2015-05-28 Roland McGrath <roland@hack.frob.com>
+ * sysdeps/nacl/exit-thread.h (__exit_thread): If not detached,
+ set THREAD_SELF->tid to a magic value and futex-wake it.
+ Pass its address to the thread_exit system call.
+ * sysdeps/nacl/pthread-pids.h (__nacl_get_tid): Assert that TID's low
+ bit is clear.
+ * sysdeps/nacl/lowlevellock.h: New file.
+ * sysdeps/nacl/lll_timedwait_tid.c: New file.
+
* sysdeps/nacl/lowlevellock-futex.h (lll_futex_timed_wait):
Add TIMEOUT to current time, don't subtract it.
License along with the GNU C Library; if not, see
<http://www.gnu.org/licenses/>. */
-#include <stddef.h>
+#include <assert.h>
+#include <atomic.h>
+#include <lowlevellock.h>
#include <nacl-interfaces.h>
+#include <nptl/pthreadP.h>
/* This causes the current thread to exit, without affecting other
threads in the process if there are any. If there are no other
static inline void __attribute__ ((noreturn, always_inline, unused))
__exit_thread (void)
{
- __nacl_irt_thread.thread_exit (NULL);
+ struct pthread *pd = THREAD_SELF;
+
+ /* The generic logic for pthread_join and stack/descriptor reuse is
+ based on the Linux kernel feature that will clear and futex-wake
+ a designated address as a final part of thread teardown. Correct
+ synchronization relies on the fact that these happen only after
+ there is no possibility of user code touching or examining the
+ late thread's stack.
+
+ The NaCl system interface implements half of this: it clears a
+ word after the thread's user stack is safely dead, but it does
+ not futex-wake the location. So, some shenanigans are required.
+ We change and futex-wake the location here, so as to wake up any
+ blocked pthread_join (i.e. lll_wait_tid) or pthread_timedjoin_np
+ (i.e. lll_timedwait_tid). However, that's before we have safely
+ vacated the stack. So instead of clearing the location, we set
+ it to a special magic value, NACL_EXITING_TID. This counts as a
+ "live thread" value for all the generic logic, but is recognized
+ specially in lll_wait_tid and lll_timedwait_tid (lowlevellock.h).
+ Once it has this value, lll_wait_tid will busy-wait for the
+ location to be cleared to zero by the NaCl system code. Only then
+ is the stack actually safe to reuse. */
+
+ if (!IS_DETACHED (pd))
+ {
+ /* The magic value must not be one that could ever be a valid
+ TID value. See pthread-pids.h about the low bit. */
+ assert (NACL_EXITING_TID & 1);
+
+ /* The magic value must not be one that has the "free" flag
+ (i.e. sign bit) set. If that bit is set, then the
+ descriptor could be reused for a new thread. */
+ assert (NACL_EXITING_TID > 0);
+
+ atomic_store_relaxed (&pd->tid, NACL_EXITING_TID);
+ lll_futex_wake (&pd->tid, 1, LLL_PRIVATE);
+ }
+
+ /* This clears PD->tid some time after the thread stack can never
+ be touched again. Unfortunately, it does not also do a
+ futex-wake at that time (as Linux does via CLONE_CHILD_CLEARTID
+ and set_tid_address). So lll_wait_tid does some busy-waiting. */
+ __nacl_irt_thread.thread_exit (&pd->tid);
/* That never returns unless something is severely and unrecoverably wrong.
If it ever does, try to make sure we crash. */
--- /dev/null
+/* Timed waiting for thread death. NaCl version.
+ Copyright (C) 2015 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+#include <assert.h>
+#include <atomic.h>
+#include <errno.h>
+#include <lowlevellock.h>
+#include <sys/time.h>
+
+int
+__lll_timedwait_tid (int *tidp, const struct timespec *abstime)
+{
+ /* Reject invalid timeouts. */
+ if (__glibc_unlikely (abstime->tv_nsec < 0)
+ || __glibc_unlikely (abstime->tv_nsec >= 1000000000))
+ return EINVAL;
+
+ /* Repeat until thread terminated. */
+ int tid;
+ while ((tid = atomic_load_relaxed (tidp)) != 0)
+ {
+ /* See exit-thread.h for details. */
+ if (tid == NACL_EXITING_TID)
+ /* The thread should now be in the process of exiting, so it will
+ finish quick enough that the timeout doesn't matter. If any
+ thread ever stays in this state for long, there is something
+ catastrophically wrong. */
+ BUSY_WAIT_NOP;
+ else
+ {
+ assert (tid > 0);
+
+ /* If *FUTEX == TID, wait until woken or timeout. */
+ int err = __nacl_irt_futex.futex_wait_abs ((volatile int *) tidp,
+ tid, abstime);
+ if (err != 0)
+ {
+ if (__glibc_likely (err == ETIMEDOUT))
+ return err;
+ assert (err == EAGAIN);
+ }
+ }
+ }
+
+ return 0;
+}
--- /dev/null
+/* Low-level lock implementation. NaCl version.
+ Copyright (C) 2015 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library. If not, see
+ <http://www.gnu.org/licenses/>. */
+
+#ifndef _LOWLEVELLOCK_H
+
+/* Everything except the exit handling is the same as the generic code. */
+# include <sysdeps/nptl/lowlevellock.h>
+
+# ifndef BUSY_WAIT_NOP
+# define BUSY_WAIT_NOP __sync_synchronize ()
+# endif
+
+/* See exit-thread.h for details. */
+# define NACL_EXITING_TID 1
+
+# undef lll_wait_tid
+# define lll_wait_tid(tid) \
+ do { \
+ __typeof (tid) __tid; \
+ volatile __typeof (tid) *__tidp = &(tid); \
+ while ((__tid = atomic_load_relaxed (__tidp)) != 0) \
+ { \
+ if (__tid == NACL_EXITING_TID) \
+ BUSY_WAIT_NOP; \
+ else \
+ lll_futex_wait (__tidp, __tid, LLL_PRIVATE); \
+ } \
+ } while (0)
+
+#endif /* lowlevellock.h */
assert ((id & 1) == 0);
assert (sizeof id == sizeof tid);
assert (tid > 0);
+ /* This ensures that NACL_EXITING_TID (lowlevellock.h) can never
+ be a valid TID value. */
+ assert ((tid & 1) == 0);
return tid;
}
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