| xf.li | bdd93d5 | 2023-05-12 07:10:14 -0700 | [diff] [blame] | 1 | /* Call to terminate the current thread. NaCl version. |
| 2 | Copyright (C) 2015-2016 Free Software Foundation, Inc. |
| 3 | This file is part of the GNU C Library. |
| 4 | |
| 5 | The GNU C Library is free software; you can redistribute it and/or |
| 6 | modify it under the terms of the GNU Lesser General Public |
| 7 | License as published by the Free Software Foundation; either |
| 8 | version 2.1 of the License, or (at your option) any later version. |
| 9 | |
| 10 | The GNU C Library is distributed in the hope that it will be useful, |
| 11 | but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU |
| 13 | Lesser General Public License for more details. |
| 14 | |
| 15 | You should have received a copy of the GNU Lesser General Public |
| 16 | License along with the GNU C Library; if not, see |
| 17 | <http://www.gnu.org/licenses/>. */ |
| 18 | |
| 19 | #include <assert.h> |
| 20 | #include <atomic.h> |
| 21 | #include <futex-internal.h> |
| 22 | #include <nacl-interfaces.h> |
| 23 | #include <nptl/pthreadP.h> |
| 24 | |
| 25 | /* This causes the current thread to exit, without affecting other |
| 26 | threads in the process if there are any. If there are no other |
| 27 | threads left, then this has the effect of _exit (0). */ |
| 28 | |
| 29 | static inline void __attribute__ ((noreturn, always_inline, unused)) |
| 30 | __exit_thread (void) |
| 31 | { |
| 32 | struct pthread *pd = THREAD_SELF; |
| 33 | |
| 34 | /* The generic logic for pthread_join and stack/descriptor reuse is |
| 35 | based on the Linux kernel feature that will clear and futex-wake |
| 36 | a designated address as a final part of thread teardown. Correct |
| 37 | synchronization relies on the fact that these happen only after |
| 38 | there is no possibility of user code touching or examining the |
| 39 | late thread's stack. |
| 40 | |
| 41 | The NaCl system interface implements half of this: it clears a |
| 42 | word after the thread's user stack is safely dead, but it does |
| 43 | not futex-wake the location. So, some shenanigans are required. |
| 44 | We change and futex-wake the location here, so as to wake up any |
| 45 | blocked pthread_join (i.e. lll_wait_tid) or pthread_timedjoin_np |
| 46 | (i.e. lll_timedwait_tid). However, that's before we have safely |
| 47 | vacated the stack. So instead of clearing the location, we set |
| 48 | it to a special magic value, NACL_EXITING_TID. This counts as a |
| 49 | "live thread" value for all the generic logic, but is recognized |
| 50 | specially in lll_wait_tid and lll_timedwait_tid (lowlevellock.h). |
| 51 | Once it has this value, lll_wait_tid will busy-wait for the |
| 52 | location to be cleared to zero by the NaCl system code. Only then |
| 53 | is the stack actually safe to reuse. */ |
| 54 | |
| 55 | if (!IS_DETACHED (pd)) |
| 56 | { |
| 57 | /* The magic value must not be one that could ever be a valid |
| 58 | TID value. See pthread-pids.h about the low bit. */ |
| 59 | assert (NACL_EXITING_TID & 1); |
| 60 | |
| 61 | /* The magic value must not be one that has the "free" flag |
| 62 | (i.e. sign bit) set. If that bit is set, then the |
| 63 | descriptor could be reused for a new thread. */ |
| 64 | assert (NACL_EXITING_TID > 0); |
| 65 | |
| 66 | atomic_store_relaxed (&pd->tid, NACL_EXITING_TID); |
| 67 | futex_wake ((unsigned int *) &pd->tid, 1, FUTEX_PRIVATE); |
| 68 | } |
| 69 | |
| 70 | /* This clears PD->tid some time after the thread stack can never |
| 71 | be touched again. Unfortunately, it does not also do a |
| 72 | futex-wake at that time (as Linux does via CLONE_CHILD_CLEARTID |
| 73 | and set_tid_address). So lll_wait_tid does some busy-waiting. */ |
| 74 | __nacl_irt_thread.thread_exit (&pd->tid); |
| 75 | |
| 76 | /* That never returns unless something is severely and unrecoverably wrong. |
| 77 | If it ever does, try to make sure we crash. */ |
| 78 | while (1) |
| 79 | __builtin_trap (); |
| 80 | } |