From: Christoph Hellwig Subject: Re: [PATCH 1/3] Implement generic freeze feature Date: Fri, 22 Aug 2008 14:14:58 -0400 Message-ID: <20080822181458.GA15469@infradead.org> References: <20080818212819t-sato@mail.jp.nec.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Andrew Morton , "linux-fsdevel@vger.kernel.org" , "dm-devel@redhat.com" , "viro@ZenIV.linux.org.uk" , "linux-ext4@vger.kernel.org" , "xfs@oss.sgi.com" , Christoph Hellwig , "axboe@kernel.dk" , "mtk.manpages@googlemail.com" , "linux-kernel@vger.kernel.org" To: Takashi Sato Return-path: Received: from bombadil.infradead.org ([18.85.46.34]:39408 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753290AbYHVSPD (ORCPT ); Fri, 22 Aug 2008 14:15:03 -0400 Content-Disposition: inline In-Reply-To: <20080818212819t-sato@mail.jp.nec.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, Aug 18, 2008 at 09:28:19PM +0900, Takashi Sato wrote: > + down(&bdev->bd_freeze_sem); > + bdev->bd_freeze_count++; > + if (bdev->bd_freeze_count > 1) { > + sb = get_super(bdev); > + drop_super(sb); > + up(&bdev->bd_freeze_sem); > + return sb; > + } > + > down(&bdev->bd_mount_sem); Now you have a reference counter of freezes which actually is pretty sensible, but also needs some documentation. What I don't understand here at all is why you do the get_super/drop_super in the already frozen case. Now that the freeze_count has replaced one of the uses of bd_mount_sem you should also replace the other use in the unmount path by simply checking for the freez_count and abort if it's set. To do so you'll need to hold the bd_mount_sem over the whole unmount operation to prevent new frezes from coming in. As others noted it should be a mutex and not a semaphore. > /* > + * ioctl_freeze - Freeze the filesystem. > + * > + * @filp: target file > + * > + * Call freeze_bdev() to freeze the filesystem. > + */ > +static int ioctl_freeze(struct file *filp) This is not quite kerneldcoc format, which would ne a /** as commnt start. But I don't think the comment is actually needed, it's a pretty obvious file scope function. (Same commnt also applies to ioctl_thaw) > + struct super_block *sb = filp->f_path.dentry->d_inode->i_sb; > + > + if (!capable(CAP_SYS_ADMIN)) > + return -EPERM; > + > + /* If filesystem doesn't support freeze feature, return. */ > + if (sb->s_op->write_super_lockfs == NULL) > + return -EOPNOTSUPP; > + > + /* If a regular file or a directory isn't specified, return. */ > + if (sb->s_bdev == NULL) > + return -EINVAL; I don't understand this commnt. What you are checking is that the filesystem has a non-NULL s_bdev, which implies a not blockdevice-backed filesystem.