Index: src/os_unix.c
==================================================================
--- src/os_unix.c
+++ src/os_unix.c
@@ -44,10 +44,13 @@
 **      plus implementations of sqlite3_os_init() and sqlite3_os_end().
 */
 #include "sqliteInt.h"
 #if SQLITE_OS_UNIX              /* This file is used on unix only */
 
+/* Turn this feature on in all builds for now */
+#define SQLITE_MUTEXFREE_SHMLOCK 1
+
 /*
 ** There are various methods for file locking used for concurrency
 ** control:
 **
 **   1. POSIX locking (the default),
@@ -4237,11 +4240,41 @@
 #ifdef SQLITE_DEBUG
   u8 exclMask;               /* Mask of exclusive locks held */
   u8 sharedMask;             /* Mask of shared locks held */
   u8 nextShmId;              /* Next available unixShm.id value */
 #endif
+
+#ifdef SQLITE_MUTEXFREE_SHMLOCK
+  /* In unix-excl mode, if SQLITE_MUTEXFREE_SHMLOCK is defined, all locks
+  ** are stored in the following 64-bit value. There are in total 8 
+  ** shm-locking slots, each of which are assigned 8-bits from the 64-bit
+  ** value. The least-significant 8 bits correspond to shm-locking slot
+  ** 0, and so on.
+  **
+  ** If the 8-bits corresponding to a shm-locking locking slot are set to
+  ** 0xFF, then a write-lock is held on the slot. Or, if they are set to
+  ** a non-zero value smaller than 0xFF, then they represent the total 
+  ** number of read-locks held on the slot. There is no way to distinguish
+  ** between a write-lock and 255 read-locks.  */
+  u64 lockmask;
+#endif
 };
+
+/*
+** Atomic CAS primitive used in multi-process mode. Equivalent to:
+** 
+**   int unixCompareAndSwap(u32 *ptr, u32 oldval, u32 newval){
+**     if( *ptr==oldval ){
+**       *ptr = newval;
+**       return 1;
+**     }
+**     return 0;
+**   }
+*/
+#define unixCompareAndSwap(ptr,oldval,newval) \
+    __sync_bool_compare_and_swap(ptr,oldval,newval)
+
 
 /*
 ** Structure used internally by this VFS to record the state of an
 ** open shared memory connection.
 **
@@ -4798,10 +4831,62 @@
   assert( pShmNode->hShm>=0 || pDbFd->pInode->bProcessLock==1 );
   assert( pShmNode->hShm<0 || pDbFd->pInode->bProcessLock==0 );
 
   mask = (1<<(ofst+n)) - (1<<ofst);
   assert( n>1 || mask==(1<<ofst) );
+
+#ifdef SQLITE_MUTEXFREE_SHMLOCK
+  if( pDbFd->pInode->bProcessLock ){
+
+    while( 1 ){
+      u64 lockmask = pShmNode->lockmask;
+      u64 newmask = lockmask;
+      int i;
+      for(i=ofst; i<n+ofst; i++){
+        int ix8 = i*8;
+        u8 v = (lockmask >> (ix8)) & 0xFF;
+        if( flags & SQLITE_SHM_UNLOCK ){
+          if( flags & SQLITE_SHM_EXCLUSIVE ){
+            if( p->exclMask & (1 << i) ){
+              newmask = newmask & ~((u64)0xFF<<ix8);
+            }
+          }else{
+            if( p->sharedMask & (1 << i) ){
+              newmask = newmask & ~((u64)0xFF<<ix8) | ((u64)(v-1)<<ix8);
+            }
+          }
+        }else{
+          if( flags & SQLITE_SHM_EXCLUSIVE ){
+            if( v ) return SQLITE_BUSY;
+            if( (p->exclMask & (1 << i))==0 ){
+              newmask = newmask | ((u64)0xFF<<ix8);
+            }
+          }else{
+            if( v==0xFF ) return SQLITE_BUSY;
+            if( (p->sharedMask & (1 << i))==0 ){
+              newmask = newmask & ~((u64)0xFF<<ix8) | ((u64)(v+1)<<ix8);
+            }
+          }
+        }
+      }
+
+      if( unixCompareAndSwap(&pShmNode->lockmask, lockmask, newmask) ) break;
+    }
+
+    if( flags & SQLITE_SHM_UNLOCK ){
+      p->sharedMask &= ~mask;
+      p->exclMask &= ~mask;
+    }else if( flags & SQLITE_SHM_EXCLUSIVE ){
+      p->exclMask |= mask;
+    }else{
+      p->sharedMask |= mask;
+    }
+
+    return SQLITE_OK;
+  }
+#endif
+
   sqlite3_mutex_enter(pShmNode->pShmMutex);
   if( flags & SQLITE_SHM_UNLOCK ){
     u16 allMask = 0; /* Mask of locks held by siblings */
 
     /* See if any siblings hold this same lock */

Index: src/test_vfs.c
==================================================================
--- src/test_vfs.c
+++ src/test_vfs.c
@@ -1560,12 +1560,66 @@
 
  bad_args:
   Tcl_WrongNumArgs(interp, 1, objv, "VFSNAME ?-noshm BOOL? ?-fullshm BOOL? ?-default BOOL? ?-mxpathname INT? ?-szosfile INT? ?-iversion INT?");
   return TCL_ERROR;
 }
+
+extern int getDbPointer(Tcl_Interp *interp, const char *zA, sqlite3 **ppDb);
+extern const char *sqlite3ErrName(int);
+
+/*
+** tclcmd: vfs_shmlock DB DBNAME (shared|exclusive) (lock|unlock) OFFSET N
+*/
+static int SQLITE_TCLAPI test_vfs_shmlock(
+  void * clientData,
+  Tcl_Interp *interp,
+  int objc,
+  Tcl_Obj *CONST objv[]
+){
+  const char *azArg1[] = {"shared", "exclusive", 0};
+  const char *azArg2[] = {"lock", "unlock", 0};
+  sqlite3 *db = 0;
+  int rc = SQLITE_OK;
+  const char *zDbname = 0;
+  int iArg1 = 0;
+  int iArg2 = 0;
+  int iOffset = 0;
+  int n = 0;
+  sqlite3_file *pFd;
+
+  if( objc!=7 ){
+    Tcl_WrongNumArgs(interp, 1, objv, 
+        "DB DBNAME (shared|exclusive) (lock|unlock) OFFSET N"
+    );
+    return TCL_ERROR;
+  }
+
+  zDbname = Tcl_GetString(objv[2]);
+  if( getDbPointer(interp, Tcl_GetString(objv[1]), &db) 
+   || Tcl_GetIndexFromObj(interp, objv[3], azArg1, "ARG", 0, &iArg1) 
+   || Tcl_GetIndexFromObj(interp, objv[4], azArg2, "ARG", 0, &iArg2) 
+   || Tcl_GetIntFromObj(interp, objv[5], &iOffset)
+   || Tcl_GetIntFromObj(interp, objv[6], &n)
+  ){
+    return TCL_ERROR;
+  }
+
+  sqlite3_file_control(db, zDbname, SQLITE_FCNTL_FILE_POINTER, (void*)&pFd);
+  if( pFd==0 ){
+    return TCL_ERROR;
+  }
+  rc = pFd->pMethods->xShmLock(pFd, iOffset, n, 
+      (iArg1==0 ? SQLITE_SHM_SHARED : SQLITE_SHM_EXCLUSIVE)
+    | (iArg2==0 ? SQLITE_SHM_LOCK : SQLITE_SHM_UNLOCK)
+  );
+  Tcl_SetObjResult(interp, Tcl_NewStringObj(sqlite3ErrName(rc), -1));
+  return TCL_OK;
+}
+
 
 int Sqlitetestvfs_Init(Tcl_Interp *interp){
   Tcl_CreateObjCommand(interp, "testvfs", testvfs_cmd, 0, 0);
+  Tcl_CreateObjCommand(interp, "vfs_shmlock", test_vfs_shmlock, 0, 0);
   return TCL_OK;
 }
 
 #endif

Index: test/lock_common.tcl
==================================================================
--- test/lock_common.tcl
+++ test/lock_common.tcl
@@ -13,22 +13,24 @@
 # processes) to test locking.
 #
 
 proc do_multiclient_test {varname script} {
 
-  foreach code [list {
+  foreach {tn code} [list 1 {
     if {[info exists ::G(valgrind)]} { db close ; continue }
     set ::code2_chan [launch_testfixture]
     set ::code3_chan [launch_testfixture]
     proc code2 {tcl} { testfixture $::code2_chan $tcl }
     proc code3 {tcl} { testfixture $::code3_chan $tcl }
-    set tn 1
-  } {
+  } 2 {
     proc code2 {tcl} { uplevel #0 $tcl }
     proc code3 {tcl} { uplevel #0 $tcl }
-    set tn 2
   }] {
+    # Do not run multi-process tests with the unix-excl VFS.
+    #
+    if {$tn==1 && [permutation]=="unix-excl"} continue
+
     faultsim_delete_and_reopen
 
