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 DBC3FEB64DD for ; Thu, 13 Jul 2023 04:05:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232557AbjGMEFR (ORCPT ); Thu, 13 Jul 2023 00:05:17 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:56296 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230054AbjGMEFP (ORCPT ); Thu, 13 Jul 2023 00:05:15 -0400 Received: from out-60.mta0.migadu.com (out-60.mta0.migadu.com [IPv6:2001:41d0:1004:224b::3c]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 5E26C11D for ; Wed, 12 Jul 2023 21:05:12 -0700 (PDT) Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1689221110; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=ph9Ck/PuFebLrZhoZJHiE3YNA72DjZg/eGCYgzyMfNA=; b=CddY4VDjuBYecZAVuxPDV38zFdG57nne3BZPdfEZGLlMb+IU/IR84ivcMthAZ+3iTsjt1d bz13OVyRe0C6VYcIX1aUelqPgXO8mUetLZsjG6f7IbrHFmTczPmPn0neNbAO6G9s8E269O ZhuplWJIPHgAsrvI17tm1I1IG53Wg1w= Date: Thu, 13 Jul 2023 12:05:00 +0800 MIME-Version: 1.0 Subject: Re: [PATCH 3/3] io_uring: add support for getdents Content-Language: en-US To: Dominique Martinet Cc: io-uring@vger.kernel.org, Jens Axboe , Pavel Begunkov , Christian Brauner , Alexander Viro , Stefan Roesch , Clay Harris , Dave Chinner , linux-fsdevel@vger.kernel.org, Wanpeng Li References: <20230711114027.59945-1-hao.xu@linux.dev> <20230711114027.59945-4-hao.xu@linux.dev> <858c3f16-ffb3-217e-b5d6-fcc63ef9c401@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Hao Xu In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: io-uring@vger.kernel.org On 7/13/23 00:10, Dominique Martinet wrote: > Hao Xu wrote on Wed, Jul 12, 2023 at 03:53:24PM +0800: >>>> + if (file_count(file) > 1) >>> I was curious about this so I found it's basically what __fdget_pos does >>> before deciding it should take the f_pos_lock, and as such this is >>> probably correct... But if someone can chime in here: what guarantees >>> someone else won't __fdget_pos (or equivalent through this) the file >>> again between this and the vfs_getdents call? >>> That second get would make file_count > 1 and it would lock, but lock >>> hadn't been taken here so the other call could get the lock without >>> waiting and both would process getdents or seek or whatever in >>> parallel. >>> >> This file_count(file) is atomic_read, so I believe no race condition here. > I don't see how that helps in the presence of another thread getting the > lock after we possibly issued a getdents without the lock, e.g. > > t1 call io_uring getdents here > t1 sees file_count(file) == 1 and skips getting lock > t1 starts issuing vfs_getdents [... processing] > t2 calls either io_uring getdents or getdents64 syscall > t2 gets the lock, since it wasn't taken by t1 it can be obtained > t2 issues another vfs_getdents > > Christian raised the same issue so I'll leave this to his part of the > thread for reply, but I hope that clarified my concern. Hi Dominique, Ah, I misunderstood your question, sorry. The thing is f_count is init-ed to be 1, and normal uring requests do fdget first, so I think it's ok for normal requests. What Christian points out is issue with fixed file, that is indeed a problem I think. > > ----- > > BTW I forgot to point out: this dropped the REWIND bit from my patch; I > believe some form of "seek" is necessary for real applications to make > use of this (for example, a web server could keep the fd open in a LRU > and keep issuing readdir over and over again everytime it gets an > indexing request); not having rewind means it'd need to close and > re-open the fd everytime which doesn't seem optimal. > > A previous iteration discussed that real seek is difficult and not > necessarily needed to I settled for rewind, but was there a reason you > decided to stop handling that? > > My very egoistical personal use case won't require it, so I can just say > I don't care here, but it would be nice to have a reason explained at > some point Yes, like Al pointed out, getdents with an offset is not the right way to do it, So a way to do seek is a must. But like what I said in the cover-letter, I do think the right thing is to import lseek/llseek to io_uring, not increment the complex of getdents. Thanks, Hao