The cRwLock class now allows nested read locks within a write lock from the same thread

3 files changed, 34 insertions, 10 deletions

diff --git a/HISTORY b/HISTORY
index 503f7707..4ae2b0e1 100644
--- a/HISTORY
+++ b/HISTORY
@@ -8839,3 +8839,7 @@ Video Disk Recorder Revision History
   instead.
 - The SVDRP command DELC now refuses to delete the very last channel in the list,
   to avoid ending up with an empty channel list.
+- The cRwLock class now allows nested read locks within a write lock from the
+  same thread. This fixes possible crashes when moving or deleting channels in
+  the menu or through SVDRP (as well as other operations that try to acquire a
+  read lock within a write lock).
diff --git a/thread.c b/thread.c
index 993d16dd..2f57f815 100644
--- a/thread.c
+++ b/thread.c
@@ -4,7 +4,7 @@
  * See the main source file 'vdr.c' for copyright information and
  * how to reach the author.
  *
- * $Id: thread.c 4.1 2015/08/29 14:43:03 kls Exp $
+ * $Id: thread.c 4.2 2016/12/08 10:18:32 kls Exp $
  */
 
 #include "thread.h"
@@ -151,6 +151,8 @@ void cCondVar::Broadcast(void)
 
 cRwLock::cRwLock(bool PreferWriter)
 {
+  locked = 0;
+  writeLockThreadId = 0;
   pthread_rwlockattr_t attr;
   pthread_rwlockattr_init(&attr);
   pthread_rwlockattr_setkind_np(&attr, PreferWriter ? PTHREAD_RWLOCK_PREFER_WRITER_NP : PTHREAD_RWLOCK_PREFER_READER_NP);
@@ -170,8 +172,15 @@ bool cRwLock::Lock(bool Write, int TimeoutMs)
      if (!GetAbsTime(&abstime, TimeoutMs))
         TimeoutMs = 0;
      }
-  if (Write)
+  if (Write) {
      Result = TimeoutMs ? pthread_rwlock_timedwrlock(&rwlock, &abstime) : pthread_rwlock_wrlock(&rwlock);
+     if (Result == 0)
+        writeLockThreadId = cThread::ThreadId();
+     }
+  else if (writeLockThreadId == cThread::ThreadId()) {
+     locked++; // there can be any number of stacked read locks, so we keep track here
+     Result = 0; // aquiring a read lock while holding a write lock within the same thread is OK
+     }
   else
      Result = TimeoutMs ? pthread_rwlock_timedrdlock(&rwlock, &abstime) : pthread_rwlock_rdlock(&rwlock);
   return Result == 0;
@@ -179,6 +188,13 @@ bool cRwLock::Lock(bool Write, int TimeoutMs)
 
 void cRwLock::Unlock(void)
 {
+  if (writeLockThreadId == cThread::ThreadId()) { // this is the thread that obtained the initial write lock
+     if (locked) { // this is the unlock of a read lock within the write lock
+        locked--;
+        return;
+        }
+     }
+  writeLockThreadId = 0;
   pthread_rwlock_unlock(&rwlock);
 }
 
@@ -206,8 +222,8 @@ void cMutex::Lock(void)
 
 void cMutex::Unlock(void)
 {
- if (!--locked)
-    pthread_mutex_unlock(&mutex);
+  if (!--locked)
+     pthread_mutex_unlock(&mutex);
 }
 
 // --- cThread ---------------------------------------------------------------
@@ -474,9 +490,11 @@ void cStateLock::Unlock(cStateKey &StateKey, bool IncState)
   if (StateKey.write && IncState && !explicitModify)
      state++;
   StateKey.state = state;
-  StateKey.write = false;
-  threadId = 0;
-  explicitModify = false;
+  if (StateKey.write) {
+     StateKey.write = false;
+     threadId = 0;
+     explicitModify = false;
+     }
   rwLock.Unlock();
 }
 
diff --git a/thread.h b/thread.h
index b5a07c79..9df677d3 100644
--- a/thread.h
+++ b/thread.h
@@ -4,7 +4,7 @@
  * See the main source file 'vdr.c' for copyright information and
  * how to reach the author.
  *
- * $Id: thread.h 4.1 2015/08/17 13:06:24 kls Exp $
+ * $Id: thread.h 4.2 2016/12/08 10:18:32 kls Exp $
  */
 
 #ifndef __THREAD_H
@@ -14,6 +14,8 @@
 #include <stdio.h>
 #include <sys/types.h>
 
+typedef pid_t tThreadId;
+
 class cCondWait {
 private:
   pthread_mutex_t mutex;
@@ -53,6 +55,8 @@ public:
 class cRwLock {
 private:
   pthread_rwlock_t rwlock;
+  int locked;
+  tThreadId writeLockThreadId;
 public:
   cRwLock(bool PreferWriter = false);
   ~cRwLock();
@@ -72,8 +76,6 @@ public:
   void Unlock(void);
   };
 
-typedef pid_t tThreadId;
-
 class cThread {
   friend class cThreadLock;
 private: