use CAS instead of swap since it's lighter for most archs, and keep
EBUSY in the lock value so that the old value obtained by CAS can be
used directly as the return value for pthread_spin_trylock.
the leak was found by static analysis (reported by Alexander Monakov),
not tested/observed, but seems to have occured both when failing due
to O_EXCL, and in a race condition with O_CREAT but not O_EXCL where a
semaphore by the same name was created concurrently.
this fixes truncation of error messages containing long pathnames or
symbol names.
the dlerror state was previously required by POSIX to be global. the
resolution of bug 97 relaxed the requirements to allow thread-safe
implementations of dlerror with thread-local state and message buffer.
previously, the dynamic tlsdesc lookup functions and the i386
special-ABI ___tls_get_addr (3 underscores) function called
__tls_get_addr when the slot they wanted was not already setup;
__tls_get_addr would then in turn also see that it's not setup and
call __tls_get_new.
calling __tls_get_new directly is both more efficient and avoids the
issue of calling a non-hidden (public API/ABI) function from asm.
for the special i386 function, a weak reference to __tls_get_new is
used since this function is not defined when static linking (the code
path that needs it is unreachable in static-linked programs).
applying the attribute to a weak_alias macro was a hack. instead use a
separate declaration to apply the visibility, and consolidate
declarations together to avoid having visibility mess all over the
file.
in a few places, non-hidden symbols were referenced from asm in ways
that assumed ld-time binding. while these is no semantic reason these
symbols need to be hidden, fixing the references without making them
hidden was going to be ugly, and hidden reduces some bloat anyway.
in the asm files, .global/.hidden directives have been moved to the
top to unclutter the actual code.
at the point of call it was declared hidden, but the definition was
not hidden. for some toolchains this inconsistency produced textrels
without ld-time binding.
since 1.1.0, musl has nominally required a thread pointer to be setup.
most of the remaining code that was checking for its availability was
doing so for the sake of being usable by the dynamic linker. as of
commit 71f099cb7d, this is no longer
necessary; the thread pointer is now valid before any libc code
(outside of dynamic linker bootstrap functions) runs.
this commit essentially concludes "phase 3" of the "transition path
for removing lazy init of thread pointer" project that began during
the 1.1.0 release cycle.
previously a new GDT slot was requested, even if one had already been
obtained by a previous call. instead extract the old slot number from
GS and reuse it if it was already set. the formula (GS-3)/8 for the
slot number automatically yields -1 (request for new slot) if GS is
zero (unset).
this global lock allows certain unlock-type primitives to exclude
mmap/munmap operations which could change the identity of virtual
addresses while references to them still exist.
the original design mistakenly assumed mmap/munmap would conversely
need to exclude the same operations which exclude mmap/munmap, so the
vmlock was implemented as a sort of 'symmetric recursive rwlock'. this
turned out to be unnecessary.
commit 25d12fc0fc already shortened the
interval during which mmap/munmap held their side of the lock, but
left the inappropriate lock design and some inefficiency.
the new design uses a separate function, __vm_wait, which does not
hold any lock itself and only waits for lock users which were already
present when it was called to release the lock. this is sufficient
because of the way operations that need to be excluded are sequenced:
the "unlock-type" operations using the vmlock need only block
mmap/munmap operations that are precipitated by (and thus sequenced
after) the atomic-unlock they perform while holding the vmlock.
this allows for a spectacular lack of synchronization in the __vm_wait
function itself.
as a result of commit 12e1e32468, kernel
processing of the robust list is only needed for process-shared
mutexes. previously the first attempt to lock any owner-tracked mutex
resulted in robust list initialization and a set_robust_list syscall.
this is no longer necessary, and since the kernel's record of the
robust list must now be cleared at thread exit time for detached
threads, optimizing it out is more worthwhile than before too.
the robust list head lies in the thread structure, which is unmapped
before exit for detached threads. this leaves the kernel unable to
process the exiting thread's robust list, and with a dangling pointer
which may happen to point to new unrelated data at the time the kernel
processes it.
userspace processing of the robust list was already needed for
non-pshared robust mutexes in order to perform private futex wakes
rather than the shared ones the kernel would do, but it was
conditional on linking pthread_mutexattr_setrobust and did not bother
processing the pshared mutexes in the list, which requires additional
logic for the robust list pending slot in case pthread_exit is
interrupted by asynchronous process termination.
the new robust list processing code is linked unconditionally (inlined
in pthread_exit), handles both private and shared mutexes, and also
removes the kernel's reference to the robust list before unmapping and
exit if the exiting thread is detached.
previously the implementation-internal signal used for multithreaded
set*id operations was left unblocked during handling of the
cancellation signal. however, on some archs, signal contexts are huge
(up to 5k) and the possibility of nested signal handlers drastically
increases the minimum stack requirement. since the cancellation signal
handler will do its job and return in bounded time before possibly
passing execution to application code, there is no need to allow other
signals to interrupt it.
