diff --git a/Lib/test/test_file2k.py b/Lib/test/test_file2k.py
index d8966e0..c73e8d8 100644
--- a/Lib/test/test_file2k.py
+++ b/Lib/test/test_file2k.py
@@ -653,18 +653,15 @@
         self._test_close_open_io(io_func)
 
     def test_iteration_torture(self):
-        # bpo-31530: Crash when concurrently iterate over a file.
+        # bpo-31530
         with open(self.filename, "wb") as fp:
             for i in xrange(2**20):
                 fp.write(b"0"*50 + b"\n")
         with open(self.filename, "rb") as f:
-            def iterate():
-                try:
-                    for l in f:
-                        pass
-                except IOError:
+            def it():
+                for l in f:
                     pass
-            self._run_workers(iterate, 10)
+            self._run_workers(it, 10)
 
     def test_iteration_seek(self):
         # bpo-31530: Crash when concurrently seek and iterate over a file.
@@ -674,17 +671,15 @@
         with open(self.filename, "rb") as f:
             it = iter([1] + [0]*10)  # one thread reads, others seek
             def iterate():
-                try:
-                    if next(it):
-                        for l in f:
-                            pass
-                    else:
-                        for i in range(100):
-                            f.seek(i*100, 0)
-                except IOError:
-                    pass
+                if next(it):
+                    for l in f:
+                        pass
+                else:
+                    for i in xrange(100):
+                        f.seek(i*100, 0)
             self._run_workers(iterate, 10)
 
+
 @unittest.skipUnless(os.name == 'posix', 'test requires a posix system.')
 class TestFileSignalEINTR(unittest.TestCase):
     def _test_reading(self, data_to_write, read_and_verify_code, method_name,
diff --git a/Misc/NEWS.d/next/Core and Builtins/2017-09-20-18-28-09.bpo-31530.CdLOM7.rst b/Misc/NEWS.d/next/Core and Builtins/2017-09-20-18-28-09.bpo-31530.CdLOM7.rst
index a6cb6c9..beb09b5 100644
--- a/Misc/NEWS.d/next/Core and Builtins/2017-09-20-18-28-09.bpo-31530.CdLOM7.rst
+++ b/Misc/NEWS.d/next/Core and Builtins/2017-09-20-18-28-09.bpo-31530.CdLOM7.rst
@@ -1,4 +1 @@
 Fixed crashes when iterating over a file on multiple threads.
-seek() and next() methods of file objects now raise an exception during
-concurrent operation on the same file object.
-A lock can be used to prevent the error.
diff --git a/Objects/fileobject.c b/Objects/fileobject.c
index 8d1c581..270b282 100644
--- a/Objects/fileobject.c
+++ b/Objects/fileobject.c
@@ -609,7 +609,12 @@
     return NULL;
 }
 
-static void drop_readahead(PyFileObject *);
+static void
+drop_file_readahead(PyFileObject *f)
+{
+    PyMem_FREE(f->f_buf);
+    f->f_buf = NULL;
+}
 
 /* Methods */
 
@@ -632,7 +637,7 @@
     Py_XDECREF(f->f_mode);
     Py_XDECREF(f->f_encoding);
     Py_XDECREF(f->f_errors);
-    drop_readahead(f);
+    drop_file_readahead(f);
     Py_TYPE(f)->tp_free((PyObject *)f);
 }
 
@@ -767,13 +772,7 @@
 
     if (f->f_fp == NULL)
         return err_closed();
-    if (f->unlocked_count > 0) {
-        PyErr_SetString(PyExc_IOError,
-            "seek() called during concurrent "
-            "operation on the same file object");
-        return NULL;
-    }
-    drop_readahead(f);
+    drop_file_readahead(f);
     whence = 0;
     if (!PyArg_ParseTuple(args, "O|i:seek", &offobj, &whence))
         return NULL;
@@ -2242,12 +2241,16 @@
     {0},
 };
 
+typedef struct {
+    char *buf, *bufptr, *bufend;
+} readaheadbuffer;
+
 static void
-drop_readahead(PyFileObject *f)
+drop_readaheadbuffer(readaheadbuffer *rab)
 {
-    if (f->f_buf != NULL) {
-        PyMem_Free(f->f_buf);
-        f->f_buf = NULL;
+    if (rab->buf != NULL) {
+        PyMem_FREE(rab->buf);
+        rab->buf = NULL;
     }
 }
 
@@ -2255,36 +2258,34 @@
    (unless at EOF) and no more than bufsize.  Returns negative value on
    error, will set MemoryError if bufsize bytes cannot be allocated. */
 static int
-readahead(PyFileObject *f, Py_ssize_t bufsize)
+readahead(PyFileObject *f, readaheadbuffer *rab, Py_ssize_t bufsize)
 {
     Py_ssize_t chunksize;
 
