[Bug gold/19044] New: Gold links single-threaded program into a deadlock on i686 Linux

[sduvan.gcc at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=19044

            Bug ID: 19044
           Summary: Gold links single-threaded program into a deadlock on
                    i686 Linux
           Product: binutils
           Version: 2.25
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gold
          Assignee: ccoutant at gmail dot com
          Reporter: sduvan.gcc at gmail dot com
                CC: ian at airs dot com
  Target Milestone: ---

Created attachment 8664
  --> https://sourceware.org/bugzilla/attachment.cgi?id=8664&action=edit
Testcase

The attached reduced testcase shows an example where single-threaded code, if
compiled with gcc 5.2.0 and linked with gold (version 2.25) on i686 Linux,
results in an executable that deadlocks when run. Looking into the details with
a debugger, it seems like the threaded mode support code in libstdc++
(__gthread_active_p()) is handled differently in the constructor and the
destructor of __gnu_cxx::___scoped_lock. While the constructor does the
appropriate test the destructor does not (equivalent to assuming no threading
support). This means that the second time __scoped_lock's constructor is called
the program deadlocks.

Linking with ld (version 2.25), there is no deadlock. Compiling and linking
with gold on x86_64 does not result in a deadlock. The testcase is sensitive to
perturbations and compiler version (the issue has also been seen with gcc 4.9.1
but the testcase was much bigger in that case).

gcc 5.2.0 was configured in the following way:

bash> /opt/sandbox/gcc/5.2.0/bin/g++ -v
Using built-in specs.
COLLECT_GCC=/opt/sandbox/gcc/5.2.0/bin/g++
COLLECT_LTO_WRAPPER=/opt/sandbox/gcc/5.2.0/lib/gcc/x86_64-suse-linux/5.2.0/lto-wrapper
Target: x86_64-suse-linux
Configured with: ../../gcc-5.2.0/configure --enable-languages=c,c++,fortran
--enable-targets=x86_64-suse-linux,i686-suse-linux
--prefix=/opt/sandbox/gcc/5.2.0 --with-gnu-as
--with-as=/opt/sandbox/gcc/binutils-2.25.1/bin/as --with-gnu-ld
--with-ld=/opt/sandbox/gcc/binutils-2.25.1/bin/ld.gold
--with-gmp=/opt/sandbox/gcc/gmp-5.0.1 --with-mpfr=/opt/sandbox/gcc/mpfr-3.0.0
--with-mpc=/opt/sandbox/gcc/mpc-0.8.2 --enable-threads=posix --enable-shared
--enable-__cxa_atexit --enable-libstdcxx-allocator=pool x86_64-suse-linux
Thread model: posix
gcc version 5.2.0 (GCC)

To see the deadlock:
bash> tar -zxvf gold-bug.tar.gz
bash> make bug
bash> ./bug
(hangs)

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