The current implementation of std::call_once uses pthread_once, which
only meets the C++ requirements when compiled with support for
exceptions. For most glibc targets and all non-glibc targets,
pthread_once does not work correctly if the init_routine exits via an
exception. The pthread_once_t object is left in the "active" state, and
any later attempts to run another init_routine will block forever.
This change makes std::call_once work correctly for Linux targets, by
replacing the use of pthread_once with a futex, based on the code from
__cxa_guard_acquire. For both glibc and musl, the Linux implementation
of pthread_once is already based on futexes, and pthread_once_t is just
a typedef for int, so this change does not alter the layout of
std::once_flag. By choosing the values for the int appropriately, the
new code is even ABI compatible. Code that calls the old implementation
of std::call_once will use pthread_once to manipulate the int, while new
code will use the new std::once_flag members to manipulate it, but they
should interoperate correctly. In both cases, the int is initially zero,
has the lowest bit set when there is an active execution, and equals 2
after a successful returning execution. The difference with the new code
is that exceptional exceptions are correctly detected and the int is
reset to zero.
The __cxa_guard_acquire code (and musl's pthread_once) use an additional
state to say there are other threads waiting. This allows the futex wake
syscall to be skipped if there is no contention. Glibc doesn't use a
waiter bit, so we have to unconditionally issue the wake in order to be
compatible with code calling the old std::call_once that uses Glibc's
pthread_once. If we know that we're using musl (and musl's pthread_once
doesn't change) it would be possible to set a waiting state and check
for it in std::once_flag::_M_finish(bool), but this patch doesn't do
that.
This doesn't fix the bug for non-linux targets. A similar approach could
be used for targets where we know the definition of pthread_once_t is a
mutex and an integer. We could make once_flag._M_activate() use
pthread_mutex_lock on the mutex member within the pthread_once_t, and
then only set the integer if the execution finishes, and then unlock the
mutex. That would require careful study of each target's pthread_once
implementation and that work is left for a later date.
This also fixes PR 55394 because pthread_once is no longer needed, and
PR 84323 because the fast path is now just an atomic load.
As a consequence of the new implementation that doesn't use
pthread_once, we can also make std::call_once work for targets with no
gthreads support. The code for the single-threaded implementation
follows the same methods as on Linux, but with no need for atomics or
futexes.
libstdc++-v3/ChangeLog:
PR libstdc++/55394
PR libstdc++/66146
PR libstdc++/84323
* config/abi/pre/gnu.ver (GLIBCXX_3.4.29): Add new symbols.
* include/std/mutex [!_GLIBCXX_HAS_GTHREADS] (once_flag): Define
even when gthreads is not supported.
(once_flag::_M_once) [_GLIBCXX_HAVE_LINUX_FUTEX]: Change type
from __gthread_once_t to int.
(once_flag::_M_passive(), once_flag::_M_activate())
(once_flag::_M_finish(bool), once_flag::_Active_execution):
Define new members for futex and non-threaded implementation.
[_GLIBCXX_HAS_GTHREADS] (once_flag::_Prepare_execution): New
RAII helper type.
(call_once): Use new members of once_flag.
* src/c++11/mutex.cc (std::once_flag::_M_activate): Define.
(std::once_flag::_M_finish): Define.
* testsuite/30_threads/call_once/39909.cc: Do not require
gthreads.
* testsuite/30_threads/call_once/49668.cc: Likewise.
* testsuite/30_threads/call_once/60497.cc: Likewise.
* testsuite/30_threads/call_once/call_once1.cc: Likewise.
* testsuite/30_threads/call_once/dr2442.cc: Likewise.
* testsuite/30_threads/call_once/once_flag.cc: Add test for
constexpr constructor.
* testsuite/30_threads/call_once/66146.cc: New test.
* testsuite/30_threads/call_once/constexpr.cc: Removed.
* testsuite/30_threads/once_flag/cons/constexpr.cc: Removed.
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