     proc code1 {tcl} { uplevel #0 $tcl }
   
     # Open connections [db2] and [db3]. Depending on which iteration this

Index: test/permutations.test
==================================================================
--- test/permutations.test
+++ test/permutations.test
@@ -604,10 +604,20 @@
   set ::G(perm:sqlite3_args) [list -vfs fs]
 } -files {
   conflict.test  insert.test   insert2.test  insert3.test
   rollback.test  select1.test  select2.test  select3.test
 }
+
+# Run some tests using the "unix-excl" VFS.
+#
+test_suite "unix-excl" -description {
+  Run some tests using the "unix-excl" VFS
+} -initialize {
+  set ::G(perm:sqlite3_args) [list -vfs unix-excl]
+} -files {
+  shmlock.test
+}
 
 # Run some tests using UTF-16 databases.
 #
 test_suite "utf16" -description {
   Run tests using UTF-16 databases

ADDED   test/shmlock.test
Index: test/shmlock.test
==================================================================
--- test/shmlock.test
+++ test/shmlock.test
@@ -0,0 +1,111 @@
+# 2018 December 6
+#
+# The author disclaims copyright to this source code.  In place of
+# a legal notice, here is a blessing:
+#
+#    May you do good and not evil.
+#    May you find forgiveness for yourself and forgive others.
+#    May you share freely, never taking more than you give.
+#
+#***********************************************************************
+#
+
+set testdir [file dirname $argv0]
+source $testdir/tester.tcl
+
+set testprefix shmlock
+
+ifcapable !wal {finish_test ; return }
+
+sqlite3 db2 test.db
+sqlite3 db3 test.db
+
+do_execsql_test 1.0 {
+  PRAGMA journal_mode = wal;
+  CREATE TABLE t1(a, b);
+  INSERT INTO t1 VALUES(1, 2);
+} {wal}
+do_test 1.1 { execsql { SELECT * FROM t1 } db2 } {1 2}
+do_test 1.2 { execsql { SELECT * FROM t1 } db3 } {1 2}
+
+foreach {tn dbhandle cmd res} {
+  1    db  {shared    lock   7 1}    OK
+  2    db2 {exclusive lock   7 1}    BUSY
+  3    db  {shared    unlock 7 1}    OK
+  4    db2 {exclusive lock   7 1}    OK
+  5    db  {shared    lock   7 1}    BUSY
+  6    db  {exclusive lock   7 1}    BUSY
+  7    db2 {exclusive unlock 7 1}    OK
+
+  8    db  {exclusive lock   0 8}    OK
+  9    db  {exclusive unlock 0 8}    OK
+  10   db2 {exclusive lock   0 8}    OK
+  11   db2 {exclusive unlock 0 8}    OK
+
+  12   db  {shared    lock   0 1}    OK
+  13   db2 {shared    lock   0 1}    OK
+  14   db3 {shared    lock   0 1}    OK
+  15   db3 {shared    unlock 0 1}    OK
+  16   db3 {exclusive lock   0 1}    BUSY
+  17   db2 {shared    unlock 0 1}    OK
+  18   db3 {exclusive lock   0 1}    BUSY
+  19   db  {shared    unlock 0 1}    OK
+  20   db3 {exclusive lock   0 1}    OK
+  21   db3 {exclusive unlock 0 1}    OK
+
+  22   db  {shared    lock   3 1}    OK
+  23   db2 {exclusive lock   2 2}    BUSY
+  24   db  {shared    lock   2 1}    OK
+  25   db2 {exclusive lock   0 5}    BUSY
+  26   db2 {exclusive lock   0 4}    BUSY
+  27   db2 {exclusive lock   0 3}    BUSY
+  28   db  {shared    unlock 3 1}    OK
+  29   db2 {exclusive lock   2 2}    BUSY
+  28   db  {shared    unlock 2 1}    OK
+  29   db2 {exclusive lock   2 2}    OK
+  29   db2 {exclusive unlock 2 2}    OK
+} {
+  do_test 1.3.$tn [list vfs_shmlock $dbhandle main {*}$cmd] "SQLITE_$res"
+}
+
+db  close
+db2 close
+db3 close
+
+if {[permutation]=="unix-excl"} {
+  do_test 2.0 {
+    for {set i 0} {$i < 256} {incr i} { 
+      sqlite3 db$i test.db 
+      execsql { SELECT * FROM t1 } db$i
+    }
+    for {set i 0} {$i < 255} {incr i} { 
+      set rc [vfs_shmlock db$i main shared lock 4 1]
+      if {$rc != "SQLITE_OK"} { error $rc }
+    }
+
+    vfs_shmlock db255 main shared lock 4 1
+  } {SQLITE_BUSY}
+
+  do_test 2.1 { vfs_shmlock db255 main exclusive lock   4 1 } SQLITE_BUSY
+  do_test 2.2 { vfs_shmlock db0   main shared    unlock 4 1 } SQLITE_OK
+  do_test 2.3 { vfs_shmlock db255 main shared    lock   4 1 } SQLITE_OK
+  do_test 2.4 { vfs_shmlock db255 main shared    unlock 4 1 } SQLITE_OK
+  do_test 2.5 { vfs_shmlock db255 main exclusive lock   4 1 } SQLITE_BUSY
+
+  do_test 2.6 {
+    for {set i 1} {$i < 255} {incr i} { 
+      set rc [vfs_shmlock db255 main exclusive lock 4 1]
+      if {$rc != "SQLITE_BUSY"} { error $rc }
+      set rc [vfs_shmlock db$i main shared unlock 4 1]
+      if {$rc != "SQLITE_OK"} { error $rc }
+    }
+
+    vfs_shmlock db255 main exclusive lock 4 1
+  } {SQLITE_OK}
+
+  vfs_shmlock db255 main exclusive unlock 4 1
+}
+
+finish_test
+
+

