From mboxrd@z Thu Jan 1 00:00:00 1970 From: Al Viro Subject: Re: Update of file offset on write() etc. is non-atomic with I/O Date: Mon, 3 Mar 2014 21:26:42 +0000 Message-ID: <20140303212642.GJ18016@ZenIV.linux.org.uk> References: <20140303210359.26624.qmail@science.horizon.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: torvalds@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org To: George Spelvin Return-path: Content-Disposition: inline In-Reply-To: <20140303210359.26624.qmail@science.horizon.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Mon, Mar 03, 2014 at 04:03:59PM -0500, George Spelvin wrote: > > struct fd { > > struct file *file; > > - int need_put; > > + unsigned need_put:1, need_pos_unlock:1; > > }; > > Since we're rounding up to 2*sizeof(struct file *) anyway, is this a case > where wasting space on a couple of char (or bool) flags would generate > better code than a bitfield? > > In particular, the code to set need_pos_unlock (which will be executed > each read/write for most files) is messy in the bitfield case. > A byte store is much cleaner. > > (If you want to use bits, why not use the two lsbits of the file pointer > for the purpose? That would save a lot of space.) Most of the cases have it kept separately in registers, actually - there's a reason why fdget() and friends are inlined.