Add and use new glibc-internal futex API.

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.

1 files changed, 203 insertions, 0 deletions

diff --git a/sysdeps/nptl/futex-internal.h b/sysdeps/nptl/futex-internal.h
new file mode 100644
index 0000000000..4f8c8fe72d
--- /dev/null
+++ b/sysdeps/nptl/futex-internal.h
@@ -0,0 +1,203 @@
+/* futex operations for glibc-internal use.  Stub version; do not include
+   this file directly.
+   Copyright (C) 2014-2015 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 STUB_FUTEX_INTERNAL_H
+#define STUB_FUTEX_INTERNAL_H
+
+#include <sys/time.h>
+#include <stdio.h>
+#include <stdbool.h>
+#include <libc-internal.h>
+
+/* This file defines futex operations used internally in glibc.  A futex
+   consists of the so-called futex word in userspace, which is of type
+   unsigned int and represents an application-specific condition, and kernel
+   state associated with this particular futex word (e.g., wait queues).  The
+   futex operations we provide are wrappers for the futex syscalls and add
+   glibc-specific error checking of the syscall return value.  We abort on
+   error codes that are caused by bugs in glibc or in the calling application,
+   or when an error code is not known.  We return error codes that can arise
+   in correct executions to the caller.  Each operation calls out exactly the
+   return values that callers need to handle.
+
+   The private flag must be either FUTEX_PRIVATE or FUTEX_SHARED.
+   FUTEX_PRIVATE is always supported, and the implementation can internally
+   use FUTEX_SHARED when FUTEX_PRIVATE is requested.  FUTEX_SHARED is not
+   necessarily supported (use futex_supports_pshared to detect this).
+
+   We expect callers to only use these operations if futexes and the
+   specific futex operations being used are supported (e.g., FUTEX_SHARED).
+
+   Given that waking other threads waiting on a futex involves concurrent
+   accesses to the futex word, you must use atomic operations to access the
+   futex word.
+
+   Both absolute and relative timeouts can be used.  An absolute timeout
+   expires when the given specific point in time on the CLOCK_REALTIME clock
+   passes, or when it already has passed.  A relative timeout expires when
+   the given duration of time on the CLOCK_MONOTONIC clock passes.  Relative
+   timeouts may be imprecise (see futex_supports_exact_relative_timeouts).
+
+   Due to POSIX requirements on when synchronization data structures such
+   as mutexes or semaphores can be destroyed and due to the futex design
+   having separate fast/slow paths for wake-ups, we need to consider that
+   futex_wake calls might effectively target a data structure that has been
+   destroyed and reused for another object, or unmapped; thus, some
+   errors or spurious wake-ups can happen in correct executions that would
+   not be possible in a program using just a single futex whose lifetime
+   does not end before the program terminates.  For background, see:
+   https://sourceware.org/ml/libc-alpha/2014-04/msg00075.html
+   https://lkml.org/lkml/2014/11/27/472  */
+
+/* Defined this way for interoperability with lowlevellock.
+   FUTEX_PRIVATE must be zero because the initializers for pthread_mutex_t,
+   pthread_rwlock_t, and pthread_cond_t initialize the respective field of
+   those structures to zero, and we want FUTEX_PRIVATE to be the default.  */
+#define FUTEX_PRIVATE LLL_PRIVATE
+#define FUTEX_SHARED  LLL_SHARED
+#if FUTEX_PRIVATE != 0
+# error FUTEX_PRIVATE must be equal to 0
+#endif
+
+/* Returns EINVAL if PSHARED is neither PTHREAD_PROCESS_PRIVATE nor
+   PTHREAD_PROCESS_SHARED; otherwise, returns 0 if PSHARED is supported, and
+   ENOTSUP if not.  */
+static __always_inline int
+futex_supports_pshared (int pshared);
+
+/* Returns true if relative timeouts are robust to concurrent changes to the
+   system clock.  If this returns false, relative timeouts can still be used
+   but might be effectively longer or shorter than requested.  */
+static __always_inline bool
+futex_supports_exact_relative_timeouts (void);
+
+/* Atomically wrt other futex operations on the same futex, this blocks iff
+   the value *FUTEX_WORD matches the expected value.  This is
+   semantically equivalent to:
+     l = <get lock associated with futex> (FUTEX_WORD);
+     wait_flag = <get wait_flag associated with futex> (FUTEX_WORD);
+     lock (l);
+     val = atomic_load_relaxed (FUTEX_WORD);
+     if (val != expected) { unlock (l); return EAGAIN; }
+     atomic_store_relaxed (wait_flag, true);
+     unlock (l);
+     // Now block; can time out in futex_time_wait (see below)
+     while (atomic_load_relaxed(wait_flag) && !<spurious wake-up>);
+
