diff --git a/src/os/unix/ngx_linux_sendfile_chain.c b/src/os/unix/ngx_linux_sendfile_chain.c
index 3c0696a..b44724c 100644
--- a/src/os/unix/ngx_linux_sendfile_chain.c
+++ b/src/os/unix/ngx_linux_sendfile_chain.c
@@ -20,8 +20,8 @@
 #error sendfile64() is required!
 #endif
 
-static ngx_int_t ngx_linux_sendfile_thread(ngx_connection_t *c, ngx_buf_t *file,
-    size_t size, size_t *sent);
+static ssize_t ngx_linux_sendfile_thread(ngx_connection_t *c, ngx_buf_t *file,
+    size_t size);
 static void ngx_linux_sendfile_thread_handler(void *data, ngx_log_t *log);
 #endif
 
@@ -56,10 +56,6 @@
     ngx_chain_t   *cl;
     ngx_iovec_t    header;
     struct iovec   headers[NGX_IOVS_PREALLOCATE];
-#if (NGX_THREADS)
-    ngx_int_t      rc;
-    ngx_uint_t     thread_handled, thread_complete;
-#endif
 
     wev = c->write;
 
@@ -82,10 +78,6 @@
 
     for ( ;; ) {
         prev_send = send;
-#if (NGX_THREADS)
-        thread_handled = 0;
-        thread_complete = 0;
-#endif
 
         /* create the iovec and coalesce the neighbouring bufs */
 
@@ -179,38 +171,19 @@
             }
 #endif
 
-#if (NGX_THREADS)
-            if (file->file->thread_handler) {
-                rc = ngx_linux_sendfile_thread(c, file, file_size, &sent);
+            n = ngx_linux_sendfile(c, file, file_size);
 
-                switch (rc) {
-                case NGX_OK:
-                    thread_handled = 1;
-                    break;
-
-                case NGX_DONE:
-                    thread_complete = 1;
-                    break;
-
-                case NGX_AGAIN:
-                    break;
-
-                default: /* NGX_ERROR */
-                    return NGX_CHAIN_ERROR;
-                }
-
-            } else
-#endif
-            {
-                n = ngx_linux_sendfile(c, file, file_size);
-
-                if (n == NGX_ERROR) {
-                    return NGX_CHAIN_ERROR;
-                }
-
-                sent = (n == NGX_AGAIN) ? 0 : n;
+            if (n == NGX_ERROR) {
+                return NGX_CHAIN_ERROR;
             }
 
+            if (n == NGX_DONE) {
+                /* thread task posted */
+                return in;
+            }
+
+            sent = (n == NGX_AGAIN) ? 0 : n;
+
         } else {
             n = ngx_writev(c, &header);
 
@@ -225,21 +198,27 @@
 
         in = ngx_chain_update_sent(in, sent);
 
-        if ((size_t) (send - prev_send) != sent) {
-#if (NGX_THREADS)
-            if (thread_handled) {
-                return in;
-            }
-
-            if (thread_complete) {
-                send = prev_send + sent;
-                continue;
-            }
-#endif
+        if (n == NGX_AGAIN) {
             wev->ready = 0;
             return in;
         }
 
+        if ((size_t) (send - prev_send) != sent) {
+
+            /*
+             * sendfile() on Linux 4.3+ might be interrupted at any time,
+             * and provides no indication if it was interrupted or not,
+             * so we have to retry till an explicit EAGAIN
+             *
+             * sendfile() in threads can also report less bytes written
+             * than we are prepared to send now, since it was started in
+             * some point in the past, so we again have to retry
+             */
+
+            send = prev_send + sent;
+            continue;
+        }
+
         if (send >= limit || in == NULL) {
             return in;
         }
@@ -258,6 +237,14 @@
     ssize_t    n;
     ngx_err_t  err;
 
+#if (NGX_THREADS)
+
+    if (file->file->thread_handler) {
+        return ngx_linux_sendfile_thread(c, file, size);
+    }
+
+#endif
+
 #if (NGX_HAVE_SENDFILE64)
     offset = file->file_pos;
 #else
@@ -324,9 +311,8 @@
 } ngx_linux_sendfile_ctx_t;
 
 
-static ngx_int_t
-ngx_linux_sendfile_thread(ngx_connection_t *c, ngx_buf_t *file, size_t size,
-    size_t *sent)
+static ssize_t
+ngx_linux_sendfile_thread(ngx_connection_t *c, ngx_buf_t *file, size_t size)
 {
     ngx_event_t               *wev;
     ngx_thread_task_t         *task;
@@ -356,10 +342,14 @@
         task->event.complete = 0;
 
         if (ctx->err == NGX_EAGAIN) {
-            *sent = 0;
+            /*
+             * if wev->complete is set, this means that a write event
+             * happened while we were waiting for the thread task, so
+             * we have to retry sending even on EAGAIN
+             */
 
             if (wev->complete) {
-                return NGX_DONE;
+                return 0;
             }
 
             return NGX_AGAIN;
@@ -384,13 +374,7 @@
             return NGX_ERROR;
         }
 
-        *sent = ctx->sent;
-
-        if (ctx->sent == ctx->size || wev->complete) {
-            return NGX_DONE;
-        }
-
-        return NGX_AGAIN;
+        return ctx->sent;
     }
 
     if (task->event.active && ctx->file == file) {
@@ -399,9 +383,7 @@
          * or multiple calls of the next body filter from a filter
          */
 
-        *sent = 0;
-
-        return NGX_OK;
+        return NGX_DONE;
     }
 
     ctx->file = file;
@@ -414,9 +396,7 @@
         return NGX_ERROR;
     }
 
-    *sent = 0;
-
-    return NGX_OK;
+    return NGX_DONE;
 }