[Bug libstdc++/101825] New: Atomic wait returns without the value having changed

["tobias.schmidt at in dot tum.de" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101825

            Bug ID: 101825
           Summary: Atomic wait returns without the value having changed
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tobias.schmidt at in dot tum.de
  Target Milestone: ---

We noticed a possible bug in gcc for std::atomic<T>::wait, where the function
returns with the atomic still having the old value.

The following combination of notify_all and wait is used to execute
initialization code in one thread while the other threads wait:

auto expected = Empty;
// Only one thread should do the initialization
if (state.compare_exchange_strong(expected, Initialize)) {
    // Do some work

    state.store(Ready);
    state.notify_all();
}
// Other threads wait here
state.wait(Initialize);

State is a 8-bit atomic value initialized with Empty and it's value always
changes from Empty to Initialize and then to Ready. Except the given code
snippet we never write to the value.

Very rarely one of the waiting threads leaves the wait function while the state
value is still Initialize. The bug seems to depend on the time it takes to
perform the work, i.e., how long the other threads have to wait. Therefore, we
were not able to reproduce the bug in a smaller example.


However, replacing state.wait(Initialize) with 

while (variant.state.load() == Initialize) {}

solves the problem. Thus, we suspect that the wait function returns at some
point without checking the state's value first.


GCC Version:
Using built-in specs.
COLLECT_GCC=gcc-11
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu
11.1.0-1ubuntu1~21.04' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr
--with-gcc-major-version-only --program-suffix=-11
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib
--enable-libphobos-checking=release --with-target-system-zlib=auto
--enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet
--with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32
--enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none=/build/gcc-11-RPS7jb/gcc-11-11.1.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-RPS7jb/gcc-11-11.1.0/debian/tmp-gcn/usr
--without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu
--host=x86_64-linux-gnu --target=x86_64-linux-gnu
--with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.1.0 (Ubuntu 11.1.0-1ubuntu1~21.04)