+   Note that no guarantee of a happens-before relation between a woken
+   futex_wait and a futex_wake is documented; however, this does not matter
+   in practice because we have to consider spurious wake-ups (see below),
+   and thus would not be able to reliably reason about which futex_wake woke
+   us.
+
+   Returns 0 if woken by a futex operation or spuriously.  (Note that due to
+   the POSIX requirements mentioned above, we need to conservatively assume
+   that unrelated futex_wake operations could wake this futex; it is easiest
+   to just be prepared for spurious wake-ups.)
+   Returns EAGAIN if the futex word did not match the expected value.
+   Returns EINTR if waiting was interrupted by a signal.
+
+   Note that some previous code in glibc assumed the underlying futex
+   operation (e.g., syscall) to start with or include the equivalent of a
+   seq_cst fence; this allows one to avoid an explicit seq_cst fence before
+   a futex_wait call when synchronizing similar to Dekker synchronization.
+   However, we make no such guarantee here.  */
+static __always_inline int
+futex_wait (unsigned int *futex_word, unsigned int expected, int private);
+
+/* Like futex_wait but does not provide any indication why we stopped waiting.
+   Thus, when this function returns, you have to always check FUTEX_WORD to
+   determine whether you need to continue waiting, and you cannot detect
+   whether the waiting was interrupted by a signal.  Example use:
+     while (atomic_load_relaxed (&futex_word) == 23)
+       futex_wait_simple (&futex_word, 23, FUTEX_PRIVATE);
+   This is common enough to make providing this wrapper worthwhile.  */
+static __always_inline void
+futex_wait_simple (unsigned int *futex_word, unsigned int expected,
+		   int private)
+{
+  ignore_value (futex_wait (futex_word, expected, private));
+}
+
+
+/* Like futex_wait but is a POSIX cancellation point.  */
+static __always_inline int
+futex_wait_cancelable (unsigned int *futex_word, unsigned int expected,
+		       int private);
+
+/* Like futex_wait, but will eventually time out (i.e., stop being
+   blocked) after the duration of time provided (i.e., RELTIME) has
+   passed.  The caller must provide a normalized RELTIME.  RELTIME can also
+   equal NULL, in which case this function behaves equivalent to futex_wait.
+
+   Returns the same values as futex_wait under those same conditions;
+   additionally, returns ETIMEDOUT if the timeout expired.
+   */
+static __always_inline int
+futex_reltimed_wait (unsigned int* futex_word, unsigned int expected,
+		     const struct timespec* reltime, int private);
+
+/* Like futex_reltimed_wait but is a POSIX cancellation point.  */
+static __always_inline int
+futex_reltimed_wait_cancelable (unsigned int* futex_word,
+				unsigned int expected,
+			        const struct timespec* reltime, int private);
+
+/* Like futex_reltimed_wait, but the provided timeout (ABSTIME) is an
+   absolute point in time; a call will time out after this point in time.  */
+static __always_inline int
+futex_abstimed_wait (unsigned int* futex_word, unsigned int expected,
+		     const struct timespec* abstime, int private);
+
+/* Like futex_reltimed_wait but is a POSIX cancellation point.  */
+static __always_inline int
+futex_abstimed_wait_cancelable (unsigned int* futex_word,
+				unsigned int expected,
+			        const struct timespec* abstime, int private);
+
+/* Atomically wrt other futex operations on the same futex, this unblocks the
+   specified number of processes, or all processes blocked on this futex if
+   there are fewer than the specified number.  Semantically, this is
+   equivalent to:
+     l = <get lock associated with futex> (FUTEX_WORD);
+     lock (l);
+     for (res = 0; PROCESSES_TO_WAKE > 0; PROCESSES_TO_WAKE--, res++) {
+       if (<no process blocked on futex>) break;
+       wf = <get wait_flag of a process blocked on futex> (FUTEX_WORD);
+       // No happens-before guarantee with woken futex_wait (see above)
+       atomic_store_relaxed (wf, 0);
+     }
+     return res;
+
+   Note that we need to support futex_wake calls to past futexes whose memory
+   has potentially been reused due to POSIX' requirements on synchronization
+   object destruction (see above); therefore, we must not report or abort
+   on most errors.  */
+static __always_inline void
+futex_wake (unsigned int* futex_word, int processes_to_wake, int private);
+
+/* Calls __libc_fatal with an error message.  Convenience function for
+   concrete implementations of the futex interface.  */
+static __always_inline __attribute__ ((__noreturn__)) void
+futex_fatal_error (void)
+{
+  __libc_fatal ("The futex facility returned an unexpected error code.");
+}
+
+#endif  /* futex-internal.h */