Date: Wed, 14 Feb 2024 08:21:13 +0000 From: bugzilla-noreply@freebsd.org To: bugs@FreeBSD.org Subject: [Bug 277047] /usr/lib/libcompiler_rt.a(atomic.o) hidden visibility causes non-atomic atomics Message-ID: <bug-277047-227@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D277047 Bug ID: 277047 Summary: /usr/lib/libcompiler_rt.a(atomic.o) hidden visibility causes non-atomic atomics Product: Base System Version: Unspecified Hardware: Any OS: Any Status: New Severity: Affects Only Me Priority: --- Component: misc Assignee: bugs@FreeBSD.org Reporter: i@maskray.me lib/libcompiler_rt/ builds llvm-project/compiler-rt including atomics (for = arm there is additional sys/arm/arm/stdatomic.c). Many packages have a dependency on lib/libcompiler_rt. lib/libcompiler_rt is installed to /usr/lib/libcompiler_rt.a.=20 The symbols are hidden, which usually lead to benign ODR violations if both= an executable and its DT_NEEDED get a copy of libcompiler_rt.a.=20 However, atomic.o has a global lock pool, and having different lock pools w= ould lead to non-synchronized atomic operations. Prepare input files as attached at the end of this post. % zsh a.sh /tmp/b-9aaa82.o: reference to __atomic_compare_exchange /usr/lib/libcompiler_rt.a(atomic.o): definition of __atomic_compare_exchange /tmp/a-864004.o: reference to __atomic_compare_exchange /usr/lib/libcompiler_rt.a(atomic.o): definition of __atomic_compare_exchange 9470401 /tmp/b-28139a.o: reference to __atomic_compare_exchange /usr/lib/libgcc.a(atomic.o): definition of __atomic_compare_exchange /tmp/a-1dceb9.o: reference to __atomic_compare_exchange /usr/lib/libgcc.a(atomic.o): definition of __atomic_compare_exchange 9481088 The expected result is 10000000. libgcc.a has the same issue. In practice it is extremely rare when two pieces of code operating on the s= ame atomic lives in different link units. --- cat > a.cc <<'eof' #include <cstdint> #include <iostream> #include <thread> #include <vector> using namespace std; #define REP(i, n) for (int i =3D 0; i < (n); i++) constexpr int kIterations =3D 500'000; __int128_t g; void loop_b(); void loop_a() { REP(i, kIterations) { for(;;) { auto e =3D __atomic_load_n(&g, __ATOMIC_RELAXED), desired =3D e+1; if (__atomic_compare_exchange_n(&g, &e, desired, 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED)) break; } } } int main() { vector<thread> ts; REP(i, 10) ts.emplace_back(loop_a), ts.emplace_back(loop_b); for (auto &t : ts) t.join(); cout << (int64_t)g << '\n'; } eof cat > a.sh <<'eof' clang++ -O1 -fpic -Wno-atomic-alignment b.cc -shared -lcompiler_rt -o b.so -Wl,-y,__atomic_compare_exchange clang++ -O1 -Wno-atomic-alignment a.cc ./b.so -lcompiler_rt -pthread -o a -Wl,-y,__atomic_compare_exchange ./a clang++ -O1 -fpic -Wno-atomic-alignment b.cc -shared -o b.gcc.so -Wl,-y,__atomic_compare_exchange clang++ -O1 -Wno-atomic-alignment a.cc ./b.gcc.so -pthread -o a.gcc -Wl,-y,__atomic_compare_exchange ./a.gcc eof cat > b.cc <<'eof' #define REP(i, n) for (int i =3D 0; i < (n); i++) extern __int128_t g; constexpr int kIterations =3D 500'000; void loop_b() { REP(i, kIterations) { for(;;) { auto e =3D __atomic_load_n(&g, __ATOMIC_RELAXED); auto desired =3D e + 1; if (__atomic_compare_exchange_n(&g, &e, desired, 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED)) break; } } } eof --=20 You are receiving this mail because: You are the assignee for the bug.=
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