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=-2.5 required=3.0 tests=MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 64407C2BA19 for ; Thu, 16 Apr 2020 01:12:52 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 260E02064A for ; Thu, 16 Apr 2020 01:12:52 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 260E02064A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id CB0508E0061; Wed, 15 Apr 2020 21:12:51 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id C3A518E0001; Wed, 15 Apr 2020 21:12:51 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id B024C8E0061; Wed, 15 Apr 2020 21:12:51 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0121.hostedemail.com [216.40.44.121]) by kanga.kvack.org (Postfix) with ESMTP id 93AAA8E0001 for ; Wed, 15 Apr 2020 21:12:51 -0400 (EDT) Received: from smtpin29.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay05.hostedemail.com (Postfix) with ESMTP id 50A41181AEF23 for ; Thu, 16 Apr 2020 01:12:51 +0000 (UTC) X-FDA: 76711943742.29.town34_5317924ef940e X-HE-Tag: town34_5317924ef940e X-Filterd-Recvd-Size: 5572 Received: from mail-pl1-f193.google.com (mail-pl1-f193.google.com [209.85.214.193]) by imf01.hostedemail.com (Postfix) with ESMTP for ; Thu, 16 Apr 2020 01:12:50 +0000 (UTC) Received: by mail-pl1-f193.google.com with SMTP id z6so719635plk.10 for ; Wed, 15 Apr 2020 18:12:50 -0700 (PDT) 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:user-agent; bh=6RGJvUJSeZs+AqGP/+hRpTOyIR7EwKBAzBmmAfw6j+g=; b=q4LMEth7oElnrqGqo72NQLv6mDP8rJrVppqzIm1XULMozn9U4V/pWP+sxVxEA0UFPs FSbxpXrrfeQRdRlKbXlLX191Gvw0JgJsN41w9lae+xauIzDglDVGlkHb6GCvlXAAQFr8 pZYJYdpnR3p2x0znLlLpK4uPNefZfOD+WyG6d3gChxeLaLK/OYsmGEFj0fus9RUbCzEy 3OKhnHKc5c1hjkiejyAb4DF821X36q11JHrCNCQEvWnSDWECVewmdSqnbsNsy8IBPR9t 1t5TTcvZes3sII9XRrfk3kNIIYSFpx6oeCuuAZ/E0JBA9vX6+bteKVykqMYxuum3A1MO /Ngw== X-Gm-Message-State: AGi0PuZArlT0Fr1B9oVG6ZO1fg6kNZ1C/guJ05th10AczPBN0B19ikmf SbasNXMNL0wV7bMFPOH8NvI= X-Google-Smtp-Source: APiQypKlxk/W+Q2C4OhNaifz/k6mqtcY/FE1S4DOTOwoBaf22/rB+RYPOVbi0ndi61BGIVJkBF/W3Q== X-Received: by 2002:a17:902:7c12:: with SMTP id x18mr7131442pll.250.1586999569702; Wed, 15 Apr 2020 18:12:49 -0700 (PDT) Received: from 42.do-not-panic.com (42.do-not-panic.com. [157.230.128.187]) by smtp.gmail.com with ESMTPSA id s74sm150560pgc.50.2020.04.15.18.12.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 15 Apr 2020 18:12:48 -0700 (PDT) Received: by 42.do-not-panic.com (Postfix, from userid 1000) id E7A8440277; Thu, 16 Apr 2020 01:12:47 +0000 (UTC) Date: Thu, 16 Apr 2020 01:12:47 +0000 From: Luis Chamberlain To: Bart Van Assche Cc: Christoph Hellwig , axboe@kernel.dk, viro@zeniv.linux.org.uk, gregkh@linuxfoundation.org, rostedt@goodmis.org, mingo@redhat.com, jack@suse.cz, ming.lei@redhat.com, nstange@suse.de, akpm@linux-foundation.org, mhocko@suse.com, yukuai3@huawei.com, linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Omar Sandoval , Hannes Reinecke , Michal Hocko Subject: Re: [PATCH 3/5] blktrace: refcount the request_queue during ioctl Message-ID: <20200416011247.GB11244@42.do-not-panic.com> References: <20200414041902.16769-1-mcgrof@kernel.org> <20200414041902.16769-4-mcgrof@kernel.org> <20200414154044.GB25765@infradead.org> <20200415061649.GS11244@42.do-not-panic.com> <20200415071425.GA21099@infradead.org> <20200415123434.GU11244@42.do-not-panic.com> <73332d32-b095-507f-fb2a-68460533eeb7@acm.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <73332d32-b095-507f-fb2a-68460533eeb7@acm.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, Apr 15, 2020 at 07:18:22AM -0700, Bart Van Assche wrote: > On 2020-04-15 05:34, Luis Chamberlain wrote: > > On Wed, Apr 15, 2020 at 12:14:25AM -0700, Christoph Hellwig wrote: > >> Btw, Isn't blk_get_queue racy as well? Shouldn't we check > >> blk_queue_dying after getting the reference and undo it if the queue is > >> indeeed dying? > > > > Yes that race should be possible: > > > > bool blk_get_queue(struct request_queue *q) > > { > > if (likely(!blk_queue_dying(q))) { > > ----------> we can get the queue to go dying here <--------- > > __blk_get_queue(q); > > return true; > > } > > > > return false; > > } > > EXPORT_SYMBOL(blk_get_queue); > > > > I'll pile up a fix. I've also considered doing a full review of callers > > outside of the core block layer using it, and maybe just unexporting > > this. It was originally exported due to commit d86e0e83b ("block: export > > blk_{get,put}_queue()") to fix a scsi bug, but I can't find such > > respective fix. I suspec that using bdgrab()/bdput() seems more likely > > what drivers should be using. That would allow us to keep this > > functionality internal. > > blk_get_queue() prevents concurrent freeing of struct request_queue but > does not prevent concurrent blk_cleanup_queue() calls. Wouldn't concurrent blk_cleanup_queue() calls be a bug? If so should I make it clear that it would be or simply prevent it? > Callers of > blk_get_queue() may encounter a change of the queue state from normal > into dying any time during the blk_get_queue() call or after > blk_get_queue() has finished. Maybe I'm overlooking something but I > doubt that modifying blk_get_queue() will help. Good point, to fix that race described by Christoph we'd have to take into consideration refcounts of the request_queue to prevent queues from changing state to dying if the refcount is > 1, however that'd also would mean not allowing the request_queue from ever dying. One way we could resolve this could be to to keep track of a quiesce/dying request, then at that point prevent blk_get_queue() from allowing increments, and once the refcount is down to 1, flip the switch to dying. Luis