From 31f88f98c97f5645c045aaca7de76dafe3e63aec Mon Sep 17 00:00:00 2001
From: Philippe Canal <pcanal@fnal.gov>
Date: Fri, 20 Oct 2017 13:47:47 -0500
Subject: [PATCH] RWLock: handle case of writer meeting a nested reader.
Prior to this update if a thread requested the write lock when another thread had two or more nested
read lock take, the code would dead-lock
---
core/thread/inc/ROOT/TReentrantRWLock.hxx | 8 ++++++++
core/thread/src/TReentrantRWLock.cxx | 17 ++++++++++++++++-
2 files changed, 24 insertions(+), 1 deletion(-)
diff --git a/core/thread/inc/ROOT/TReentrantRWLock.hxx b/core/thread/inc/ROOT/TReentrantRWLock.hxx
index 6f6a3879a6c..a616b8d9669 100644
--- a/core/thread/inc/ROOT/TReentrantRWLock.hxx
+++ b/core/thread/inc/ROOT/TReentrantRWLock.hxx
@@ -50,6 +50,10 @@ struct UniqueLockRecurseCount {
template <typename MutexT>
void DecrementReadCount(local_t &local, MutexT &) { DecrementReadCount(local); }
+ void ResetReadCount(local_t &local, int newvalue) {
+ local->fReadersCount = newvalue;
+ }
+
bool IsNotCurrentWriter(local_t &local) { return !local->fIsWriter; }
void SetIsWriter(local_t &local)
@@ -97,6 +101,10 @@ struct RecurseCounts {
DecrementReadCount(local);
}
+ void ResetReadCount(local_t &local, int newvalue) {
+ fReadersCount[local] = newvalue;
+ }
+
bool IsNotCurrentWriter(local_t &local) { return fWriterThread != local; }
void SetIsWriter(local_t &local)
diff --git a/core/thread/src/TReentrantRWLock.cxx b/core/thread/src/TReentrantRWLock.cxx
index f04ab29c84b..457fd87b130 100644
--- a/core/thread/src/TReentrantRWLock.cxx
+++ b/core/thread/src/TReentrantRWLock.cxx
@@ -35,6 +35,7 @@ thus preventing starvation.
#include "ROOT/TSpinMutex.hxx"
#include "TMutex.h"
#include "TError.h"
+#include <assert.h>
using namespace ROOT;
@@ -75,7 +76,21 @@ void TReentrantRWLock<MutexT, RecurseCountsT>::ReadLock()
std::unique_lock<MutexT> lock(fMutex);
// Wait for writers, if any
- if (fWriter && fRecurseCounts.IsNotCurrentWriter(local)) fCond.wait(lock, [this] { return !fWriter; });
+ if (fWriter && fRecurseCounts.IsNotCurrentWriter(local)) {
+ auto readerCount = fRecurseCounts.GetLocalReadersCount(local);
+ if (readerCount == 0)
+ fCond.wait(lock, [this] { return !fWriter; });
+ // else
+ // There is a writer **but** we have outstanding readers
+ // locks, this must mean that the writer is actually
+ // waiting on this thread to release its read locks.
+ // This can be done in only two ways:
+ // * request the writer lock
+ // * release the reader lock
+ // Either way, this thread needs to proceed to
+ // be able to reach a point whether it does one
+ // of the two.
+ }
fRecurseCounts.IncrementReadCount(local);
--
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