLS has not been initialized and thread cleanup can be skipped.
+///
+/// The index into this register is the `key`. This key is identical
+/// between all threads, but indexes a different offset within this
+/// pointer.
+pub type Key = usize;
+
+pub type Dtor = unsafe extern "C" fn(*mut u8);
+
+const TLS_MEMORY_SIZE: usize = 4096;
+
+/// TLS keys start at `1` to mimic POSIX.
+static TLS_KEY_INDEX: AtomicUsize = AtomicUsize::new(1);
+
+fn tls_ptr_addr() -> *mut usize {
+    let mut tp: usize;
+    unsafe {
+        asm!(
+            "mv {}, tp",
+            out(reg) tp,
+        );
+    }
+    core::ptr::from_exposed_addr_mut::<usize>(tp)
+}
+
+/// Create an area of memory that's unique per thread. This area will
+/// contain all thread local pointers.
+fn tls_ptr() -> *mut usize {
+    let mut tp = tls_ptr_addr();
+
+    // If the TP register is `0`, then this thread hasn't initialized
+    // its TLS yet. Allocate a new page to store this memory.
+    if tp.is_null() {
+        tp = unsafe {
+            map_memory(
+                None,
+                None,
+                TLS_MEMORY_SIZE / core::mem::size_of::<usize>(),
+                MemoryFlags::R | MemoryFlags::W,
+            )
+        }
+        .expect("Unable to allocate memory for thread local storage")
+        .as_mut_ptr();
+
+        unsafe {
+            // Key #0 is currently unused.
+            (tp).write_volatile(0);
+
+            // Set the thread's `$tp` register
+            asm!(
+                "mv tp, {}",
+                in(reg) tp as usize,
+            );
+        }
+    }
+    tp
+}
+
+/// Allocate a new TLS key. These keys are shared among all threads.
+fn tls_alloc() -> usize {
+    TLS_KEY_INDEX.fetch_add(1, SeqCst)
+}
+
+#[inline]
+pub unsafe fn create(dtor: Option<Dtor>) -> Key {
+    let key = tls_alloc();
+    if let Some(f) = dtor {
+        unsafe { register_dtor(key, f) };
+    }
+    key
+}
+
+#[inline]
+pub unsafe fn set(key: Key, value: *mut u8) {
+    assert!((key < 1022) && (key >= 1));
+    unsafe { tls_ptr().add(key).write_volatile(value as usize) };
+}
+
+#[inline]
+pub unsafe fn get(key: Key) -> *mut u8 {
+    assert!((key < 1022) && (key >= 1));
+    core::ptr::from_exposed_addr_mut::<u8>(unsafe { tls_ptr().add(key).read_volatile() })
+}
+
+#[inline]
+pub unsafe fn destroy(_key: Key) {
+    panic!("can't destroy keys on Xous");
+}
+
+// -------------------------------------------------------------------------
+// Dtor registration (stolen from Windows)
+//
+// Xous has no native support for running destructors so we manage our own
+// list of destructors to keep track of how to destroy keys. We then install a
+// callback later to get invoked whenever a thread exits, running all
+// appropriate destructors.
+//
+// Currently unregistration from this list is not supported. A destructor can be
+// registered but cannot be unregistered. There's various simplifying reasons
+// for doing this, the big ones being:
+//
+// 1. Currently we don't even support deallocating TLS keys, so normal operation
+//    doesn't need to deallocate a destructor.
+// 2. There is no point in time where we know we can unregister a destructor
+//    because it could always be getting run by some remote thread.
+//
+// Typically processes have a statically known set of TLS keys which is pretty
+// small, and we'd want to keep this memory alive for the whole process anyway
+// really.
+//
+// Perhaps one day we can fold the `Box` here into a static allocation,
+// expanding the `StaticKey` structure to contain not only a slot for the TLS
+// key but also a slot for the destructor queue on windows. An optimization for
+// another day!
+
+static DTORS: AtomicPtr<Node> = AtomicPtr::new(ptr::null_mut());
+
+struct Node {
+    dtor: Dtor,
+    key: Key,
+    next: *mut Node,
+}
+
+unsafe fn register_dtor(key: Key, dtor: Dtor) {
+    let mut node = ManuallyDrop::new(Box::new(Node { key, dtor, next: ptr::null_mut() }));
+
+    let mut head = DTORS.load(SeqCst);
+    loop {
+        node.next = head;
+        match DTORS.compare_exchange(head, &mut **node, SeqCst, SeqCst) {
+            Ok(_) => return, // nothing to drop, we successfully added the node to the list
+            Err(cur) => head = cur,
+        }
+    }
+}
+
+pub unsafe fn destroy_tls() {
+    let tp = tls_ptr_addr();
+
+    // If the pointer address is 0, then this thread has no TLS.
+    if tp.is_null() {
+        return;
+    }
+    unsafe { run_dtors() };
+
+    // Finally, free the TLS array
+    unsafe {
+        unmap_memory(core::slice::from_raw_parts_mut(
+            tp,
+            TLS_MEMORY_SIZE / core::mem::size_of::<usize>(),
+        ))
+        .unwrap()
+    };
+}
+
+unsafe fn run_dtors() {
+    let mut any_run = true;
+    for _ in 0..5 {
+        if !any_run {
+            break;
+        }
+        any_run = false;
+        let mut cur = DTORS.load(SeqCst);
+        while !cur.is_null() {
+            let ptr = unsafe { get((*cur).key) };
+
+            if !ptr.is_null() {
+                unsafe { set((*cur).key, ptr::null_mut()) };
+                unsafe { ((*cur).dtor)(ptr as *mut _) };
+                any_run = true;
+            }
+
+            unsafe { cur = (*cur).next };
+        }
+    }
+}