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=-2.2 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, HEADER_FROM_DIFFERENT_DOMAINS,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 7323EC33CA9 for ; Mon, 13 Jan 2020 17:09:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 46E0021739 for ; Mon, 13 Jan 2020 17:09:07 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=kernel-dk.20150623.gappssmtp.com header.i=@kernel-dk.20150623.gappssmtp.com header.b="WKqIN2x4" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728928AbgAMRJG (ORCPT ); Mon, 13 Jan 2020 12:09:06 -0500 Received: from mail-io1-f65.google.com ([209.85.166.65]:34133 "EHLO mail-io1-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728915AbgAMRJE (ORCPT ); Mon, 13 Jan 2020 12:09:04 -0500 Received: by mail-io1-f65.google.com with SMTP id z193so10607571iof.1 for ; Mon, 13 Jan 2020 09:09:04 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel-dk.20150623.gappssmtp.com; s=20150623; h=subject:to:cc:references:from:message-id:date:user-agent :mime-version:in-reply-to:content-language:content-transfer-encoding; bh=HwOMSNU0BOcmSRdPEWKuyZX5aZw+rCZHEo3YxXXPbFY=; b=WKqIN2x4arw2oKUauJg3IzH0Qjl/mB5DQfhbHOo9RwqOxk1NJAI/DXrWhbYvac3FUU 6z14DZZMo1fvXjZbp/NMHMaMWa3xcDbxdnXLXMKhGK0DYjmHTKtTpAyFbGxMVFVSK4mQ 1M6o9th63zQJ7uVxLbmFHDx+V1IVk0PBWnWfHNjj+DAROwY9Sz1zLTOGUkAyjytc3LXw TNAFCePAGIquBokzfdvjK5TWHwCsRtCc2XevcbVtPPT5OePT53zNlRKrZNAEC982It9U 98EWBsgK2gb1Hf8NPxoFa/rIm/s4UpeMeR4NOEZDdBcoKet82/f4Me+lSg19PNK4sKHP i49A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:subject:to:cc:references:from:message-id:date :user-agent:mime-version:in-reply-to:content-language :content-transfer-encoding; bh=HwOMSNU0BOcmSRdPEWKuyZX5aZw+rCZHEo3YxXXPbFY=; b=OH3n56R6usOVSFxARnZnsh8h0zV4GWoPUfWYNeAc7CmC/4f2OObd2IbmVSsUrdpyMd 64F4jrOqs02rExySt7/iUdhmo6UcbPNONGWg92IUAAqKcQTrES+whX0tGGkfosJJci1o 8H92D9eHCR6rltND3Bd8CsdhfgNfDNMg2KFfazua/wMZhXPoCntqbiLSA28SNnoTLeHy oVZq8Zq5sLtIxaDouLS0nPnWjvp/qBNuhYPp3RGgAAGHpe1wHsZFUy9ftOuRenauWGdh 4rC2cyd6DDVp5IhcJA5u54wFsQTZqf3HobnBrF2yvM4RblXDKbfUpWbXzRaFb9B+DIg0 zplA== X-Gm-Message-State: APjAAAWmlN9JqClPXaraIHFntsmF1H+GtVGk6wrPKrH++0lDKMs76+vu CvcaGpmRoX21XoZ4u5uxE2MxJDYinQY= X-Google-Smtp-Source: APXvYqy68d9Ng4v1S+3XbifY4an9Sl2hEGBXR3rUtx/ooElUc+fi5+WPRSFy8jmCisZsF0cjR9P+zg== X-Received: by 2002:a6b:c986:: with SMTP id z128mr13473801iof.8.1578935343131; Mon, 13 Jan 2020 09:09:03 -0800 (PST) Received: from [192.168.1.159] ([65.144.74.34]) by smtp.gmail.com with ESMTPSA id x62sm4000954ill.86.2020.01.13.09.09.02 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 13 Jan 2020 09:09:02 -0800 (PST) Subject: Re: linux-next: build failure after merge of the block tree To: Al Viro Cc: Stephen Rothwell , Linux Next Mailing List , Linux Kernel Mailing List , Aleksa Sarai References: <20200106123027.1a162197@canb.auug.org.au> <20200112183235.GO8904@ZenIV.linux.org.uk> From: Jens Axboe Message-ID: Date: Mon, 13 Jan 2020 10:09:01 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: <20200112183235.GO8904@ZenIV.linux.org.uk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 1/12/20 11:32 AM, Al Viro wrote: > On Mon, Jan 06, 2020 at 09:04:01PM -0700, Jens Axboe wrote: >> On 1/5/20 6:30 PM, Stephen Rothwell wrote: >>> Hi all, >>> >>> After merging the block tree, today's linux-next build (arm >>> multi_v7_defconfig) failed like this: >>> >>> fs/open.c:977:12: error: conflicting types for 'build_open_flags' >>> 977 | inline