From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Steve French (smfltc)" Subject: i_mutex and deadlock Date: Fri, 23 Feb 2007 10:02:16 -0600 Message-ID: <45DF1008.3090903@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-cifs-client@lists.samba.org To: linux-fsdevel@vger.kernel.org Return-path: Received: from e6.ny.us.ibm.com ([32.97.182.146]:36342 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932719AbXBWQCL (ORCPT ); Fri, 23 Feb 2007 11:02:11 -0500 Received: from d01relay04.pok.ibm.com (d01relay04.pok.ibm.com [9.56.227.236]) by e6.ny.us.ibm.com (8.13.8/8.13.8) with ESMTP id l1NG2ona005570 for ; Fri, 23 Feb 2007 11:02:50 -0500 Received: from d01av03.pok.ibm.com (d01av03.pok.ibm.com [9.56.224.217]) by d01relay04.pok.ibm.com (8.13.8/8.13.8/NCO v8.2) with ESMTP id l1NG23gq187818 for ; Fri, 23 Feb 2007 11:02:06 -0500 Received: from d01av03.pok.ibm.com (loopback [127.0.0.1]) by d01av03.pok.ibm.com (8.12.11.20060308/8.13.3) with ESMTP id l1NG23iU004632 for ; Fri, 23 Feb 2007 11:02:03 -0500 Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org A field in i_size_write (i_size_seqcount) must be protected against simultaneous update otherwise we risk looping in i_size_read. The suggestion in fs.h is to use i_mutex which seems too dangerous due to the possibility of deadlock. There are 65 places in the fs directory which lock an i_mutex, and seven more in the mm directory. The vfs does clearly lock file inodes in some paths before calling into a particular filesystem (nfs, ext3, cifs etc.) - in particular for fsync but probably for others that are harder to trace. This seems to introduce the possibility of deadlock if a filesystem also uses i_mutex to protect file size updates Documentation/filesystems/Locking describes the use of i_mutex (was "i_sem" previously) and indicates that it is held by the vfs on three additional calls on file inodes which concern me (for deadlock possibility), setattr, truncate and unlink. nfs seems to limit its use of i_mutex to llseek and invalidate_mapping, and does not appear to grab the i_mutex (or any sem for that matter) to protect i_size_write (nfs calls i_size_write in nfs_grow_file) - and for the case of nfs_fhget (in which they bypass i_size_write and set i_size directly) does not seem to grab i_mutex either. ext3 also does not use i_mutex for this purpose (protecting i_size_write) - ony to protect a journalling ioctl. I am concerned about using i_mutex to protect the cifs calls to i_size_write (although it seems to fix a problem reported in i_size_read under stress) because of the following: 1) no one else calls i_size_write AFAIK (on our file inodes) 2) we don't block inside i_size_write do we ... (so why in the world do they take a slow mutex instead of a fast spinlock) 3) we don't really know what happens inside fsync (the paths through the page cache code seem complex and we don't want to reenter writepage in low memory conditions and deadlock updating the file size), and there is some concern that the vfs takes the i_mutex in other paths on file inodes before entering our code and could deadlock. Any reason, why an fs shouldn't simply use something else (a spinlock) other than i_mutex to protect the i_size_write call?