diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index 7b6cc4e..506be86 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -674,9 +674,7 @@ zstream_expand_buffer(struct zstream *z)
 	        rb_obj_reveal(z->buf, rb_cString);
             }
 
-            rb_mutex_unlock(z->mutex);
-	    rb_protect(rb_yield, z->buf, &state);
-            rb_mutex_lock(z->mutex);
+            rb_protect(rb_yield, z->buf, &state);
 
             if (ZSTREAM_REUSE_BUFFER_P(z)) {
                 rb_str_modify(z->buf);
@@ -720,15 +718,14 @@ zstream_expand_buffer_into(struct zstream *z, unsigned long size)
     }
 }
 
-static void *
-zstream_expand_buffer_protect(void *ptr)
+static int
+zstream_expand_buffer_protect(struct zstream *z)
 {
-    struct zstream *z = (struct zstream *)ptr;
     int state = 0;
 
     rb_protect((VALUE (*)(VALUE))zstream_expand_buffer, (VALUE)z, &state);
 
-    return (void *)(VALUE)state;
+    return state;
 }
 
 static int
@@ -1023,57 +1020,14 @@ zstream_ensure_end(VALUE v)
 }
 
 static void *
-zstream_run_func(void *ptr)
+zstream_run_once(void *_arguments)
 {
-    struct zstream_run_args *args = (struct zstream_run_args *)ptr;
-    int err, state, flush = args->flush;
-    struct zstream *z = args->z;
-    uInt n;
-
-    err = Z_OK;
-    while (!args->interrupt) {
-	n = z->stream.avail_out;
-	err = z->func->run(&z->stream, flush);
-	rb_str_set_len(z->buf, ZSTREAM_BUF_FILLED(z) + (n - z->stream.avail_out));
-
-	if (err == Z_STREAM_END) {
-	    z->flags &= ~ZSTREAM_FLAG_IN_STREAM;
-	    z->flags |= ZSTREAM_FLAG_FINISHED;
-	    break;
-	}
-
-	if (err != Z_OK && err != Z_BUF_ERROR)
-	    break;
-
-	if (z->stream.avail_out > 0) {
-	    z->flags |= ZSTREAM_FLAG_IN_STREAM;
-	    break;
-	}
-
-	if (z->stream.avail_in == 0 && z->func == &inflate_funcs) {
-	    /* break here because inflate() return Z_BUF_ERROR when avail_in == 0. */
-	    /* but deflate() could be called with avail_in == 0 (there's hidden buffer
-	       in zstream->state) */
-	    z->flags |= ZSTREAM_FLAG_IN_STREAM;
-	    break;
-	}
-
-	if (args->stream_output) {
-	    state = (int)(VALUE)rb_thread_call_with_gvl(zstream_expand_buffer_protect,
-							(void *)z);
-	}
-	else {
-	    state = zstream_expand_buffer_non_stream(z);
-	}
+    struct zstream_run_args *arguments = (struct zstream_run_args *)_arguments;
+    struct zstream *z = arguments->z;
 
-	if (state) {
-	    err = Z_OK; /* buffer expanded but stream processing was stopped */
-	    args->jump_state = state;
-	    break;
-	}
-    }
+    uintptr_t error = z->func->run(&z->stream, arguments->flush);
 
-    return (void *)(VALUE)err;
+    return (void*)error;
 }
 
 /*
@@ -1088,6 +1042,86 @@ zstream_unblock_func(void *ptr)
     args->interrupt = 1;
 }
 