This adds complete aarch64 target support including bigendian subarch.
Some of the long double math functions are known to be broken otherwise
interfaces should be fully functional, but at this point consider this
port experimental.
Initial work on this port was done by Sireesh Tripurari and Kevin Bortis.
due to a logic error in the use of masked cancellation mode,
pthread_cond_wait did not honor PTHREAD_CANCEL_DISABLE but instead
failed with ECANCELED when cancellation was pending.
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.
like close, pthread_join is a resource-deallocation function which is
also a cancellation point. the intent of masked cancellation mode is
to exempt such functions from failure with ECANCELED.
previously, the __timedwait function was optionally a cancellation
point depending on whether it was passed a pointer to a cleaup
function and context to register. as of now, only one caller actually
used such a cleanup function (and it may face removal soon); most
callers either passed a null pointer to disable cancellation or a
dummy cleanup function.
now, __timedwait is never a cancellation point, and __timedwait_cp is
the cancellable version. this makes the intent of the calling code
more obvious and avoids ugly dummy functions and long argument lists.
as part of abstracting the futex wait, this function suppresses all
futex error values which callers should not see using a whitelist
approach. when the masked cancellation mode was added, the new
ECANCELED error was not whitelisted. this omission caused the new
pthread_cond_wait code using masked cancellation to exhibit a spurious
wake (rather than acting on cancellation) when the request arrived
after blocking on the cond var.
due to accidental use of = instead of ==, the error code was always
set to zero in the signaled wake case for non-shared cv waits.
suppressing ETIMEDOUT (the only possible wait error) is harmless and
actually permitted in this case, but suppressing mutex errors could
give the caller false information about the state of the mutex.
commit 8741ffe625 introduced this
regression and commit d9da1fb8c5
preserved it when reorganizing the code.
it's possible that signaling a waiter races with cancellation of that
same waiter. previously, cancellation was acted upon, causing the
signal to be consumed with no waiter returning. by using the new
masked cancellation state, it's possible to refuse to act on the
cancellation request and instead leave it pending.
to ease review and understanding of the changes made, this commit
leaves the unwait function, which was previously the cancellation
cleanup handler, in place. additional simplifications could be made by
removing it.
this is a new extension which is presently intended only for
experimental and internal libc use. interface and behavior details may
change subject to feedback and experience from using it internally.
the basic concept for the new PTHREAD_CANCEL_MASKED state is that the
first cancellation point to observe the cancellation request fails
with an errno value of ECANCELED rather than acting on cancellation,
allowing the caller to process the status and choose whether/how to
act upon it.
in practice this was probably a non-issue, because the necessary
barrier almost certainly exists in kernel space -- implementing signal
delivery without such a barrier seems impossible -- but for the sake
of correctness, it should be done here too.
in principle, without a barrier, it is possible that the thread to be
cancelled does not see the store of its cancellation flag performed by
another thread. this affects both the case where the signal arrives
before entering the critical program counter range from __cp_begin to
__cp_end (in which case both the signal handler and the inline check
fail to see the value which was already stored) and the case where the
signal arrives during the critical range (in which case the signal
handler should be responsible for cancellation, but when it does not
see the cancellation flag, it assumes the signal is spurious and
refuses to act on it).
in the fix, the barrier is placed only in the signal handler, not in
the inline check at the beginning of the critical program counter
range. if the signal handler runs before the critical range is
entered, it will of course take no action, but its barrier will ensure
that the inline check subsequently sees the store. if on the other
hand the inline check runs first, it may miss seeing the store, but
the subsequent signal handler in the critical range will act upon the
cancellation request. this strategy avoids adding a memory barrier in
the common, non-cancellation code path.