Index: test/wal.test
==================================================================
--- test/wal.test
+++ test/wal.test
@@ -1295,55 +1295,57 @@
 #      the data is present and the database is not corrupt.
 #
 # At one point, SQLite was failing to grow the mapping of the wal-index
 # file in step 3 and the checkpoint was corrupting the database file.
 #
-do_test wal-20.1 {
-  catch {db close}
-  forcedelete test.db test.db-wal test.db-journal
-  sqlite3 db test.db
-  execsql {
-    PRAGMA journal_mode = WAL;
-    CREATE TABLE t1(x);
-    INSERT INTO t1 VALUES(randomblob(900));
-    SELECT count(*) FROM t1;
-  }
-} {wal 1}
-do_test wal-20.2 {
-  set ::buddy [launch_testfixture]
-  testfixture $::buddy {
+if {[permutation]!="unix-excl"} {
+  do_test wal-20.1 {
+    catch {db close}
+    forcedelete test.db test.db-wal test.db-journal
+    sqlite3 db test.db
+    execsql {
+      PRAGMA journal_mode = WAL;
+      CREATE TABLE t1(x);
+      INSERT INTO t1 VALUES(randomblob(900));
+      SELECT count(*) FROM t1;
+    }
+  } {wal 1}
+  do_test wal-20.2 {
+    set ::buddy [launch_testfixture]
+    testfixture $::buddy {
+      sqlite3 db test.db
+      db transaction { db eval {
+        PRAGMA wal_autocheckpoint = 0;
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 2 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 4 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 8 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 16 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 32 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 64 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 128 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 256 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 512 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 1024 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 2048 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 4096 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 8192 */
+        INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 16384 */
+      } }
+    }
+  } {0}
+  do_test wal-20.3 {
+    close $::buddy
+    execsql { PRAGMA wal_checkpoint }
+    execsql { SELECT count(*) FROM t1 }
+  } {16384}
+  do_test wal-20.4 {
+    db close
     sqlite3 db test.db
-    db transaction { db eval {
-      PRAGMA wal_autocheckpoint = 0;
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 2 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 4 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 8 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 16 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 32 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 64 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 128 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 256 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 512 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 1024 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 2048 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 4096 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 8192 */
-      INSERT INTO t1 SELECT randomblob(900) FROM t1;       /* 16384 */
-    } }
-  }
-} {0}
-do_test wal-20.3 {
-  close $::buddy
-  execsql { PRAGMA wal_checkpoint }
-  execsql { SELECT count(*) FROM t1 }
-} {16384}
-do_test wal-20.4 {
-  db close
-  sqlite3 db test.db
-  execsql { SELECT count(*) FROM t1 }
-} {16384}
-integrity_check wal-20.5
+    execsql { SELECT count(*) FROM t1 }
+  } {16384}
+  integrity_check wal-20.5
+}
 
 catch { db2 close }
 catch { db close }
 
 do_test wal-21.1 {