int build_open_flags(const struct open_how *how, >>> | ^~~~~~~~~~~~~~~~ >>> In file included from /home/sfr/next/next/fs/open.c:36: >>> fs/internal.h:127:12: note: previous declaration of 'build_open_flags' was here >>> 127 | extern int build_open_flags(int flags, umode_t mode, struct open_flags *op); >>> | ^~~~~~~~~~~~~~~~ >>> >>> Caused by commits >>> >>> 4e9e15c9426e ("fs: make build_open_flags() available internally") >>> 3bba3e571bc8 ("io_uring: add support for IORING_OP_OPENAT") >>> >>> interacting with commit >>> >>> 0a51692d49ec ("open: introduce openat2(2) syscall") >>> >>> from the vfs tree. >>> >>> I have applied the following fix up patch for today: >> >> Thanks Stephen - I'll pull in the VFS tree and rebase the 5.6 io_uring >> bits on that. Then I'll send it out for review again, haven't heard from >> Al on the non-block open change. > > FWIW, I don't believe that your approach is workable. First of all, > *ANY* transition out of RCU mode can lead to blocking. You need to > acquire several references (mount and dentry, at the very least). > Suppose the last one fails (->d_seq mismatch). Now you suddenly > have to drop the one(s) you've acquired. And both dput() and mntput() > are fundamentally blocking operations. > > It simply does not work. You could cobble up something that kinda-sorta > works, if your added flag had > * caused hard failure on unlazy_child() > * caused hard failure on unlazy_walk() with any symlinks in stack > * caused hard failure on unlazy_walk() if it would've been required > to grab root > * made unlazy_walk() go through very careful dance if it's just > about nd->path; I'm not sure how well that could be done, but theoretically > it's not impossible. > > But for open() it's not going to work at all. Any open for write => you > will have to wait if you run into fs freeze. O_TRUNC => you've got IO > to do. Worst of all, once you've dropped out of RCU mode, *YOU* *CAN'T* > *FAIL*. Because that means blocking operations. So you need to verify > that you won't run into a blocking ->open(), IMA deciding to play silly > buggers and read through the entire file, etc., etc. _before_ dropping > out of RCU mode. > > do_last() is messy enough as it is; adding _this_ is completely out of > question. Thanks Al, that's useful! Sounds like the lookup is doable, but the open part is just a wasp nest of "don't even go there". For now, I'll drop the lookup change and just have the io_uring open punt to async. With that, I don't need any non-blocking guarantees. That is workable for now. > Jens, if you have a workable plan on that non-blocking open of yours, > post it in full details. Until then - NAK, and that's about as hard one > as I ever had to give. It's like the other io_uring opcodes - we prefer to try a non-blocking attempt first, and if that fails, then we go async. I have no grand async open design, was just hoping I could make it work with minimal effort. That's obviously not doable. I would not mind working on actually making it doable, but that's a bigger project than I originally wanted to take on. So the most likely outcome longer term is for io_uring to adopt a syslet type of approach to this, where we always just just call the open helper, and if we need to block/reschedule, then we move context to an appropriate worker thread. Time is better spent there rather than trying to make every useful system call provide a sane non-blocking path, I think. -- Jens Axboe