diff --git a/class.rkt b/class.rkt
index 4f38dce..1df5502 100644
--- a/class.rkt
+++ b/class.rkt
@@ -56,5 +56,8 @@
 
     (define/public (key-values)
       (ks-key-values ksh))
+
+    (define/public (with-lock proc)
+      (ks-with-lock ksh proc))
     )
   )
diff --git a/keystore.rkt b/keystore.rkt
index 6d64c54..e0af688 100644
--- a/keystore.rkt
+++ b/keystore.rkt
@@ -27,8 +27,10 @@
          ks-abort-transaction
          )
 
+(define (any? a) #t)
+
 (define-struct keystore
-  (file path dbh)
+  (file path dbh lock-sem (in-lock #:mutable) (entered #:mutable))
   #:transparent
   )
 
@@ -46,7 +48,7 @@
     (let ((dbh (sqlite3-connect #:database path #:mode 'create)))
       (query-exec dbh "CREATE TABLE IF NOT EXISTS keystore(key varchar primary key, value varchar, str_key varchar);")
       (query-exec dbh "CREATE INDEX IF NOT EXISTS keystore_idx on keystore(str_key);")
-      (make-keystore file path dbh))))
+      (make-keystore file path dbh (make-semaphore 1) #f 0))))
 
 (define/contract (ks-close ksh)
   (-> keystore? boolean?)
@@ -54,6 +56,27 @@
     (disconnect dbh)
     #t))
 
+(define/contract (ks-with-lock ksh proc)
+  (-> keystore? procedure? any?)
+  (dynamic-wind
+   (λ ()
+     (cond
+       ((eq? (keystore-in-lock ksh) (current-thread))
+        (set-keystore-entered! ksh (+ keystore-entered ksh) 1))
+       (else 
+        (semaphore-wait (keystore-lock-sem ksh))
+        (set-keystore-entered! ksh 1)
+        ))
+     (set-keystore-in-lock! ksh (current-thread)))
+   proc
+   (λ ()
+     (set-keystore-entered! (- (keystore-entered ksh) 1))
+     (when (= (keystore-entered ksh) 0)
+       (set-keystore-in-lock! ksh #f)
+       (semaphore-post (keystore-lock-sem ksh))))
+   )
+  )
+
 (define/contract (ks-set! ksh key value)
   (-> keystore? any/c any/c boolean?)
    (ks-set!* (keystore-dbh ksh) (key->string key) (format "~a" key) (value->string value)))
diff --git a/scrbl/class.scrbl b/scrbl/class.scrbl
index 517c9f4..124a663 100644
--- a/scrbl/class.scrbl
+++ b/scrbl/class.scrbl
@@ -83,6 +83,18 @@ Returns all keys.
 
 Returns all key-value pairs.
 }
+
+@defmethod[(with-lock [proc (-> any/c)]) any/c]{
+
+Calls @racket[proc] while holding the lock belonging to this keystore
+object and returns the value produced by @racket[proc]. Other threads
+using @method[keystore% with-lock] on the same object wait until the lock is
+released.
+
+The lock is reentrant for nested calls on the same thread and is
+released when control leaves @racket[proc], including when an exception
+is raised. Keystore methods do not acquire this lock automatically.
+}
 }
 
 @section{Examples}
@@ -115,4 +127,13 @@ Returns all key-value pairs.
 @racketblock[
 (send ks glob "*b*")
 (send ks glob-kv "*b*")
-]
\ No newline at end of file
+]
+
+@subsection{Locking}
+
+@racketblock[
+(send ks with-lock
+      (λ ()
+        (send ks set! 'counter
+              (add1 (send ks get 'counter 0)))))
+]
diff --git a/scrbl/keystore.scrbl b/scrbl/keystore.scrbl
index 6ae20e6..58a9ef5 100644
--- a/scrbl/keystore.scrbl
+++ b/scrbl/keystore.scrbl
@@ -19,11 +19,18 @@ stringified lowercase form, which allows glob-style queries.
 @defstruct*[keystore
   ([file any/c]
    [path path?]
-   [dbh any/c])]{
+   [dbh any/c]
+   [lock-sem semaphore?]
+   [in-lock (or/c thread? #f)]
+   [entered exact-nonnegative-integer?])]{
 
 Represents an open keystore. The @racket[file] field contains the
 original argument, @racket[path] is the resolved database path, and
-@racket[dbh] is the SQLite connection.
+@racket[dbh] is the SQLite connection. The remaining fields implement
+the reentrant lock used by @racket[ks-with-lock]: @racket[lock-sem]
+serializes access, @racket[in-lock] records the thread that holds the
+lock, and @racket[entered] records that thread's nesting depth. The
+@racket[in-lock] and @racket[entered] fields are mutable internal state.
 
 The @tt{keystore?} predicate is altered to also check if the
 database connection is (still) valid.
@@ -149,7 +156,25 @@ Commits a transaction with "COMMIT".
 @defproc[(ks-abort-transaction [ks keystore?]) boolean?]{
 
 Aborts a transaction with "ROLLBACK".
-}                                                         
+}
+
+@section{Locking}
+
+@defproc[(ks-with-lock [ks keystore?] [proc (-> any/c)]) any/c]{
+
+Calls @racket[proc] while holding the lock belonging to @racket[ks] and
+returns the value produced by @racket[proc]. Other threads using
+@racket[ks-with-lock] with the same keystore handle wait until the lock
+is released.
+
+The lock is reentrant: @racket[proc] may call @racket[ks-with-lock]
+again on the same handle and thread. The lock is released when control
+leaves @racket[proc], including when an exception is raised.
+
+The lock coordinates only code that uses the same keystore handle and
+explicitly calls @racket[ks-with-lock]. Individual keystore operations
+do not acquire it automatically.
+}
 
 @section{Examples}
 
@@ -180,4 +205,14 @@ Aborts a transaction with "ROLLBACK".
 
 @racketblock[
 (ks-keys-glob ks "*b*")
-]
\ No newline at end of file
+]
+
+@subsection{Locking Example}
+
+@racketblock[
+(ks-with-lock
+ ks
+ (λ ()
+   (ks-set! ks 'counter
+            (add1 (ks-get ks 'counter 0)))))
+]