multi-threaded set*id and setrlimit use the internal __synccall
function to work around the kernel's wrongful treatment of these
process properties as thread-local. the old implementation of
__synccall failed to be AS-safe, despite POSIX requiring setuid and
setgid to be AS-safe, and was not rigorous in assuring that all
threads were caught. in a worst case, threads late in the process of
exiting could retain permissions after setuid reported success, in
which case attacks to regain dropped permissions may have been
possible under the right conditions.
the new implementation of __synccall depends on the presence of
/proc/self/task and will fail if it can't be opened, but is able to
determine that it has caught all threads, and does not use any locks
except its own. it thereby achieves AS-safety simply by blocking
signals to preclude re-entry in the same thread.
with this commit, all known conformance and safety issues in set*id
functions should be fixed.
per POSIX, the EINTR condition is an optional error for these
functions, not a mandatory one. since old kernels (pre-2.6.22) failed
to honor SA_RESTART for the futex syscall, it's dangerous to trust
EINTR from the kernel. thankfully POSIX offers an easy way out.
calls to __aeabi_read_tp may be generated by the compiler to access
TLS on pre-v6 targets. previously, this function was hard-coded to
call the kuser helper, which would crash on kernels with kuser helper
removed.
to fix the problem most efficiently, the definition of __aeabi_read_tp
is moved so that it's an alias for the new __a_gettp. however, on v7+
targets, code to initialize the runtime choice of thread-pointer
loading code is not even compiled, meaning that defining
__aeabi_read_tp would have caused an immediate crash due to using the
default implementation of __a_gettp with a HCF instruction.
fortunately there is an elegant solution which reduces overall code
size: putting the native thread-pointer loading instruction in the
default code path for __a_gettp, so that separate default/native code
paths are not needed. this function should never be called before
__set_thread_area anyway, and if it is called early on pre-v6
hardware, the old behavior (crashing) is maintained.
ideally __aeabi_read_tp would not be called at all on v7+ targets
anyway -- in fact, prior to the overhaul, the same problem existed,
but it was never caught by users building for v7+ with kuser disabled.
however, it's possible for calls to __aeabi_read_tp to end up in a v7+
binary if some of the object files were built for pre-v7 targets, e.g.
in the case of static libraries that were built separately, so this
case needs to be handled.
previously, builds for pre-armv6 targets hard-coded use of the "kuser
helper" system for atomics and thread-pointer access, resulting in
binaries that fail to run (crash) on systems where this functionality
has been disabled (as a security/hardening measure) in the kernel.
additionally, builds for armv6 hard-coded an outdated/deprecated
memory barrier instruction which may require emulation (extremely
slow) on future models.
this overhaul replaces the behavior for all pre-armv7 builds (both of
the above cases) to perform runtime detection of the appropriate
mechanisms for barrier, atomic compare-and-swap, and thread pointer
access. detection is based on information provided by the kernel in
auxv: presence of the HWCAP_TLS bit for AT_HWCAP and the architecture
version encoded in AT_PLATFORM. direct use of the instructions is
preferred when possible, since probing for the existence of the kuser
helper page would be difficult and would incur runtime cost.
for builds targeting armv7 or later, the runtime detection code is not
compiled at all, and much more efficient versions of the non-cas
atomic operations are provided by using ldrex/strex directly rather
than wrapping cas.
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.
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.
based on patch by Jens Gustedt.
the main difficulty here is handling the difference between start
function signatures and thread return types for C11 threads versus
POSIX threads. pointers to void are assumed to be able to represent
faithfully all values of int. the function pointer for the thread
start function is cast to an incorrect type for passing through
pthread_create, but is cast back to its correct type before calling so
that the behavior of the call is well-defined.
changes to the existing threads implementation were kept minimal to
reduce the risk of regressions, and duplication of code that carries
implementation-specific assumptions was avoided for ease and safety of
future maintenance.
These all have POSIX equivalents, but aside from tss_get, they all
have minor changes to the signature or return value and thus need to
exist as separate functions.
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.
per POSIX these functions are both cancellation points, so they must
act on any cancellation request which is pending prior to the call.
previously, only the code path where actual waiting took place could
act on cancellation.
if there is already a waiter for a lock, spinning on the lock is
essentially an attempt to steal it from whichever waiter would obtain
it via any priority rules in place, and is therefore undesirable. in
the current implementation, there is always an inherent race window at
unlock during which a newly-arriving thread may steal the lock from
the existing waiters, but we should aim to keep this window minimal
rather than enlarging it.
Rich Felker