make all objects used with atomic operations volatile

the memory model we use internally for atomics permits plain loads of
values which may be subject to concurrent modification without
requiring that a special load function be used. since a compiler is
free to make transformations that alter the number of loads or the way
in which loads are performed, the compiler is theoretically free to
break this usage. the most obvious concern is with atomic cas
constructs: something of the form tmp=*p;a_cas(p,tmp,f(tmp)); could be
transformed to a_cas(p,*p,f(*p)); where the latter is intended to show
multiple loads of *p whose resulting values might fail to be equal;
this would break the atomicity of the whole operation. but even more
fundamental breakage is possible.

with the changes being made now, objects that may be modified by
atomics are modeled as volatile, and the atomic operations performed
on them by other threads are modeled as asynchronous stores by
hardware which happens to be acting on the request of another thread.
such modeling of course does not itself address memory synchronization
between cores/cpus, but that aspect was already handled. this all
seems less than ideal, but it's the best we can do without mandating a
C11 compiler and using the C11 model for atomics.

in the case of pthread_once_t, the ABI type of the underlying object
is not volatile-qualified. so we are assuming that accessing the
object through a volatile-qualified lvalue via casts yields volatile
access semantics. the language of the C standard is somewhat unclear
on this matter, but this is an assumption the linux kernel also makes,
and seems to be the correct interpretation of the standard.

manually "shrink wrap" fast path in pthread_once

this change is a workaround for the inability of current compilers to
perform "shrink wrapping" optimizations. in casual testing, it roughly
doubled the performance of pthread_once when called on an
already-finished once control object.

eliminate global waiters count in pthread_once

fix missing barrier in pthread_once/call_once shortcut path

these functions need to be fast when the init routine has already run,
since they may be called very often from code which depends on global
initialization having taken place. as such, a fast path bypassing
atomic cas on the once control object was used to avoid heavy memory
contention. however, on archs with weakly ordered memory, the fast
path failed to ensure that the caller actually observes the side
effects of the init routine.

preliminary performance testing showed that simply removing the fast
path was not practical; a performance drop of roughly 85x was observed
with 20 threads hammering the same once control on a 24-core machine.
so the new explicit barrier operation from atomic.h is used to retain
the fast path while ensuring memory visibility.

performance may be reduced on some archs where the barrier actually
makes a difference, but the previous behavior was unsafe and incorrect
on these archs. future improvements to the implementation of a_barrier
should reduce the impact.

use weak symbols for the POSIX functions that will be used by C threads

The intent of this is to avoid name space pollution of the C threads
implementation.

This has two sides to it. First we have to provide symbols that wouldn't
pollute the name space for the C threads implementation. Second we have
to clean up some internal uses of POSIX functions such that they don't
implicitly drag in such symbols.

make futex operations use private-futex mode when possible

private-futex uses the virtual address of the futex int directly as
the hash key rather than requiring the kernel to resolve the address
to an underlying backing for the mapping in which it lies. for certain
usage patterns it improves performance significantly.

in many places, the code using futex __wake and __wait operations was
already passing a correct fixed zero or nonzero flag for the priv
argument, so no change was needed at the site of the call, only in the
__wake and __wait functions themselves. in other places, especially
where the process-shared attribute for a synchronization object was
not previously tracked, additional new code is needed. for mutexes,
the only place to store the flag is in the type field, so additional
bit masking logic is needed for accessing the type.

for non-process-shared condition variable broadcasts, the futex
requeue operation is unable to requeue from a private futex to a
process-shared one in the mutex structure, so requeue is simply
disabled in this case by waking all waiters.

for robust mutexes, the kernel always performs a non-private wake when
the owner dies. in order not to introduce a behavioral regression in
non-process-shared robust mutexes (when the owning thread dies), they
are simply forced to be treated as process-shared for now, giving
correct behavior at the expense of performance. this can be fixed by
adding explicit code to pthread_exit to do the right thing for
non-shared robust mutexes in userspace rather than relying on the
kernel to do it, and will be fixed in this way later.

since not all supported kernels have private futex support, the new
code detects EINVAL from the futex syscall and falls back to making
the call without the private flag. no attempt to cache the result is
made; caching it and using the cached value efficiently is somewhat
difficult, and not worth the complexity when the benefits would be
seen only on ancient kernels which have numerous other limitations and
bugs anyway.

fix deadlock race in pthread_once

at the end of successful pthread_once, there was a race window during
which another thread calling pthread_once would momentarily change the
state back from 2 (finished) to 1 (in-progress). in this case, the
status was immediately changed back, but with no wake call, meaning
that waiters which arrived during this short window could block
forever. there are two possible fixes. one would be adding the wake to
the code path where it was missing. but it's better just to avoid
reverting the status at all, by using compare-and-swap instead of
swap.

fix major breakage in pthread_once (it was always deadlocking)

the issue was a break statement that was breaking only from the
switch, not the enclosing for loop, and a failure to set the final
success state.

initial check-in, version 0.5.0