From owner-freebsd-current@FreeBSD.ORG Mon May 4 14:53:02 2015 Return-Path: Delivered-To: current@FreeBSD.ORG Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 47EE2302 for ; Mon, 4 May 2015 14:53:02 +0000 (UTC) Received: from vps1.elischer.org (vps1.elischer.org [204.109.63.16]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client CN "vps1.elischer.org", Issuer "CA Cert Signing Authority" (not verified)) by mx1.freebsd.org (Postfix) with ESMTPS id 26ADE18BC for ; Mon, 4 May 2015 14:53:01 +0000 (UTC) Received: from Julian-MBP3.local (ppp121-45-241-118.lns20.per4.internode.on.net [121.45.241.118]) (authenticated bits=0) by vps1.elischer.org (8.14.9/8.14.9) with ESMTP id t44EqmVg025602 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES128-SHA bits=128 verify=NO); Mon, 4 May 2015 07:52:52 -0700 (PDT) (envelope-from julian@freebsd.org) Message-ID: <554787BA.1020105@freebsd.org> Date: Mon, 04 May 2015 22:52:42 +0800 From: Julian Elischer User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: Jilles Tjoelker , Konstantin Belousov CC: "current@freebsd.org" Subject: Re: seekdir/readdir patch .. Call for Review. References: <55432593.6050709@freebsd.org> <20150501161742.GW2390@kib.kiev.ua> <20150503143345.GB70576@stack.nl> In-Reply-To: <20150503143345.GB70576@stack.nl> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 04 May 2015 14:53:02 -0000 On 5/3/15 10:33 PM, Jilles Tjoelker wrote: > On Fri, May 01, 2015 at 07:17:42PM +0300, Konstantin Belousov wrote: >> On Fri, May 01, 2015 at 03:04:51PM +0800, Julian Elischer wrote: >>> if you are interested in readdir(3), seekdir(3) and telldir(3) then >>> you should look at >>> https://reviews.freebsd.org/D2410 >>> this patches around a problem in seekdir() that breaks Samba. >>> Seekdir(3) will not work as expected when files prior to the point of >>> interest in directory have been deleted since the directory was opened. >>> Windows clients using Samba cause both these things to happen, causing >>> the next readdir(3) after the bad seekdir(3) to skip some entries and >>> return the wrong file. >>> Samba only needs to step back a single directory entry in the case >>> where it reads an entry and then discovers it can't fit it into the >>> buffer it is sending to the windows client. It turns out we can >>> reliably cater to Samba's requirement because the "last returned >>> element" is always still in memory, so with a little care, we can >>> set our filepointer back to it safely. (once) >>> seekdir and readdir (and telldir()) need a complete rewrite along with >>> getdirentries() but that is more than a small edit like this. >> Can you explain your expectations from the whole readdir() vs. parallel >> directory modifications interaction ? From what I understood so far, >> there is unlocked modification of the container and parallel iterator >> over the same container. IMO, in such situation, whatever tweaks you >> apply to the iterator, it is still cannot be made reliable. >> Before making single-purpose changes to the libc readdir and seekdir >> code, or to the kernel code, it would be useful to state exact behaviour >> of the dirent machinery we want to see. No, 'make samba works in my >> situation' does not sound good enough. > Consider the subsequence of entries that existed at opendir() time and > were not removed until now. This subsequence is clearly defined and does > not have concurrency problems. The order of this subsequence must remain > unchanged and seekdir() must be correct with respect to this > subsequence. > > Additionally, two other kinds of entries may be returned. New entries > may be inserted anywhere in between the entries of the subsequence, and > removed entries may be returned as if they were still part of the > subsequence (so that not every readdir() needs a system call). > > A simple implementation for UFS-style directories is to store the offset > in the directory (all bits of it, not masking off the lower 9 bits). > This needs d_off or similar in struct dirent. The kernel getdirentries() > then needs a similar loop as the old libc seekdir() to go from the start > of the 512-byte directory block to the desired entry (since an entry may > not exist at the stored offset within the directory block). > > This means that a UFS-style directory cannot be compacted (existing > entries moved from higher to lower offsets to fill holes) while it is > open for reading. An NFS exported directory is always open for reading. > > This also means that duplicate entries can only be returned if that > particular filename was deleted and created again. > > Without kernel support, it is hard to get telldir/seekdir completely > reliable. The current libc implementation is wrong since the "holes" > within the block just disappear and change the offsets of the following > entries; the kernel cannot fix this using entries with d_fileno = 0 > since it cannot know, in the general case, how long the deleted entry > was in the filesystem-independent dirent format. My previous idea of > storing one d_fileno during telldir() is wrong since it will fail if > that entry is deleted. > > If you do not care about memory usage (which probably is already > excessive with the current libc implementation), you could store at > telldir() time the offset of the current block returned by > getdirentries() and the d_fileno of all entries already returned in the > current block. > > The D2410 patch can conceptually work for what Samba needs, stepping > back one directory entry. I will comment on it. > how long do I have to wait until I can commit this and was kib's comment a "do not commit"?