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a2f0363f81
This adds new functions for futex operations, starting with wait, abstimed_wait, reltimed_wait, wake. They add documentation and error checking according to the current draft of the Linux kernel futex manpage. Waiting with absolute or relative timeouts is split into separate functions. This allows for removing a few cases of code duplication in pthreads code, which uses absolute timeouts; also, it allows us to put platform-specific code to go from an absolute to a relative timeout into the platform-specific futex abstractions.. Futex operations that can be canceled are also split out into separate functions suffixed by "_cancelable". There are separate versions for both Linux and NaCl; while they currently differ only slightly, my expectation is that the separate versions of lowlevellock-futex.h will eventually be merged into futex-internal.h when we get to move the lll_ functions over to the new futex API.
81 lines
3.5 KiB
C
81 lines
3.5 KiB
C
/* Call to terminate the current thread. NaCl version.
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Copyright (C) 2015 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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#include <assert.h>
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#include <atomic.h>
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#include <futex-internal.h>
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#include <nacl-interfaces.h>
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#include <nptl/pthreadP.h>
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/* This causes the current thread to exit, without affecting other
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threads in the process if there are any. If there are no other
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threads left, then this has the effect of _exit (0). */
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static inline void __attribute__ ((noreturn, always_inline, unused))
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__exit_thread (void)
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{
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struct pthread *pd = THREAD_SELF;
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/* The generic logic for pthread_join and stack/descriptor reuse is
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based on the Linux kernel feature that will clear and futex-wake
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a designated address as a final part of thread teardown. Correct
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synchronization relies on the fact that these happen only after
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there is no possibility of user code touching or examining the
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late thread's stack.
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The NaCl system interface implements half of this: it clears a
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word after the thread's user stack is safely dead, but it does
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not futex-wake the location. So, some shenanigans are required.
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We change and futex-wake the location here, so as to wake up any
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blocked pthread_join (i.e. lll_wait_tid) or pthread_timedjoin_np
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(i.e. lll_timedwait_tid). However, that's before we have safely
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vacated the stack. So instead of clearing the location, we set
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it to a special magic value, NACL_EXITING_TID. This counts as a
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"live thread" value for all the generic logic, but is recognized
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specially in lll_wait_tid and lll_timedwait_tid (lowlevellock.h).
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Once it has this value, lll_wait_tid will busy-wait for the
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location to be cleared to zero by the NaCl system code. Only then
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is the stack actually safe to reuse. */
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if (!IS_DETACHED (pd))
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{
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/* The magic value must not be one that could ever be a valid
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TID value. See pthread-pids.h about the low bit. */
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assert (NACL_EXITING_TID & 1);
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/* The magic value must not be one that has the "free" flag
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(i.e. sign bit) set. If that bit is set, then the
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descriptor could be reused for a new thread. */
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assert (NACL_EXITING_TID > 0);
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atomic_store_relaxed (&pd->tid, NACL_EXITING_TID);
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futex_wake ((unsigned int *) &pd->tid, 1, FUTEX_PRIVATE);
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}
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/* This clears PD->tid some time after the thread stack can never
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be touched again. Unfortunately, it does not also do a
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futex-wake at that time (as Linux does via CLONE_CHILD_CLEARTID
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and set_tid_address). So lll_wait_tid does some busy-waiting. */
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__nacl_irt_thread.thread_exit (&pd->tid);
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/* That never returns unless something is severely and unrecoverably wrong.
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If it ever does, try to make sure we crash. */
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while (1)
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__builtin_trap ();
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}
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