diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index c7ffb7528..1d654cd67 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -2,7 +2,6 @@
 #include "misc.hh"
 #include "pathlocks.hh"
 #include "local-store.hh"
-#include "util.hh"
 
 #include <boost/shared_ptr.hpp>
 
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 769b3be7f..9d3cea7a0 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1,6 +1,5 @@
 #include "config.h"
 #include "local-store.hh"
-#include "util.hh"
 #include "globals.hh"
 #include "archive.hh"
 #include "pathlocks.hh"
@@ -43,40 +42,37 @@ LocalStore::LocalStore()
 {
     substitutablePathsLoaded = false;
     
+    schemaPath = nixDBPath + "/schema";
+    
     if (readOnlyMode) return;
 
     checkStoreNotSymlink();
 
-    try {
-        createDirs(nixDBPath + "/info");
-        createDirs(nixDBPath + "/referrer");
-    } catch (Error & e) {
-        // !!! fix access check
-        printMsg(lvlTalkative, "cannot access Nix database; continuing anyway");
-        readOnlyMode = true;
-        return;
-    }
+    Path globalLockPath = nixDBPath + "/big-lock";
+    globalLock = open(globalLockPath.c_str(), O_RDWR | O_CREAT, 0666);
+    if (globalLock == -1) throw SysError(format("opening file `%1%'") % globalLockPath);
     
-    int curSchema = 0;
-    Path schemaFN = nixDBPath + "/schema";
-    if (pathExists(schemaFN)) {
-        string s = readFile(schemaFN);
-        if (!string2Int(s, curSchema))
-            throw Error(format("`%1%' is corrupt") % schemaFN);
+    if (!lockFile(globalLock, ltRead, false)) {
+        printMsg(lvlError, "waiting for the big Nix store lock...");
+        lockFile(globalLock, ltRead, true);
     }
 
+    createDirs(nixDBPath + "/info");
+    createDirs(nixDBPath + "/referrer");
+
+    //printMsg(lvlTalkative, "cannot access Nix database; continuing anyway");
+    //readOnlyMode = true;
+
+    int curSchema = getSchema();
     if (curSchema > nixSchemaVersion)
         throw Error(format("current Nix store schema is version %1%, but I only support %2%")
             % curSchema % nixSchemaVersion);
-
-    if (curSchema < nixSchemaVersion) {
-        if (curSchema == 0) /* new store */
-            curSchema = nixSchemaVersion;
-        if (curSchema <= 1)
-            throw Error("your Nix store is no longer supported");
-        if (curSchema <= 4) upgradeStore12();
-        writeFile(schemaFN, (format("%1%") % nixSchemaVersion).str());
+    if (curSchema == 0) { /* new store */
+        curSchema = nixSchemaVersion; 
+        writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str());
     }
+    if (curSchema == 1) throw Error("your Nix store is no longer supported");
+    if (curSchema < nixSchemaVersion) upgradeStore12();
 }
 
 
@@ -90,6 +86,18 @@ LocalStore::~LocalStore()
 }
 
 
+int LocalStore::getSchema()
+{
+    int curSchema = 0;
+    if (pathExists(schemaPath)) {
+        string s = readFile(schemaPath);
+        if (!string2Int(s, curSchema))
+            throw Error(format("`%1%' is corrupt") % schemaPath);
+    }
+    return curSchema;
+}
+
+
 void copyPath(const Path & src, const Path & dst, PathFilter & filter)
 {
     debug(format("copying `%1%' to `%2%'") % src % dst);
diff --git a/src/libstore/local-store.hh b/src/libstore/local-store.hh
index ead49baae..57d9d765e 100644
--- a/src/libstore/local-store.hh
+++ b/src/libstore/local-store.hh
@@ -4,6 +4,7 @@
 #include <string>
 
 #include "store-api.hh"
+#include "util.hh"
 
 
 namespace nix {
@@ -114,6 +115,11 @@ public:
 
 private:
 
+    Path schemaPath;
+
+    /* Lock file used for upgrading. */
+    AutoCloseFD globalLock;
+
     /* !!! The cache can grow very big.  Maybe it should be pruned
        every once in a while. */
     std::map<Path, ValidPathInfo> pathInfoCache;
@@ -121,6 +127,8 @@ private:
     /* Store paths for which the referrers file must be purged. */
     PathSet delayedUpdates;
 
+    int getSchema();
+
     void registerValidPath(const ValidPathInfo & info, bool ignoreValidity = false);
 
     ValidPathInfo queryPathInfo(const Path & path);
diff --git a/src/libstore/optimise-store.cc b/src/libstore/optimise-store.cc
index d03c4513e..c260f253e 100644
--- a/src/libstore/optimise-store.cc
+++ b/src/libstore/optimise-store.cc
@@ -1,5 +1,5 @@
-#include "local-store.hh"
 #include "util.hh"
+#include "local-store.hh"
 
 #include <sys/types.h>
 #include <sys/stat.h>
diff --git a/src/libstore/upgrade-schema.cc b/src/libstore/upgrade-schema.cc
index b3d691485..6b8358582 100644
--- a/src/libstore/upgrade-schema.cc
+++ b/src/libstore/upgrade-schema.cc
@@ -3,6 +3,7 @@
 #include "util.hh"
 #include "local-store.hh"
 #include "globals.hh"
+#include "pathlocks.hh"
 
 #include <iostream>
 
@@ -18,8 +19,15 @@ Hash parseHashField(const Path & path, const string & s);
    meta-information in files. */
 void LocalStore::upgradeStore12()
 {
+    if (!lockFile(globalLock, ltWrite, false)) {
+        printMsg(lvlError, "waiting for exclusive access to the Nix store...");
+        lockFile(globalLock, ltWrite, true);
+    }
+
     printMsg(lvlError, "upgrading Nix store to new schema (this may take a while)...");
 
+    if (getSchema() >= nixSchemaVersion) return; /* somebody else beat us to it */
+
     /* Open the old Nix database and tables. */
     Database nixDB;
     nixDB.open(nixDBPath);
@@ -76,6 +84,10 @@ void LocalStore::upgradeStore12()
     }
 
     std::cerr << std::endl;
+
+    writeFile(schemaPath, (format("%1%") % nixSchemaVersion).str());
+
+    lockFile(globalLock, ltRead, true);
 }