+static VALUE
+zstream_run_once_begin(VALUE _arguments)
+{
+    struct zstream_run_args *arguments = (struct zstream_run_args *)_arguments;
+    struct zstream *z = arguments->z;
+
+    rb_str_locktmp(z->buf);
+
+#ifndef RB_NOGVL_UBF_ASYNC_SAFE
+    return (VALUE)rb_thread_call_without_gvl(zstream_run_once, (void *)arguments, zstream_unblock_func, (void *)arguments);
+#else
+    return (VALUE)rb_nogvl(zstream_run_once, (void *)arguments, zstream_unblock_func, (void *)arguments, RB_NOGVL_UBF_ASYNC_SAFE);
+#endif
+}
+
+static VALUE
+zstream_run_once_ensure(VALUE _arguments)
+{
+    struct zstream_run_args *arguments = (struct zstream_run_args *)_arguments;
+    struct zstream *z = arguments->z;
+
+    rb_str_unlocktmp(z->buf);
+
+    return Qnil;
+}
+
+static int
+zstream_run_func(struct zstream_run_args *args)
+{
+    struct zstream *z = args->z;
+    int state;
+    uInt n;
+
+    int err = Z_OK;
+    while (!args->interrupt) {
+        n = z->stream.avail_out;
+
+        err = (int)(VALUE)rb_ensure(zstream_run_once_begin, (VALUE)args, zstream_run_once_ensure, (VALUE)args);
+
+        rb_str_set_len(z->buf, ZSTREAM_BUF_FILLED(z) + (n - z->stream.avail_out));
+
+        if (err == Z_STREAM_END) {
+            z->flags &= ~ZSTREAM_FLAG_IN_STREAM;
+            z->flags |= ZSTREAM_FLAG_FINISHED;
+            break;
+        }
+
+        if (err != Z_OK && err != Z_BUF_ERROR)
+            break;
+
+        if (z->stream.avail_out > 0) {
+            z->flags |= ZSTREAM_FLAG_IN_STREAM;
+            break;
+        }
+
+        if (z->stream.avail_in == 0 && z->func == &inflate_funcs) {
+            /* break here because inflate() return Z_BUF_ERROR when avail_in == 0. */
+            /* but deflate() could be called with avail_in == 0 (there's hidden buffer
+                in zstream->state) */
+            z->flags |= ZSTREAM_FLAG_IN_STREAM;
+            break;
+        }
+
+        if (args->stream_output) {
+            state = zstream_expand_buffer_protect(z);
+        }
+        else {
+            state = zstream_expand_buffer_non_stream(z);
+        }
+
+        if (state) {
+            err = Z_OK; /* buffer expanded but stream processing was stopped */
+            args->jump_state = state;
+            break;
+        }
+    }
+
+    return err;
+}
+
 static VALUE
 zstream_run_try(VALUE value_arg)
 {
@@ -1126,14 +1160,7 @@ zstream_run_try(VALUE value_arg)
     }
 
 loop:
-#ifndef RB_NOGVL_UBF_ASYNC_SAFE
-    err = (int)(VALUE)rb_thread_call_without_gvl(zstream_run_func, (void *)args,
-						 zstream_unblock_func, (void *)args);
-#else
-    err = (int)(VALUE)rb_nogvl(zstream_run_func, (void *)args,
-                               zstream_unblock_func, (void *)args,
-                               RB_NOGVL_UBF_ASYNC_SAFE);
-#endif
+    err = zstream_run_func(args);
 
     /* retry if no exception is thrown */
     if (err == Z_OK && args->interrupt) {
@@ -1184,7 +1211,6 @@ zstream_run_ensure(VALUE value_arg)
 
     /* Remove ZSTREAM_IN_PROGRESS flag to signal that this zstream is not in use. */
     z->flags &= ~ZSTREAM_IN_PROGRESS;
-    rb_mutex_unlock(z->mutex);
 
     return Qnil;
 }
@@ -1201,7 +1227,7 @@ zstream_run(struct zstream *z, Bytef *src, long len, int flush)
         .jump_state = 0,
         .stream_output = !ZSTREAM_IS_GZFILE(z) && rb_block_given_p(),
     };
-    rb_mutex_lock(z->mutex);
+
     rb_ensure(zstream_run_try, (VALUE)&args, zstream_run_ensure, (VALUE)&args);
     if (args.jump_state)
 	rb_jump_tag(args.jump_state);
@@ -1770,6 +1796,22 @@ do_deflate(struct zstream *z, VALUE src, int flush)
     }
 }
 
+struct rb_zlib_deflate_arguments {
+    struct zstream *z;
+    VALUE src;
+    int flush;
+};
+
+static VALUE
+rb_deflate_deflate_body(VALUE args)
+{
+    struct rb_zlib_deflate_arguments *arguments = (struct rb_zlib_deflate_arguments *)args;
+
+    do_deflate(arguments->z, arguments->src, arguments->flush);
+
+    return zstream_detach_buffer(arguments->z);
+}
+
 /*
  * Document-method: Zlib::Deflate#deflate
  *
@@ -1801,11 +1843,10 @@ rb_deflate_deflate(int argc, VALUE *argv, VALUE obj)
 {
     struct zstream *z = get_zstream(obj);
     VALUE src, flush;
-
     rb_scan_args(argc, argv, "11", &src, &flush);
-    do_deflate(z, src, ARG_FLUSH(flush));
+    struct rb_zlib_deflate_arguments arguments = {z, src, ARG_FLUSH(flush)};
 
-    return zstream_detach_buffer(z);
+    return rb_mutex_synchronize(z->mutex, rb_deflate_deflate_body, (VALUE)&arguments);
 }
 
 /*
@@ -2101,56 +2142,19 @@ rb_inflate_add_dictionary(VALUE obj, VALUE dictionary)
     return obj;
 }
 
