[clang] bbb3baf - Thread safety analysis: Improve documentation for scoped capabilities

[Aaron Puchert via cfe-commits]

Author: Aaron Puchert
Date: 2020-09-06T20:37:42+02:00
New Revision: bbb3baf6205c54231257f64fd18661a13a5c97ee

URL: https://github.com/llvm/llvm-project/commit/bbb3baf6205c54231257f64fd18661a13a5c97ee
DIFF: https://github.com/llvm/llvm-project/commit/bbb3baf6205c54231257f64fd18661a13a5c97ee.diff

LOG: Thread safety analysis: Improve documentation for scoped capabilities

They are for more powerful than the current documentation implies, this
adds

* adopting a lock,
* deferring a lock,
* manually unlocking the scoped capability,
* relocking the scoped capability, possibly in a different mode,
* try-relocking the scoped capability.

Also there is now a generic explanation how attributes on scoped
capabilities work. There has been confusion in the past about how to
annotate them (see e.g. PR33504), hopefully this clears things up.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D87066

Added: 
    

Modified: 
    clang/docs/ThreadSafetyAnalysis.rst

Removed: 
    


################################################################################
diff  --git a/clang/docs/ThreadSafetyAnalysis.rst b/clang/docs/ThreadSafetyAnalysis.rst
index e6ce0e04e70a..e4a3342c02bd 100644
--- a/clang/docs/ThreadSafetyAnalysis.rst
+++ b/clang/docs/ThreadSafetyAnalysis.rst
@@ -402,6 +402,13 @@ the destructor.  Such classes require special handling because the constructor
 and destructor refer to the capability via 
diff erent names; see the
 ``MutexLocker`` class in :ref:`mutexheader`, below.
 
+Scoped capabilities are treated as capabilities that are implicitly acquired
+on construction and released on destruction. They are associated with
+the set of (regular) capabilities named in thread safety attributes on the
+constructor. Acquire-type attributes on other member functions are treated as
+applying to that set of associated capabilities, while ``RELEASE`` implies that
+a function releases all associated capabilities in whatever mode they're held.
+
 
 TRY_ACQUIRE(<bool>, ...), TRY_ACQUIRE_SHARED(<bool>, ...)
 ---------------------------------------------------------
@@ -886,19 +893,78 @@ implementation.
     const Mutex& operator!() const { return *this; }
   };
 
+  // Tag types for selecting a constructor.
+  struct adopt_lock_t {} inline constexpr adopt_lock = {};
+  struct defer_lock_t {} inline constexpr defer_lock = {};
+  struct shared_lock_t {} inline constexpr shared_lock = {};
 
   // MutexLocker is an RAII class that acquires a mutex in its constructor, and
   // releases it in its destructor.
   class SCOPED_CAPABILITY MutexLocker {
   private:
     Mutex* mut;
+    bool locked;
 
   public:
-    MutexLocker(Mutex *mu) ACQUIRE(mu) : mut(mu) {
+    // Acquire mu, implicitly acquire *this and associate it with mu.
+    MutexLocker(Mutex *mu) ACQUIRE(mu) : mut(mu), locked(true) {
       mu->Lock();
     }
+
+    // Assume mu is held, implicitly acquire *this and associate it with mu.
+    MutexLocker(Mutex *mu, adopt_lock_t) REQUIRES(mu) : mut(mu), locked(true) {}
+
+    // Acquire mu in shared mode, implicitly acquire *this and associate it with mu.
+    MutexLocker(Mutex *mu, shared_lock_t) ACQUIRE_SHARED(mu) : mut(mu), locked(true) {
+      mu->ReaderLock();
+    }
+
+    // Assume mu is held in shared mode, implicitly acquire *this and associate it with mu.
+    MutexLocker(Mutex *mu, adopt_lock_t, shared_lock_t) REQUIRES_SHARED(mu)
+      : mut(mu), locked(true) {}
+
+    // Assume mu is not held, implicitly acquire *this and associate it with mu.
+    MutexLocker(Mutex *mu, defer_lock_t) EXCLUDES(mu) : mut(mu), locked(false) {}
+
+    // Release *this and all associated mutexes, if they are still held.
+    // There is no warning if the scope was already unlocked before.
     ~MutexLocker() RELEASE() {
+      if (locked)
+        mut->GenericUnlock();
+    }
+
+    // Acquire all associated mutexes exclusively.
+    void Lock() ACQUIRE() {
+      mut->Lock();
+      locked = true;
+    }
+
+    // Try to acquire all associated mutexes exclusively.
+    bool TryLock() TRY_ACQUIRE(true) {
+      return locked = mut->TryLock();
+    }
+
+    // Acquire all associated mutexes in shared mode.
+    void ReaderLock() ACQUIRE_SHARED() {
+      mut->ReaderLock();
+      locked = true;
+    }
+
+    // Try to acquire all associated mutexes in shared mode.
+    bool ReaderTryLock() TRY_ACQUIRE_SHARED(true) {
+      return locked = mut->ReaderTryLock();
+    }
+
+    // Release all associated mutexes. Warn on double unlock.
+    void Unlock() RELEASE() {
       mut->Unlock();
+      locked = false;
+    }
+
+    // Release all associated mutexes. Warn on double unlock.
+    void ReaderUnlock() RELEASE() {
+      mut->ReaderUnlock();
+      locked = false;
     }
   };