From: Dmitry Monakhov Subject: Re: [PATCH 04/11] ext4: completed_io locking cleanup V4 Date: Tue, 02 Oct 2012 16:42:39 +0400 Message-ID: <87obklavuo.fsf@openvz.org> References: <1348847051-6746-1-git-send-email-dmonakhov@openvz.org> <1348847051-6746-5-git-send-email-dmonakhov@openvz.org> <20121001183833.GF32092@quack.suse.cz> <87vcetbay1.fsf@openvz.org> <20121002103141.GA22777@quack.suse.cz> <87r4phb0q5.fsf@openvz.org> <20121002111106.GB22777@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Jan Kara , linux-ext4@vger.kernel.org, tytso@mit.edu, lczerner@redhat.com To: Jan Kara Return-path: Received: from mail-lb0-f174.google.com ([209.85.217.174]:35005 "EHLO mail-lb0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753054Ab2JBMmm (ORCPT ); Tue, 2 Oct 2012 08:42:42 -0400 Received: by mail-lb0-f174.google.com with SMTP id n3so5060994lbo.19 for ; Tue, 02 Oct 2012 05:42:42 -0700 (PDT) In-Reply-To: <20121002111106.GB22777@quack.suse.cz> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, 2 Oct 2012 13:11:06 +0200, Jan Kara wrote: > On Tue 02-10-12 14:57:22, Dmitry Monakhov wrote: > > On Tue, 2 Oct 2012 12:31:41 +0200, Jan Kara wrote: > > > On Tue 02-10-12 11:16:38, Dmitry Monakhov wrote: > > > > > > + spin_lock_irqsave(&ei->i_completed_io_lock, flags); > > > > > > + while (!list_empty(&complete)) { > > > > > > + io = list_entry(complete.next, ext4_io_end_t, list); > > > > > > + io->flag &= ~EXT4_IO_END_UNWRITTEN; > > > > > > + /* end_io context can not be destroyed now because it still > > > > > > + * used by queued worker. Worker thread will destroy it later */ > > > > > > + if (io->flag & EXT4_IO_END_QUEUED) > > > > > > + list_del_init(&io->list); > > > > > > + else > > > > > > + list_move(&io->list, &to_free); > > > > > > + } > > > > > > + /* If we are called from worker context, it is time to clear queued > > > > > > + * flag, and destroy it's end_io if it was converted already */ > > > > > > + if (work_io) { > > > > > > + work_io->flag &= ~EXT4_IO_END_QUEUED; > > > > > > + if (!(work_io->flag & EXT4_IO_END_UNWRITTEN)) > > > > > > + list_add_tail(&work_io->list, &to_free); > > > > > > } > > > > > > - list_del_init(&io->list); > > > > > > spin_unlock_irqrestore(&ei->i_completed_io_lock, flags); > > > > > > - (void) ext4_end_io_nolock(io); > > > > > > - mutex_unlock(&inode->i_mutex); > > > > > > -free: > > > > > > - ext4_free_io_end(io); > > > > > > + > > > > > > + while (!list_empty(&to_free)) { > > > > > > + io = list_entry(to_free.next, ext4_io_end_t, list); > > > > > > + list_del_init(&io->list); > > > > > > + ext4_free_io_end(io); > > > > > > + } > > > > > > + return ret; > > > > > > +} > > > > > > + > > > > > > +/* > > > > > > + * work on completed aio dio IO, to convert unwritten extents to extents > > > > > > + */ > > > > > > +static void ext4_end_io_work(struct work_struct *work) > > > > > > +{ > > > > > > + ext4_io_end_t *io = container_of(work, ext4_io_end_t, work); > > > > > > + ext4_do_flush_completed_IO(io->inode, io); > > > > > > +} > > > > > > + > > > > > > +int ext4_flush_completed_IO(struct inode *inode) > > > > > > +{ > > > > > > + return ext4_do_flush_completed_IO(inode, NULL); > > > > > > } > > > > > Also it seems that when ext4_flush_completed_IO() is called, workqueue > > > > > can have several IO structures queued in its local lists thus we miss them > > > > > here and don't properly wait for all conversions? > > > > No it is not. Because list drained atomically, and > > > > add_complete_io will queue work only if list is empty. > > > > > > > > Race between conversion and dequeue-process is not possible because > > > > we hold lock during entire walk of complete_list, so from external > > > > point of view we mark list as conversed(clear unwritten flag) > > > > happens atomically. I've drawn all possible situations and race not > > > > happen. If you know any please let me know. > > > I guess I'm missing something obvious. So let's go step by step: > > > 1) ext4_flush_completed_IO() must make sure there is no outstanding > > > conversion for the inode. > > > 2) Now assume we have non-empty i_completed_io_list - thus work is queued. > > > 3) The following situation seems to be possible: > > > > > > CPU1 CPU2 > > > (worker thread) (truncate) > > > ext4_end_io_work() > > > ext4_do_flush_completed_IO() > > > spin_lock_irqsave(&ei->i_completed_io_lock, flags); > > > dump_completed_IO(inode); > > > list_replace_init(&ei->i_completed_io_list, &unwritten); > > > spin_unlock_irqrestore(&ei->i_completed_io_lock, flags); > > > > > > ext4_flush_completed_IO() > > > ext4_do_flush_completed_IO() > > > - sees empty i_completed_io_list > > > => exits > > > > > > But we still have some conversions pending in 'unwritten' list. What am > > > I missing? > > Indeed, I've simply missed that case. The case which result silently > > broke integrity sync ;( > > Thank you for spotting this. I'll be back with updated version. > Umm, actually, I was thinking about it and ext4_flush_completed_IO() > seems to be unnecessary in fsync these days. We don't call aio_complete() > until we perform the conversion so what fsync does to such IO is undefined. > Such optimization is a separate matter though. Yes aio is ok, but integrity fsync after buffered write to unwritten extent is broken. fsync() blkdev_completion kwork ->filemap_write_and_wait_range ->ext4_end_bio ->end_page_writeback <-- filemap_write_and_wait_range return ->ext4_add_complete_io ->ext4_do_flush_completed_IO ->list_replace_init ->ext4_flush_completed_IO sees empty i_comleted_io_list but pended conversion still exist ->ext4_end_io > > But for truncate or punch hole, it is critical that all conversions are > really flushed. > > Honza > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html