src/libstd/sys/unix/fast_thread_local.rs

   1 // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 #![cfg(target_thread_local)]
  12 #![unstable(feature = "thread_local_internals", issue = "0")]
  13
  14 use cell::{Cell, UnsafeCell};
  15 use intrinsics;
  16 use ptr;
  17
  18 pub struct Key<T> {
  19     inner: UnsafeCell<Option<T>>,
  20
  21     // Metadata to keep track of the state of the destructor. Remember that
  22     // these variables are thread-local, not global.
  23     dtor_registered: Cell<bool>,
  24     dtor_running: Cell<bool>,
  25 }
  26
  27 unsafe impl<T> ::marker::Sync for Key<T> { }
  28
  29 impl<T> Key<T> {
  30     pub const fn new() -> Key<T> {
  31         Key {
  32             inner: UnsafeCell::new(None),
  33             dtor_registered: Cell::new(false),
  34             dtor_running: Cell::new(false)
  35         }
  36     }
  37
  38     pub fn get(&'static self) -> Option<&'static UnsafeCell<Option<T>>> {
  39         unsafe {
  40             if intrinsics::needs_drop::<T>() && self.dtor_running.get() {
  41                 return None
  42             }
  43             self.register_dtor();
  44         }
  45         Some(&self.inner)
  46     }
  47
  48     unsafe fn register_dtor(&self) {
  49         if !intrinsics::needs_drop::<T>() || self.dtor_registered.get() {
  50             return
  51         }
  52
  53         register_dtor(self as *const _ as *mut u8,
  54                       destroy_value::<T>);
  55         self.dtor_registered.set(true);
  56     }
  57 }
  58
  59 #[cfg(any(target_os = "linux", target_os = "fuchsia"))]
  60 unsafe fn register_dtor_fallback(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
  61     // The fallback implementation uses a vanilla OS-based TLS key to track
  62     // the list of destructors that need to be run for this thread. The key
  63     // then has its own destructor which runs all the other destructors.
  64     //
  65     // The destructor for DTORS is a little special in that it has a `while`
  66     // loop to continuously drain the list of registered destructors. It
  67     // *should* be the case that this loop always terminates because we
  68     // provide the guarantee that a TLS key cannot be set after it is
  69     // flagged for destruction.
  70     use sys_common::thread_local as os;
  71
  72     static DTORS: os::StaticKey = os::StaticKey::new(Some(run_dtors));
  73     type List = Vec<(*mut u8, unsafe extern fn(*mut u8))>;
  74     if DTORS.get().is_null() {
  75         let v: Box<List> = box Vec::new();
  76         DTORS.set(Box::into_raw(v) as *mut u8);
  77     }
  78     let list: &mut List = &mut *(DTORS.get() as *mut List);
  79     list.push((t, dtor));
  80
  81     unsafe extern fn run_dtors(mut ptr: *mut u8) {
  82         while !ptr.is_null() {
  83             let list: Box<List> = Box::from_raw(ptr as *mut List);
  84             for &(ptr, dtor) in list.iter() {
  85                 dtor(ptr);
  86             }
  87             ptr = DTORS.get();
  88             DTORS.set(ptr::null_mut());
  89         }
  90     }
  91 }
  92
  93 // Since what appears to be glibc 2.18 this symbol has been shipped which
  94 // GCC and clang both use to invoke destructors in thread_local globals, so
  95 // let's do the same!
  96 //
  97 // Note, however, that we run on lots older linuxes, as well as cross
  98 // compiling from a newer linux to an older linux, so we also have a
  99 // fallback implementation to use as well.
 100 //
 101 // Due to rust-lang/rust#18804, make sure this is not generic!
 102 #[cfg(target_os = "linux")]
 103 unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
 104     use mem;
 105     use libc;
 106
 107     extern {
 108         #[linkage = "extern_weak"]
 109         static __dso_handle: *mut u8;
 110         #[linkage = "extern_weak"]
 111         static __cxa_thread_atexit_impl: *const libc::c_void;
 112     }
 113     if !__cxa_thread_atexit_impl.is_null() {
 114         type F = unsafe extern fn(dtor: unsafe extern fn(*mut u8),
 115                                   arg: *mut u8,
 116                                   dso_handle: *mut u8) -> libc::c_int;
 117         mem::transmute::<*const libc::c_void, F>(__cxa_thread_atexit_impl)
 118             (dtor, t, &__dso_handle as *const _ as *mut _);
 119         return
 120     }
 121     register_dtor_fallback(t, dtor);
 122 }
 123
 124 // OSX's analog of the above linux function is this _tlv_atexit function.
 125 // The disassembly of thread_local globals in C++ (at least produced by
 126 // clang) will have this show up in the output.
 127 #[cfg(target_os = "macos")]
 128 unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
 129     extern {
 130         fn _tlv_atexit(dtor: unsafe extern fn(*mut u8),
 131                        arg: *mut u8);
 132     }
 133     _tlv_atexit(dtor, t);
 134 }
 135
 136 // Just use the thread_local fallback implementation, at least until there's
 137 // a more direct implementation.
 138 #[cfg(target_os = "fuchsia")]
 139 unsafe fn register_dtor(t: *mut u8, dtor: unsafe extern fn(*mut u8)) {
 140     register_dtor_fallback(t, dtor);
 141 }
 142
 143 pub unsafe extern fn destroy_value<T>(ptr: *mut u8) {
 144     let ptr = ptr as *mut Key<T>;
 145     // Right before we run the user destructor be sure to flag the
 146     // destructor as running for this thread so calls to `get` will return
 147     // `None`.
 148     (*ptr).dtor_running.set(true);
 149
 150     // The OSX implementation of TLS apparently had an odd aspect to it
 151     // where the pointer we have may be overwritten while this destructor
 152     // is running. Specifically if a TLS destructor re-accesses TLS it may
 153     // trigger a re-initialization of all TLS variables, paving over at
 154     // least some destroyed ones with initial values.
 155     //
 156     // This means that if we drop a TLS value in place on OSX that we could
 157     // revert the value to its original state halfway through the
 158     // destructor, which would be bad!
 159     //
 160     // Hence, we use `ptr::read` on OSX (to move to a "safe" location)
 161     // instead of drop_in_place.
 162     if cfg!(target_os = "macos") {
 163         ptr::read((*ptr).inner.get());
 164     } else {
 165         ptr::drop_in_place((*ptr).inner.get());
 166     }
 167 }