-    assert(f->unlocked_count == 0);
-    if (f->f_buf != NULL) {
-        if( (f->f_bufend - f->f_bufptr) >= 1)
+    if (rab->buf != NULL) {
+        if ((rab->bufend - rab->bufptr) >= 1)
             return 0;
         else
-            drop_readahead(f);
+            drop_readaheadbuffer(rab);
     }
-    if ((f->f_buf = (char *)PyMem_Malloc(bufsize)) == NULL) {
+    if ((rab->buf = PyMem_MALLOC(bufsize)) == NULL) {
         PyErr_NoMemory();
         return -1;
     }
     FILE_BEGIN_ALLOW_THREADS(f)
     errno = 0;
-    chunksize = Py_UniversalNewlineFread(
-        f->f_buf, bufsize, f->f_fp, (PyObject *)f);
+    chunksize = Py_UniversalNewlineFread(rab->buf, bufsize, f->f_fp, (PyObject *)f);
     FILE_END_ALLOW_THREADS(f)
     if (chunksize == 0) {
         if (ferror(f->f_fp)) {
             PyErr_SetFromErrno(PyExc_IOError);
             clearerr(f->f_fp);
-            drop_readahead(f);
+            drop_readaheadbuffer(rab);
             return -1;
         }
     }
-    f->f_bufptr = f->f_buf;
-    f->f_bufend = f->f_buf + chunksize;
+    rab->bufptr = rab->buf;
+    rab->bufend = rab->buf + chunksize;
     return 0;
 }
 
@@ -2294,51 +2295,43 @@
    logarithmic buffer growth to about 50 even when reading a 1gb line. */
 
 static PyStringObject *
-readahead_get_line_skip(PyFileObject *f, Py_ssize_t skip, Py_ssize_t bufsize)
+readahead_get_line_skip(PyFileObject *f, readaheadbuffer *rab, Py_ssize_t skip, Py_ssize_t bufsize)
 {
     PyStringObject* s;
     char *bufptr;
     char *buf;
     Py_ssize_t len;
 
-    if (f->unlocked_count > 0) {
-        PyErr_SetString(PyExc_IOError,
-            "next() called during concurrent "
-            "operation on the same file object");
-        return NULL;
-    }
-    if (f->f_buf == NULL)
-        if (readahead(f, bufsize) < 0)
+    if (rab->buf == NULL)
+        if (readahead(f, rab, bufsize) < 0)
             return NULL;
 
-    len = f->f_bufend - f->f_bufptr;
+    len = rab->bufend - rab->bufptr;
     if (len == 0)
-        return (PyStringObject *)
-            PyString_FromStringAndSize(NULL, skip);
-    bufptr = (char *)memchr(f->f_bufptr, '\n', len);
+        return (PyStringObject *)PyString_FromStringAndSize(NULL, skip);
+    bufptr = (char *)memchr(rab->bufptr, '\n', len);
     if (bufptr != NULL) {
         bufptr++;                               /* Count the '\n' */
-        len = bufptr - f->f_bufptr;
-        s = (PyStringObject *)
-            PyString_FromStringAndSize(NULL, skip + len);
+        len = bufptr - rab->bufptr;
+        s = (PyStringObject *)PyString_FromStringAndSize(NULL, skip + len);
         if (s == NULL)
             return NULL;
-        memcpy(PyString_AS_STRING(s) + skip, f->f_bufptr, len);
-        f->f_bufptr = bufptr;
-        if (bufptr == f->f_bufend)
-            drop_readahead(f);
+        memcpy(PyString_AS_STRING(s) + skip, rab->bufptr, len);
+        rab->bufptr = bufptr;
+        if (bufptr == rab->bufend)
+            drop_readaheadbuffer(rab);
     } else {
-        bufptr = f->f_bufptr;
-        buf = f->f_buf;
-        f->f_buf = NULL;                /* Force new readahead buffer */
+        bufptr = rab->bufptr;
+        buf = rab->buf;
+        rab->buf = NULL;                /* Force new readahead buffer */
         assert(len <= PY_SSIZE_T_MAX - skip);
-        s = readahead_get_line_skip(f, skip + len, bufsize + (bufsize>>2));
+        s = readahead_get_line_skip(f, rab, skip + len, bufsize + (bufsize>>2));
         if (s == NULL) {
-            PyMem_Free(buf);
+            PyMem_FREE(buf);
             return NULL;
         }
         memcpy(PyString_AS_STRING(s) + skip, bufptr, len);
-        PyMem_Free(buf);
+        PyMem_FREE(buf);
     }
     return s;
 }
@@ -2356,7 +2349,30 @@
     if (!f->readable)
         return err_mode("reading");
 
-    l = readahead_get_line_skip(f, 0, READAHEAD_BUFSIZE);
+    {
+        /*
+          Multiple threads can enter this method while the GIL is released
+          during file read and wreak havoc on the file object's readahead
+          buffer. To avoid dealing with cross-thread coordination issues, we
+          cache the file buffer state locally and only set it back on the file
+          object when we're done.
+        */
+        readaheadbuffer rab = {f->f_buf, f->f_bufptr, f->f_bufend};
+        f->f_buf = NULL;
+        l = readahead_get_line_skip(f, &rab, 0, READAHEAD_BUFSIZE);
+        /*
+          Make sure the file's internal read buffer is cleared out. This will
+          only do anything if some other thread interleaved with us during
+          readahead. We want to drop any changeling buffer, so we don't leak
+          memory. We may lose data, but that's what you get for reading the same
+          file object in multiple threads.
+        */
+        drop_file_readahead(f);
+        f->f_buf = rab.buf;
+        f->f_bufptr = rab.bufptr;
+        f->f_bufend = rab.bufend;
+    }
+
     if (l == NULL || PyString_GET_SIZE(l) == 0) {
         Py_XDECREF(l);
         return NULL;