[libc-commits] [libc] [libc][thread] detect self-join and mutual-join deadlock (PR #194891)

[Michael Jones via libc-commits]

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@@ -340,6 +342,24 @@ int Thread::run(ThreadStyle style, ThreadRunner runner, void *arg, void *stack,
 }
 
 int Thread::join(ThreadReturnValue &retval) {
+  if (self.attrib) {
+    // Reject self join.
+    if (self.attrib == attrib)
+      return EDEADLK;
+
+    // Do a best-effort check of concurrent/repeated join.
+    ThreadAttributes *expected = nullptr;
+    if (!attrib->joiner.compare_exchange_strong(expected, self.attrib,
+                                                cpp::MemoryOrder::ACQ_REL))
+      return EINVAL;
+
+    // Reject mutual join.
+    if (self.attrib->joiner.load(cpp::MemoryOrder::ACQUIRE) == attrib) {
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michaelrj-google wrote:

nit: From what I understand this is where the `joiner` is set, as well as where it's compared. It's a little unclear, could you add a comment noting that this is where the variable is set?

https://github.com/llvm/llvm-project/pull/194891