From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Chinner Subject: Re: [PATCH 1/3] dio: track and serialise unaligned direct IO Date: Tue, 9 Nov 2010 09:55:48 +1100 Message-ID: <20101108225548.GR2715@dastard> References: <1289202023-15102-1-git-send-email-david@fromorbit.com> <1289202023-15102-2-git-send-email-david@fromorbit.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org To: Jeff Moyer Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Mon, Nov 08, 2010 at 10:28:29AM -0500, Jeff Moyer wrote: > Dave Chinner writes: >=20 > > +struct dio_zero_block { > > + struct list_head dio_list; /* list of io in progress */ > > + sector_t zero_block; /* block being zeroed */ > > + struct dio *dio; /* owner dio */ > > + wait_queue_head_t wq; /* New IO block here */ > New IOs block here, or new IO blocks here? >=20 > > +/* > > + * Add a filesystem block to the list of blocks we are tracking. > > + */ > > +static void > > +dio_start_zero_block(struct dio *dio, sector_t zero_block) > > +{ > > + struct dio_zero_block *zb; > > + > > + zb =3D kmalloc(sizeof(*zb), GFP_NOIO); > > + if (!zb) > > + return; > > + INIT_LIST_HEAD(&zb->dio_list); > > + init_waitqueue_head(&zb->wq); > > + zb->zero_block =3D zero_block; > > + zb->dio =3D dio; > > + atomic_set(&zb->ref, 1); > > + > > + spin_lock(&dio_zero_block_lock); > > + list_add(&zb->dio_list, &dio_zero_block_list); > > + spin_unlock(&dio_zero_block_lock); >=20 > What protects from two processes getting here at the same time, and > hence adding the same block to the list? i_mutex? The wait in dio_zero_block() called before this function is called is supposed to serialise them, but now that you point it out, it's not an atomic wait-and-add so ther eis a small window where two IOs could pass through here. Easy enoug to fix by combining the search and in=D1=95ert - I'll restructure it along those lines. Cheers, Dave. --=20 Dave Chinner david@fromorbit.com