[clang] Thread safety analysis: Don't warn on acquiring reentrant capability (PR #150857)

[Aaron Puchert via cfe-commits]

https://github.com/aaronpuchert created https://github.com/llvm/llvm-project/pull/150857

The point of reentrant capabilities is that they can be acquired multiple times, so they should probably be excluded from requiring a negative capability on acquisition via -Wthread-safety-negative.

However, we still propagate explicit negative requirements.

>From 26716f3b21d39f9cf3eaced3cbc6fff9e8f27be0 Mon Sep 17 00:00:00 2001
From: Aaron Puchert <aaronpuchert at alice-dsl.net>
Date: Sun, 27 Jul 2025 23:24:51 +0200
Subject: [PATCH] Thread safety analysis: Don't warn on acquiring reentrant
 capability

The point of reentrant capabilities is that they can be acquired
multiple times, so they should probably be excluded from requiring a
negative capability on acquisition via -Wthread-safety-negative.

However, we still propagate explicit negative requirements.
---
 clang/lib/Analysis/ThreadSafety.cpp           |  2 +-
 .../SemaCXX/warn-thread-safety-negative.cpp   | 32 +++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Analysis/ThreadSafety.cpp b/clang/lib/Analysis/ThreadSafety.cpp
index 80e7c8eff671a..dadb0b757a2c8 100644
--- a/clang/lib/Analysis/ThreadSafety.cpp
+++ b/clang/lib/Analysis/ThreadSafety.cpp
@@ -1331,7 +1331,7 @@ void ThreadSafetyAnalyzer::addLock(FactSet &FSet,
       FSet.removeLock(FactMan, NegC);
     }
     else {
-      if (inCurrentScope(*Entry) && !Entry->asserted())
+      if (inCurrentScope(*Entry) && !Entry->asserted() && !Entry->reentrant())
         Handler.handleNegativeNotHeld(Entry->getKind(), Entry->toString(),
                                       NegC.toString(), Entry->loc());
     }
diff --git a/clang/test/SemaCXX/warn-thread-safety-negative.cpp b/clang/test/SemaCXX/warn-thread-safety-negative.cpp
index 9eabd67e4fc76..0caf6d6139e58 100644
--- a/clang/test/SemaCXX/warn-thread-safety-negative.cpp
+++ b/clang/test/SemaCXX/warn-thread-safety-negative.cpp
@@ -21,6 +21,15 @@ class LOCKABLE Mutex {
   void AssertReaderHeld() ASSERT_SHARED_LOCK();
 };
 
+class LOCKABLE REENTRANT_CAPABILITY ReentrantMutex {
+public:
+  void Lock() EXCLUSIVE_LOCK_FUNCTION();
+  void Unlock() UNLOCK_FUNCTION();
+
+  // for negative capabilities
+  const ReentrantMutex& operator!() const { return *this; }
+};
+
 class SCOPED_LOCKABLE MutexLock {
 public:
   MutexLock(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu);
@@ -89,6 +98,29 @@ class Foo {
   }
 };
 
+class Reentrant {
+  ReentrantMutex mu;
+
+public:
+  void acquire() {
+    mu.Lock();   // no warning -- reentrant mutex
+    mu.Unlock();
+  }
+
+  void requireNegative() EXCLUSIVE_LOCKS_REQUIRED(!mu) { // warning?
+    mu.Lock();
+    mu.Unlock();
+  }
+
+  void callRequireNegative() {
+    requireNegative(); // expected-warning{{calling function 'requireNegative' requires negative capability '!mu'}}
+  }
+
+  void callHaveNegative() EXCLUSIVE_LOCKS_REQUIRED(!mu) {
+    requireNegative();
+  }
+};
+
 }  // end namespace SimpleTest
 
 Mutex globalMutex;