From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.4 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 09C92C07E96 for ; Fri, 9 Jul 2021 00:50:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D85EA61464 for ; Fri, 9 Jul 2021 00:50:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229843AbhGIAw4 (ORCPT ); Thu, 8 Jul 2021 20:52:56 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:50209 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229637AbhGIAwz (ORCPT ); Thu, 8 Jul 2021 20:52:55 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1625791812; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=LT1OUQIRT9x4UNZQgTUwxS3/kHnBRplw+Byqjig8gR0=; b=dojc/WOmoWmWallvHE+bX6hoYqCyAnYSpvduQ/X/qic8ESsufouL7rGQL5MF/ZCT8+YmZM AktQuyE7CA5VOtwAiZg4iPijZu6oF6PoaGHs1nWPmVg5q0hPPJ79wuXNFfpsPBGke5uISe iLv/cFDWHGJYRplcGg+uFCJd++vIIh8= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-364-m_HgjgDNMj-NYpcxBy_tXQ-1; Thu, 08 Jul 2021 20:50:11 -0400 X-MC-Unique: m_HgjgDNMj-NYpcxBy_tXQ-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.phx2.redhat.com [10.5.11.16]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 7566E362FB; Fri, 9 Jul 2021 00:50:10 +0000 (UTC) Received: from T590 (ovpn-12-111.pek2.redhat.com [10.72.12.111]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 7E9045C1D5; Fri, 9 Jul 2021 00:50:00 +0000 (UTC) Date: Fri, 9 Jul 2021 08:49:55 +0800 From: Ming Lei To: Dan Schatzberg Cc: Jens Axboe , linux-block@vger.kernel.org, Christoph Hellwig , Michal =?iso-8859-1?Q?Koutn=FD?= Subject: Re: [PATCH 6/6] loop: don't add worker into idle list Message-ID: References: <20210705102607.127810-1-ming.lei@redhat.com> <20210705102607.127810-7-ming.lei@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Thu, Jul 08, 2021 at 11:15:13AM -0400, Dan Schatzberg wrote: > On Thu, Jul 08, 2021 at 11:01:54PM +0800, Ming Lei wrote: > > On Thu, Jul 08, 2021 at 10:16:50AM -0400, Dan Schatzberg wrote: > > > On Thu, Jul 08, 2021 at 02:58:36PM +0800, Ming Lei wrote: > > > > On Wed, Jul 07, 2021 at 09:55:34AM -0400, Dan Schatzberg wrote: > > > > > On Wed, Jul 07, 2021 at 11:19:14AM +0800, Ming Lei wrote: > > > > > > On Tue, Jul 06, 2021 at 09:55:36AM -0400, Dan Schatzberg wrote: > > > > > > > On Mon, Jul 05, 2021 at 06:26:07PM +0800, Ming Lei wrote: > > > > > > > > } > > > > > > > > + > > > > > > > > + spin_lock(lock); > > > > > > > > list_add_tail(&cmd->list_entry, cmd_list); > > > > > > > > + spin_unlock(lock); > > > > > > > > queue_work(lo->workqueue, work); > > > > > > > > - spin_unlock(&lo->lo_work_lock); > > > > > > > > } > > > > > > > > > > > > > > > > static void loop_update_rotational(struct loop_device *lo) > > > > > > > > @@ -1131,20 +1159,18 @@ static void loop_set_timer(struct loop_device *lo) > > > > > > > > > > > > > > > > static void __loop_free_idle_workers(struct loop_device *lo, bool force) > > > > > > > > { > > > > > > > > - struct loop_worker *pos, *worker; > > > > > > > > + struct loop_worker *worker; > > > > > > > > + unsigned long id; > > > > > > > > > > > > > > > > spin_lock(&lo->lo_work_lock); > > > > > > > > - list_for_each_entry_safe(worker, pos, &lo->idle_worker_list, > > > > > > > > - idle_list) { > > > > > > > > + xa_for_each(&lo->workers, id, worker) { > > > > > > > > if (!force && time_is_after_jiffies(worker->last_ran_at + > > > > > > > > LOOP_IDLE_WORKER_TIMEOUT)) > > > > > > > > break; > > > > > > > > - list_del(&worker->idle_list); > > > > > > > > - xa_erase(&lo->workers, worker->blkcg_css->id); > > > > > > > > - css_put(worker->blkcg_css); > > > > > > > > - kfree(worker); > > > > > > > > + if (refcount_dec_and_test(&worker->refcnt)) > > > > > > > > + loop_release_worker(worker); > > > > > > > > > > > > > > This one is puzzling to me. Can't you hit this refcount decrement > > > > > > > superfluously each time the loop timer fires? > > > > > > > > > > > > Not sure I get your point. > > > > > > > > > > > > As I mentioned above, this one is the counter pair of INIT reference, > > > > > > but one new lo_cmd may just grab it when queueing rq before erasing the > > > > > > worker from xarray, so we can't release worker here until the command is > > > > > > completed. > > > > > > > > > > Suppose at this point there's still an outstanding