-/*
- * Document-method: Zlib::Inflate#inflate
- *
- * call-seq:
- *   inflate(deflate_string, buffer: nil)                 -> String
- *   inflate(deflate_string, buffer: nil) { |chunk| ... } -> nil
- *
- * Inputs +deflate_string+ into the inflate stream and returns the output from
- * the stream.  Calling this method, both the input and the output buffer of
- * the stream are flushed.  If string is +nil+, this method finishes the
- * stream, just like Zlib::ZStream#finish.
- *
- * If a block is given consecutive inflated chunks from the +deflate_string+
- * are yielded to the block and +nil+ is returned.
- *
- * If a :buffer keyword argument is given and not nil:
- *
- * * The :buffer keyword should be a String, and will used as the output buffer.
- *   Using this option can reuse the memory required during inflation.
- * * When not passing a block, the return value will be the same object as the
- *   :buffer keyword argument.
- * * When passing a block, the yielded chunks will be the same value as the
- *   :buffer keyword argument.
- *
- * Raises a Zlib::NeedDict exception if a preset dictionary is needed to
- * decompress.  Set the dictionary by Zlib::Inflate#set_dictionary and then
- * call this method again with an empty string to flush the stream:
- *
- *   inflater = Zlib::Inflate.new
- *
- *   begin
- *     out = inflater.inflate compressed
- *   rescue Zlib::NeedDict
- *     # ensure the dictionary matches the stream's required dictionary
- *     raise unless inflater.adler == Zlib.adler32(dictionary)
- *
- *     inflater.set_dictionary dictionary
- *     inflater.inflate ''
- *   end
- *
- *   # ...
- *
- *   inflater.close
- *
- * See also Zlib::Inflate.new
- */
+struct rb_zlib_inflate_arguments {
+    struct zstream *z;
+    int argc;
+    VALUE *argv;
+};
+
 static VALUE
-rb_inflate_inflate(int argc, VALUE* argv, VALUE obj)
+rb_inflate_inflate_body(VALUE _arguments)
 {
-    struct zstream *z = get_zstream(obj);
+    struct rb_zlib_inflate_arguments *arguments = (struct rb_zlib_inflate_arguments*)_arguments;
+    struct zstream *z = arguments->z;
+    int argc = arguments->argc;
+    VALUE *argv = arguments->argv;
     VALUE dst, src, opts, buffer = Qnil;
 
     if (OPTHASH_GIVEN_P(opts)) {
@@ -2205,6 +2209,61 @@ rb_inflate_inflate(int argc, VALUE* argv, VALUE obj)
     return dst;
 }
 
+/*
+ * Document-method: Zlib::Inflate#inflate
+ *
+ * call-seq:
+ *   inflate(deflate_string, buffer: nil)                 -> String
+ *   inflate(deflate_string, buffer: nil) { |chunk| ... } -> nil
+ *
+ * Inputs +deflate_string+ into the inflate stream and returns the output from
+ * the stream.  Calling this method, both the input and the output buffer of
+ * the stream are flushed.  If string is +nil+, this method finishes the
+ * stream, just like Zlib::ZStream#finish.
+ *
+ * If a block is given consecutive inflated chunks from the +deflate_string+
+ * are yielded to the block and +nil+ is returned.
+ *
+ * If a :buffer keyword argument is given and not nil:
+ *
+ * * The :buffer keyword should be a String, and will used as the output buffer.
+ *   Using this option can reuse the memory required during inflation.
+ * * When not passing a block, the return value will be the same object as the
+ *   :buffer keyword argument.
+ * * When passing a block, the yielded chunks will be the same value as the
+ *   :buffer keyword argument.
+ *
+ * Raises a Zlib::NeedDict exception if a preset dictionary is needed to
+ * decompress.  Set the dictionary by Zlib::Inflate#set_dictionary and then
+ * call this method again with an empty string to flush the stream:
+ *
+ *   inflater = Zlib::Inflate.new
+ *
+ *   begin
+ *     out = inflater.inflate compressed
+ *   rescue Zlib::NeedDict
+ *     # ensure the dictionary matches the stream's required dictionary
+ *     raise unless inflater.adler == Zlib.adler32(dictionary)
+ *
+ *     inflater.set_dictionary dictionary
+ *     inflater.inflate ''
+ *   end
+ *
+ *   # ...
+ *
+ *   inflater.close
+ *
+ * See also Zlib::Inflate.new
+ */
+static VALUE
+rb_inflate_inflate(int argc, VALUE* argv, VALUE obj)
+{
+    struct zstream *z = get_zstream(obj);
+    struct rb_zlib_inflate_arguments arguments = {z, argc, argv};
+    
+    return rb_mutex_synchronize(z->mutex, rb_inflate_inflate_body, (VALUE)&arguments);
+}
+
 /*
  * call-seq: << string
  *
diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index 15e5bd8..5a8463a 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -545,7 +545,7 @@ def test_recursive_deflate
       zd = Zlib::Deflate.new
 
       s = SecureRandom.random_bytes(1024**2)
-      assert_raise(Zlib::InProgressError) do
+      assert_raise(ThreadError) do
         zd.deflate(s) do
           zd.deflate(s)
         end
@@ -563,7 +563,7 @@ def test_recursive_inflate
 
       s = Zlib.deflate(SecureRandom.random_bytes(1024**2))
 
-      assert_raise(Zlib::InProgressError) do
+      assert_raise(ThreadError) do
         zi.inflate(s) do
           zi.inflate(s)
         end