1 /*
   2  * Copyright (c) 1999, 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "runtime/thread.inline.hpp"
  27 #include "runtime/threadLocalStorage.hpp"
  28 
  29 // Map stack pointer (%esp) to thread pointer for faster TLS access
  30 //
  31 // Here we use a flat table for better performance. Getting current thread
  32 // is down to one memory access (read _sp_map[%esp>>12]) in generated code
  33 // and two in runtime code (-fPIC code needs an extra load for _sp_map).
  34 //
  35 // This code assumes stack page is not shared by different threads. It works
  36 // in 32-bit VM when page size is 4K (or a multiple of 4K, if that matters).
  37 //
  38 // Notice that _sp_map is allocated in the bss segment, which is ZFOD
  39 // (zero-fill-on-demand). While it reserves 4M address space upfront,
  40 // actual memory pages are committed on demand.
  41 //
  42 // If an application creates and destroys a lot of threads, usually the
  43 // stack space freed by a thread will soon get reused by new thread.
  44 // No memory page in _sp_map is wasted.
  45 //
  46 // However, it's still possible that we might end up populating &
  47 // committing a large fraction of the 4M table over time, but the actual
  48 // amount of live data in the table could be quite small. The max wastage
  49 // is less than 4M bytes. If it becomes an issue, we could use madvise()
  50 // with MADV_DONTNEED to reclaim unused (i.e. all-zero) pages in _sp_map.
  51 // MADV_DONTNEED on Linux keeps the virtual memory mapping, but zaps the
  52 // physical memory page (i.e. similar to MADV_FREE on Solaris).
  53 
  54 #if !defined(AMD64) && !defined(MINIMIZE_RAM_USAGE)
  55 Thread* ThreadLocalStorage::_sp_map[1UL << (SP_BITLENGTH - PAGE_SHIFT)];
  56 
  57 void ThreadLocalStorage::generate_code_for_get_thread() {
  58     // nothing we can do here for user-level thread
  59 }
  60 
  61 void ThreadLocalStorage::pd_init() {
  62   assert(align_size_down(os::vm_page_size(), PAGE_SIZE) == os::vm_page_size(),
  63          "page size must be multiple of PAGE_SIZE");
  64 }
  65 
  66 void ThreadLocalStorage::pd_set_thread(Thread* thread) {
  67   os::thread_local_storage_at_put(ThreadLocalStorage::thread_index(), thread);
  68   address stack_top = os::current_stack_base();
  69   size_t stack_size = os::current_stack_size();
  70 
  71   for (address p = stack_top - stack_size; p < stack_top; p += PAGE_SIZE) {
  72     // pd_set_thread() is called with non-NULL value when a new thread is
  73     // created/attached, or with NULL value when a thread is about to exit.
  74     // If both "thread" and the corresponding _sp_map[] entry are non-NULL,
  75     // they should have the same value. Otherwise it might indicate that the
  76     // stack page is shared by multiple threads. However, a more likely cause
  77     // for this assertion to fail is that an attached thread exited without
  78     // detaching itself from VM, which is a program error and could cause VM
  79     // to crash.
  80     assert(thread == NULL || _sp_map[(uintptr_t)p >> PAGE_SHIFT] == NULL ||
  81            thread == _sp_map[(uintptr_t)p >> PAGE_SHIFT],
  82            "thread exited without detaching from VM??");
  83     _sp_map[(uintptr_t)p >> PAGE_SHIFT] = thread;
  84   }
  85 }
  86 #else
  87 
  88 void ThreadLocalStorage::generate_code_for_get_thread() {
  89     // nothing we can do here for user-level thread
  90 }
  91 
  92 void ThreadLocalStorage::pd_init() {
  93 }
  94 
  95 void ThreadLocalStorage::pd_set_thread(Thread* thread) {
  96   os::thread_local_storage_at_put(ThreadLocalStorage::thread_index(), thread);
  97 }
  98 #endif // !AMD64 && !MINIMIZE_RAM_USAGE