From: Marco Stornelli Subject: Re: [PATCH, RFC] xip: use i_mutex for xip_file_fault Date: Sun, 11 Sep 2011 17:57:36 +0200 Message-ID: <4E6CDA70.5080205@gmail.com> References: <4E6B82DB.50803@gmail.com> <20110910155639.GY2203@ZenIV.linux.org.uk> <4E6C705F.8010101@gmail.com> <20110911111504.GA2203@ZenIV.linux.org.uk> <20110911112552.GB2203@ZenIV.linux.org.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Linux FS Devel , Linux Kernel To: Al Viro Return-path: Received: from mail-fx0-f46.google.com ([209.85.161.46]:40676 "EHLO mail-fx0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754935Ab1IKQDq (ORCPT ); Sun, 11 Sep 2011 12:03:46 -0400 In-Reply-To: <20110911112552.GB2203@ZenIV.linux.org.uk> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Il 11/09/2011 13:25, Al Viro ha scritto: > On Sun, Sep 11, 2011 at 12:15:04PM +0100, Al Viro wrote: > >> write() grabs ->i_mutex on the file it's going to write to. It uses >> copy_from_user() while holding ->i_mutex; that can end up calling ->fault(). >> If your data comes from the same file mmapped in your address space, you >> have xip_write_fault() called while you are in xip_file_write() and *already* >> are holding ->i_mutex on the same inode. With your patch it will, AFAICS, >> cheerfully deadlock. > > Oh, wait... You are only doing that to write side of pagefault? That's > better, but not much: > > thread 1: mmap the file, modify mapping > thread 2: write() to file > > The former will do xip_write_fault() while holding ->mmap_sem. > The latter will do copy_from_user() from xip_file_write(), getting > pagefaults while holding ->i_mutex. > > Note that we are grabbing ->mmap_sem and ->i_mutex in opposite orders. > I.e. that will deadlock on you - all you need is threads sharing the > address space. > Ok, thank you very much for the on-line debug :) So i_mutex is not a good lock to use in this situation. It was a common sync point, but it has some collateral effect on the write path that we must avoid. At this point, what can be a good strategy? Any opinion is welcome. Marco