From: Joel Becker Subject: Re: [PATCH 4/8] fs: kill i_alloc_sem Date: Mon, 20 Jun 2011 14:32:03 -0700 Message-ID: <20110620213203.GB26204@noexit.corp.google.com> References: <20110620201533.847236272@bombadil.infradead.org> <20110620202031.175620498@bombadil.infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: viro@zeniv.linux.org.uk, tglx@linutronix.de, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org, hirofumi@mail.parknet.co.jp, mfasheh@suse.com To: Christoph Hellwig Return-path: Content-Disposition: inline In-Reply-To: <20110620202031.175620498@bombadil.infradead.org> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon, Jun 20, 2011 at 04:15:37PM -0400, Christoph Hellwig wrote: > i_alloc_sem is a rather special rw_semaphore. It's the last one that may > be released by a non-owner, and it's write side is always mirrored by > real exclusion. It's intended use it to wait for all pending direct I/O > requests to finish before starting a truncate. > > Replace it with a hand-grown construct: > > - exclusion for truncates is already guaranteed by i_mutex, so it can > simply fall way > - the reader side is replaced by an i_dio_count member in struct inode > that counts the number of pending direct I/O requests. Truncate can't > proceed as long as it's non-zero > - when i_dio_count reaches non-zero we wake up a pending truncate using > wake_up_bit on a new bit in i_flags > - new references to i_dio_count can't appear while we are waiting for > it to read zero because the direct I/O count always needs i_mutex > (or an equivalent like XFS's i_iolock) for starting a new operation. > > This scheme is much simpler, and saves the space of a spinlock_t and a > struct list_head in struct inode (typically 160 bytes on a non-debug 64-bit > system). Are we guaranteed that all allocation changes are locked out by i_dio_count>0? I don't think we are. The ocfs2 code very strongly assumes the state of a file's allocation when it holds i_alloc_sem. I feel like we lose that here. Joel -- "I don't even butter my bread; I consider that cooking." - Katherine Cebrian http://www.jlbec.org/ jlbec@evilplan.org