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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 93172C4167B for ; Thu, 7 Dec 2023 06:44:03 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231130AbjLGGny (ORCPT ); Thu, 7 Dec 2023 01:43:54 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45652 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229456AbjLGGnw (ORCPT ); Thu, 7 Dec 2023 01:43:52 -0500 Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 1F570D5E for ; Wed, 6 Dec 2023 22:43:59 -0800 (PST) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5281EC433C8; Thu, 7 Dec 2023 06:43:55 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1701931438; bh=sksjh5ChWPYyf6bJcZNzXZ+iAfli+dlUwRYAmrfj+b4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=FGCUCUWcJvKOTVj4/UhBQV3+duLk5NBfUf+iBvA/pGMddZAWUFejwRoenQGTdiFB8 jh3kowD1Pnm6Y/BuPrZuLzUM/0IlVy4VxiTCRJYLj0ZboHlug/kXfpOlUJWlck0nc0 lAwwqhESwBEk9co5FBp8wS9GfL7tgr2LVcOAf40ZTi7kWsH0PFu7/JqOv9KLj1s6vZ tT5z1tgp5f2gxpJrq3BF/kxMEDeUphwCHrjd5XhLNqO00heAon9kWeCm56H6dICLqw rTYxrnUaXEv+iy4TyvAorx8f4EaB2h8mCYjBV+SyYGi6V4qFADgQ2aD01PUZqQIURi SIQ3TfWsrZIXQ== Date: Thu, 7 Dec 2023 12:13:47 +0530 From: Manivannan Sadhasivam To: Qiang Yu Cc: quic_jhugo@quicinc.com, mhi@lists.linux.dev, linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org, quic_cang@quicinc.com, quic_mrana@quicinc.com Subject: Re: [PATCH v4 2/4] bus: mhi: host: Drop chan lock before queuing buffers Message-ID: <20231207064347.GE2932@thinkpad> References: <1699939661-7385-1-git-send-email-quic_qianyu@quicinc.com> <1699939661-7385-3-git-send-email-quic_qianyu@quicinc.com> <20231124100403.GA4536@thinkpad> <639d6008-bdfa-4b6e-b622-e916003ec908@quicinc.com> <20231128133252.GX3088@thinkpad> <5eb0a521-0b72-4d15-9a65-429c4c123833@quicinc.com> <20231130053157.GB3043@thinkpad> <9873ee7b-7ef1-4327-8e22-49e1cd3872f1@quicinc.com> <20231206134848.GG12802@thinkpad> <947af144-e2a2-49bc-9f39-31f6ee5b1aa1@quicinc.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <947af144-e2a2-49bc-9f39-31f6ee5b1aa1@quicinc.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Dec 07, 2023 at 01:27:19PM +0800, Qiang Yu wrote: > > On 12/6/2023 9:48 PM, Manivannan Sadhasivam wrote: > > On Wed, Dec 06, 2023 at 10:25:12AM +0800, Qiang Yu wrote: > > > On 11/30/2023 1:31 PM, Manivannan Sadhasivam wrote: > > > > On Wed, Nov 29, 2023 at 11:29:07AM +0800, Qiang Yu wrote: > > > > > On 11/28/2023 9:32 PM, Manivannan Sadhasivam wrote: > > > > > > On Mon, Nov 27, 2023 at 03:13:55PM +0800, Qiang Yu wrote: > > > > > > > On 11/24/2023 6:04 PM, Manivannan Sadhasivam wrote: > > > > > > > > On Tue, Nov 14, 2023 at 01:27:39PM +0800, Qiang Yu wrote: > > > > > > > > > Ensure read and write locks for the channel are not taken in succession by > > > > > > > > > dropping the read lock from parse_xfer_event() such that a callback given > > > > > > > > > to client can potentially queue buffers and acquire the write lock in that > > > > > > > > > process. Any queueing of buffers should be done without channel read lock > > > > > > > > > acquired as it can result in multiple locks and a soft lockup. > > > > > > > > > > > > > > > > > Is this patch trying to fix an existing issue in client drivers or a potential > > > > > > > > issue in the future drivers? > > > > > > > > > > > > > > > > Even if you take care of disabled channels, "mhi_event->lock" acquired during > > > > > > > > mhi_mark_stale_events() can cause deadlock, since event lock is already held by > > > > > > > > mhi_ev_task(). > > > > > > > > > > > > > > > > I'd prefer not to open the window unless this patch is fixing a real issue. > > > > > > > > > > > > > > > > - Mani > > > > > > > In [PATCH v4 1/4] bus: mhi: host: Add spinlock to protect WP access when > > > > > > > queueing > > > > > > > TREs,  we add > > > > > > > write_lock_bh(&mhi_chan->lock)/write_unlock_bh(&mhi_chan->lock) > > > > > > > in mhi_gen_tre, which may be invoked as part of mhi_queue in client xfer > > > > > > > callback, > > > > > > > so we have to use read_unlock_bh(&mhi_chan->lock) here to avoid acquiring > > > > > > > mhi_chan->lock > > > > > > > twice. > > > > > > > > > > > > > > Sorry for confusing you. Do you think we need to sqush this two patch into > > > > > > > one? > > > > > > Well, if patch 1 is introducing a potential deadlock, then we should fix patch > > > > > > 1 itself and not introduce a follow up patch. > > > > > > > > > > > > But there is one more issue that I pointed out in my previous reply. > > > > > Sorry, I can not understand why "mhi_event->lock" acquired during > > > > > mhi_mark_stale_events() can cause deadlock. In mhi_ev_task(), we will > > > > > not invoke mhi_mark_stale_events(). Can you provide some interpretation? > > > > Going by your theory that if a channel gets disabled while processing the event, > > > > the process trying to disable the channel will try to acquire "mhi_event->lock" > > > > which is already held by the process processing the event. > > > > > > > > - Mani > > > OK, I get you. Thank you for kind explanation. Hopefully I didn't intrude > > > too much. > > Not at all. Btw, did you actually encounter any issue that this patch is trying > > to fix? Or just fixing based on code inspection. > > > > - Mani > Yes, we actually meet the race issue in downstream driver. But I can not > find more details about the issue. Hmm. I think it is OK to accept this patch and ignore the channel disabling concern since the event lock is in place to prevent that. There would be no deadlock as I mentioned above, since the process that is parsing the xfer event is not the one that is going to disable the channel in parallel. Could you please respin this series dropping patch 3/4 and also addressing the issue I mentioned in patch 4/4? - Mani -- மணிவண்ணன் சதாசிவம்