From 7e746ad90c88f8708f7ea9177d2728d38f93068f Mon Sep 17 00:00:00 2001
From: Roman Nozdrin <rnozdrin@mariadb.com>
Date: Tue, 22 Aug 2023 08:36:37 +0000
Subject: [PATCH] fix(extent-map): MCOL-5559 This patch extends the scope of
 the crit section to cover remap operation to avoid access races running
 EMIndex operations

---
 versioning/BRM/extentmap.cpp | 52 +++++++++++++++++++++++++++++-------
 versioning/BRM/extentmap.h   |  2 ++
 2 files changed, 45 insertions(+), 9 deletions(-)

diff --git a/versioning/BRM/extentmap.cpp b/versioning/BRM/extentmap.cpp
index e59af8603..df4bbfc52 100644
--- a/versioning/BRM/extentmap.cpp
+++ b/versioning/BRM/extentmap.cpp
@@ -2080,20 +2080,54 @@ void ExtentMap::grabEMIndex(OPS op)
     }
     else
     {
-      // Sending down current Managed Shmem size. If EMIndexImpl instance size doesn't match
-      // fEMIndexShminfo->allocdSize makeExtentMapIndexImpl will remap managed shmem segment.
-      fPExtMapIndexImpl_ =
-          ExtentMapIndexImpl::makeExtentMapIndexImpl(getInitialEMIndexShmkey(), fEMIndexShminfo->allocdSize);
+      if (op == READ)
+      {
+        fMST.getTable_upgrade(MasterSegmentTable::EMIndex);
+        emIndexLocked = true;
 
-      if (r_only)
-        fPExtMapIndexImpl_->makeReadOnly();
+        // Sending down current Managed Shmem size. If EMIndexImpl instance size doesn't match
+        // fEMIndexShminfo->allocdSize makeExtentMapIndexImpl will remap managed shmem segment.
+        fPExtMapIndexImpl_ = ExtentMapIndexImpl::makeExtentMapIndexImpl(getInitialEMIndexShmkey(),
+                                                                        fEMIndexShminfo->allocdSize);
+
+        if (r_only)
+          fPExtMapIndexImpl_->makeReadOnly();
+
+        emIndexLocked = false;
+        fMST.getTable_downgrade(MasterSegmentTable::EMIndex);
+      }
+      else
+      {
+        fPExtMapIndexImpl_ = ExtentMapIndexImpl::makeExtentMapIndexImpl(getInitialEMIndexShmkey(),
+                                                                        fEMIndexShminfo->allocdSize);
+
+        if (r_only)
+          fPExtMapIndexImpl_->makeReadOnly();
+      }
     }
   }
   else if (fPExtMapIndexImpl_->getShmemImplSize() != (unsigned)fEMIndexShminfo->allocdSize)
   {
-    fPExtMapIndexImpl_->refreshShm();
-    fPExtMapIndexImpl_ =
-        ExtentMapIndexImpl::makeExtentMapIndexImpl(getInitialEMIndexShmkey(), fEMIndexShminfo->allocdSize);
+    if (op == READ)
+    {
+      fMST.getTable_upgrade(MasterSegmentTable::EMIndex);
+      emIndexLocked = true;
+
+      fPExtMapIndexImpl_->refreshShm();
+
+      fPExtMapIndexImpl_ =
+          ExtentMapIndexImpl::makeExtentMapIndexImpl(getInitialEMIndexShmkey(), fEMIndexShminfo->allocdSize);
+
+      emIndexLocked = false;
+      fMST.getTable_downgrade(MasterSegmentTable::EMIndex);
+    }
+    else
+    {
+      fPExtMapIndexImpl_->refreshShm();
+
+      fPExtMapIndexImpl_ =
+          ExtentMapIndexImpl::makeExtentMapIndexImpl(getInitialEMIndexShmkey(), fEMIndexShminfo->allocdSize);
+    }
   }
 }
 
diff --git a/versioning/BRM/extentmap.h b/versioning/BRM/extentmap.h
index 0f16d3018..5cba29ab3 100644
--- a/versioning/BRM/extentmap.h
+++ b/versioning/BRM/extentmap.h
@@ -372,6 +372,8 @@ class ExtentMapIndexImpl
   {
     if (fInstance_)
     {
+      // effectively umpas a mapped managed shmem segment changing the segment VA address
+      // when mapped next time.
       delete fInstance_;
       fInstance_ = nullptr;
     }