From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:58843 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754737AbeFNKLI (ORCPT ); Thu, 14 Jun 2018 06:11:08 -0400 Date: Thu, 14 Jun 2018 12:11:05 +0200 From: Jan Kara To: Tetsuo Handa Cc: Jan Kara , Linus Torvalds , Tejun Heo , Dmitry Vyukov , Jens Axboe , syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com, syzkaller-bugs@googlegroups.com, linux-fsdevel , Linux Kernel Mailing List , Al Viro , Dave Chinner , linux-block Subject: Re: [PATCH] bdi: Fix another oops in wb_workfn() Message-ID: <20180614101105.mcs7b75olai7gfxp@quack2.suse.cz> References: <20180611160131.GQ1351649@devbig577.frc2.facebook.com> <20180611162920.mwapvuqotvhkntt3@quack2.suse.cz> <20180611172053.GR1351649@devbig577.frc2.facebook.com> <20180612155754.x5k2yndh5t6wlmpy@quack2.suse.cz> <20180613144606.nvbcyg2rdjpxhf7s@quack2.suse.cz> <7f4ae045-dfe4-6677-7418-f6f60b6c26f1@i-love.sakura.ne.jp> <20180613164509.oeb3fsjylfpfzxuh@quack2.suse.cz> <4a70ca66-3352-10aa-d351-e3fa3baebffc@i-love.sakura.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4a70ca66-3352-10aa-d351-e3fa3baebffc@i-love.sakura.ne.jp> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu 14-06-18 06:04:04, Tetsuo Handa wrote: > On 2018/06/14 1:45, Jan Kara wrote: > > On Wed 13-06-18 09:25:03, Linus Torvalds wrote: > >> On Wed, Jun 13, 2018 at 9:21 AM Tetsuo Handa > >> wrote: > >>> > >>> Since multiple addresses share bit_wait_table[256], isn't it possible that > >>> cgwb_start_shutdown() prematurely returns false due to wake_up_bit() by > >>> hash-conflicting addresses (i.e. not limited to clear_and_wake_up_bit() from > >>> wb_shutdown())? I think that we cannot be sure without confirming that > >>> test_bit(WB_shutting_down, &wb->state) == false after returning from schedule(). > >> > >> Right. > >> > >> That's _always_ true, btw. Something else entirely could have woken > >> you up. TASK_UNINTERRUPTIBLE does not mean "nothing else wakes me", it > >> just means "_signals_ don't wake me". > >> > >> So every single sleep always needs to be in a loop. Always. > > > > Agreed and in my patch it actually is in a loop - the one iterating the > > list of active writeback structures. If we get a false wakeup, we find the > > same structure in the list again and wait again... > > Indeed. I overlooked that wb = list_first_entry() will select same wb again > if cgwb_remove_from_bdi_list() is not yet called. Well, we could update > "(in which case we also wait for it to finish)" part or move the body of > cgwb_start_shutdown() to cgwb_bdi_unregister() so that it becomes clear > that false wake-up is not a problem in this case. I prefer to keep the wb shutdown in a separate function but I've added some comments to explain that. Honza -- Jan Kara SUSE Labs, CR