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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - libcxx: data race in shared_ptr"
href="http://llvm.org/bugs/show_bug.cgi?id=22836">22836</a>
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<th>Summary</th>
<td>libcxx: data race in shared_ptr
</td>
</tr>
<tr>
<th>Product</th>
<td>libc++
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>All Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>dvyukov@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu, mclow.lists@gmail.com
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>ThreadSanitizer says:
WARNING: ThreadSanitizer: data race (pid=19491)
Read of size 8 at 0x7d4800017b88 by thread T4 (mutexes: write M188):
#0 std::__1::__shared_weak_count::lock()
/work/libc++/libcxx/src/memory.cpp:102 (libc++.so.1+0x000000086cc7)
#1 std::__1::weak_ptr<base::Stream>::lock() const
/opt/clang36/bin/../include/c++/v1/memory:5197 (block_test+0x0000005ae3c9)
#2 operator() // std::shared_ptr is passed through std::function (via the
capture list)
.....
Previous atomic write of size 8 at 0x7d4800017b88 by thread T3:
#0 __tsan_atomic64_fetch_add
/work/x/llvm/projects/compiler-rt/lib/tsan/rtl/tsan_interface_atomic.cc:618
(block_test+0x000000526d07)
#1 __release_shared /work/libc++/libcxx/src/memory.cpp:61
(libc++.so.1+0x000000086bf6)
#2 ~shared_ptr /opt/clang36/bin/../include/c++/v1/memory:4462
(block_test+0x00000063b21f)
#3 operator() // std::shared_ptr is passed through std::function (via the
capture list)
....
Both sites contain this pattern:
void f(std::shared_ptr<Thing> b)
{
Invoke(
[b]() {
do_something(b); // runs on a
different thread
});
}
I don't have a full reproducer as it is reported by a ThreadSanitizer user. But
it should be easy to reproduce.
Looking at the shared_ptr sources:
<a href="http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/memory.cpp?revision=224095&view=markup">http://llvm.org/viewvc/llvm-project/libcxx/trunk/src/memory.cpp?revision=224095&view=markup</a>
99 __shared_weak_count*
100 __shared_weak_count::lock() _NOEXCEPT
101 {
102 long object_owners = __shared_owners_;
103 while (object_owners != -1)
104 {
105 if (__sync_bool_compare_and_swap(&__shared_owners_,
106 object_owners,
107 object_owners+1))
108 return this;
109 object_owners = __shared_owners_;
110 }
111 return 0;
112 }
There is non-atomic load of __shared_owners_ on line 102.
On a related note, why does libcxx use the rudimentary atomic operations
interface instead of std::atomic or at least __atomic_load_n family of
functions? The __sync builtins do not even allow to fix this data race in a
reasonable way.</pre>
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