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=-8.0 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS autolearn=no 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 D1077C07E96 for ; Thu, 8 Jul 2021 15:15:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id ACC1561414 for ; Thu, 8 Jul 2021 15:15:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231849AbhGHPR7 (ORCPT ); Thu, 8 Jul 2021 11:17:59 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59600 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229738AbhGHPR7 (ORCPT ); Thu, 8 Jul 2021 11:17:59 -0400 Received: from mail-qt1-x82e.google.com (mail-qt1-x82e.google.com [IPv6:2607:f8b0:4864:20::82e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 883CDC061574 for ; Thu, 8 Jul 2021 08:15:16 -0700 (PDT) Received: by mail-qt1-x82e.google.com with SMTP id z12so5073606qtj.3 for ; Thu, 08 Jul 2021 08:15:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to; bh=nZmeif0+dO/gCppEiVHJPKNDHfZkNHwAKTBKNNHFTX0=; b=GHDRo66h/ZHdiRietj7GT81fGnTG9Cc9E69RoaY4TayfTco1XLRJzRVjtTdqyQTd43 J3Jafh0jwiYysKCv4UDdppD1tHX8DbsrIPxtcMRVZ0Tl7bA0ecMZQkrVH/aTj69qtmTi DEsoXJp7hytxFW35E4mqd0/PcJk023L47x1PXMtgTV4Kn5vKh4T8oyVn5rgZ12G3FWyS 6gelt26qsyfBXoJfsrs6CK+PLNk96+WG6LD6lR0ZvKUvFXwY2mLY9w6YCXpvKykPr157 kq9l5LXhDTQnVhQJKfaG2x0udsfYVfN3LO5q905/lEH+hxiOwDXZQddoSxVZl5d8tPho Kt8A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to; bh=nZmeif0+dO/gCppEiVHJPKNDHfZkNHwAKTBKNNHFTX0=; b=nGwi97L0EseZGgjESNpQHbaNSLg6520tk0iGNfycKYbLR5KmoUPvLjtIORS4lHvXUW gY3ML4b/HUuEuuNVYSq/BgJvT9834IYfyxcnGFU676abBx7xoFGf2GX0ZZEN0Q+fqlUX CUc9w9PtCg7HilAR1KC6vyM+chJS73SYonmRTzmtAQYcjJXSZPdVSnwoKSJedrBD8V8/ doq0DwrtA8walrYpNtYtcGXzLd3gHGJP4xCuYgQKm293sboQPCdaUBbjjcosP+KPFDVZ PgdclqOPFSukzC4B6yJjK11q8b9b92oGL5nOJOCiqP8GET2fqVeSsaNcMg2I2RJtP31y bINA== X-Gm-Message-State: AOAM532d6XbpZgFrtJgiq/O3KWRa0QtGTCpMwSASgnm93wPExSuavNMC /rC4gY2LwORzcEHB7D8cjoQ= X-Google-Smtp-Source: ABdhPJx3lRrpSEFZ8hDi09zi5jc7lCV3BRbLAcsfjyOZBLHswT329iGG/0+g4pzUe8paTxPDn8xkRw== X-Received: by 2002:aed:306f:: with SMTP id 102mr28680511qte.197.1625757315703; Thu, 08 Jul 2021 08:15:15 -0700 (PDT) Received: from dschatzberg-fedora-PC0Y6AEN ([2620:10d:c091:480::1:ee15]) by smtp.gmail.com with ESMTPSA id x16sm1086849qkn.130.2021.07.08.08.15.14 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 08 Jul 2021 08:15:15 -0700 (PDT) Date: Thu, 8 Jul 2021 11:15:13 -0400 From: Dan Schatzberg To: Ming Lei 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: Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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. > > > > > 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. > > > 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.