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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Insufficient locking in std::atomic_compare_exchange_strong for std::shared_ptr"
href="https://bugs.llvm.org/show_bug.cgi?id=43813">43813</a>
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<th>Summary</th>
<td>Insufficient locking in std::atomic_compare_exchange_strong for std::shared_ptr
</td>
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<th>Product</th>
<td>libc++
</td>
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<th>Version</th>
<td>9.0
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>lahav.sch.llvm@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mclow.lists@gmail.com
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<p>
<div>
<pre>Hi,
The implementation of 'std::atomic_compare_exchange_strong(shared_ptr* p,
shared_ptr* v, shared_ptr w)' only locks the mutex related to 'p', but not the
one for 'v' as well.
This means the following code (Which should have no race conditions):
#include <memory>
#include <atomic>
#include <unistd.h>
#include <pthread.h>
std::shared_ptr<int> g_p;
static void* thread_run(void*) {
auto w = std::make_shared<int>();
while (true) {
auto p = std::make_shared<int>();
std::atomic_compare_exchange_strong(&p, &g_p, w);
}
return nullptr;
}
int main() {
pthread_t tid;
pthread_create(&tid, nullptr, thread_run, nullptr);
auto w = std::make_shared<int>();
while(true) {
std::atomic_store(&g_p, w);
}
}
may attempt to access the same 'g_p' instance from multiple threads - The main
thread (Via std::atomic_store()) will properly lock the matching mutex, but the
2nd thread will not, causing a race condition.
I believe the mutex for 'v' should be locked as well, with special care as to
not introduce a deadlock now that 2 mutexes are being locked.</pre>
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