loop_cmd to be > > > > > serviced for this worker. The refcount_dec_and_test should decrement > > > > > the refcount and then fail the conditional, not calling > > > > > loop_release_worker. What happens if __loop_free_idle_workers fires > > > > > again before the loop_cmd is processed? Won't you decrement the > > > > > refcount again, and then end up calling loop_release_worker before the > > > > > loop_cmd is processed? > > > > > > > > Good catch! > > > > > > > > The following one line change should avoid the issue: > > > > > > > > diff --git a/drivers/block/loop.c b/drivers/block/loop.c > > > > index 146eaa03629b..3cd51bddfec9 100644 > > > > --- a/drivers/block/loop.c > > > > +++ b/drivers/block/loop.c > > > > @@ -980,7 +980,6 @@ static struct loop_worker *loop_alloc_or_get_worker(struct loop_device *lo, > > > > > > > > static void loop_release_worker(struct loop_worker *worker) > > > > { > > > > - xa_erase(&worker->lo->workers, worker->blkcg_css->id); > > > > css_put(worker->blkcg_css); > > > > kfree(worker); > > > > } > > > > @@ -1167,6 +1166,7 @@ static void __loop_free_idle_workers(struct loop_device *lo, bool force) > > > > if (!force && time_is_after_jiffies(worker->last_ran_at + > > > > LOOP_IDLE_WORKER_TIMEOUT)) > > > > break; > > > > + xa_erase(&worker->lo->workers, worker->blkcg_css->id); > > > > if (refcount_dec_and_test(&worker->refcnt)) > > > > loop_release_worker(worker); > > > > } > > > > > > Yeah, I think this resolves the issue. You could end up repeatedly > > > allocating workers for the same blkcg in the event that you're keeping > > > the worker busy for the entire LOOP_IDLE_WORKER_TIMEOUT (since it only > > > updates the last_ran_at when idle). You may want to add a racy check > > > if the refcount is > 1 to avoid that. > > > > Given the event is very unlikely to trigger, I think we can live > > with that. > > It doesn't seem unlikely to me - any workload that saturates the > backing device would keep the loop worker constantly with at least one > loop_cmd queued and trigger a free and allocate every > LOOP_IDLE_WORKER_TIMEOUT. Another way to solve this is to just update > last_ran_at before or after each loop_cmd. In any event, I'll defer to > your decision, it's not a critical difference. Sorry, I missed that ->last_ran_at is only set when the work isn't pending, then we can cleanup/simplify the reclaim a bit by: 1) keep lo->idle_work to be scheduled in 60 period if there is any active worker allocated, which is scheduled when allocating/reclaiming one worker 2) always set ->last_ran_at after retrieving the worker from xarray, which can be done lockless via WRITE_ONCE(), and it is cheap 3) inside __loop_free_idle_workers(), reclaim one worker only if the worker is expired and hasn't commands in worker->cmd_list > > > > > > > > > I think there might be a separate issue with the locking here though - > > > you acquire the lo->lo_work_lock in __loop_free_idle_workers and then > > > check worker->last_ran_at for each worker. However you only protect > > > the write to worker->last_ran_at (in loop_process_work) with the > > > worker->lock which I think means there's a potential data race on > > > worker->last_ran_at. > > > > It should be fine since both WRITE and READ on worker->last_ran_at is > > atomic. Even though the race is triggered, we still can live with that. > > True, though in this case I think last_ran_at should be atomic_t with > atomic_set and atomic_read. I think READ_ONCE()/WRITE_ONCE() should be enough, and we can set/get last_ran_at lockless. > > > > > > > On Thu, Jul 8, 2021 at 10:41 PM Dan Schatzberg wrote: > > > > > > On Thu, Jul 08, 2021 at 02:58:36PM +0800, Ming Lei wrote: > > ... > > > Another thought - do you need to change the kfree here to kfree_rcu? > > > I'm concerned about the scenario where loop_queue_work's xa_load finds > > > the worker and subsequently __loop_free_idle_workers erases and calls > > > loop_release_worker. If the worker is freed then the subsequent > > > refcount_inc_not_zero in loop_queue_work would be a use after free. > > > > Good catch, will fix it in next version. > > Thanks, you can go ahead and add my Acked-by to the updated version of > this patch as well. Thanks for the review! -- Ming