From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753276AbaBYWDw (ORCPT ); Tue, 25 Feb 2014 17:03:52 -0500 Received: from out03.mta.xmission.com ([166.70.13.233]:44729 "EHLO out03.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751356AbaBYWDs (ORCPT ); Tue, 25 Feb 2014 17:03:48 -0500 From: ebiederm@xmission.com (Eric W. Biederman) To: "J. Bruce Fields" Cc: Miklos Szeredi , Al Viro , "Serge E. Hallyn" , Linux-Fsdevel , Kernel Mailing List , Andy Lutomirski , Rob Landley , Linus Torvalds , Christoph Hellwig , Karel Zak References: <87a9kkax0j.fsf@xmission.com> <8761v7h2pt.fsf@tw-ebiederman.twitter.com> <87li281wx6.fsf_-_@xmission.com> <87ob28kqks.fsf_-_@xmission.com> <87eh34jbsl.fsf_-_@xmission.com> <20140218174053.GE4026@tucsk.piliscsaba.szeredi.hu> <87y510dpra.fsf@xmission.com> <20140225151349.GA19981@fieldses.org> Date: Tue, 25 Feb 2014 14:03:36 -0800 In-Reply-To: <20140225151349.GA19981@fieldses.org> (J. Bruce Fields's message of "Tue, 25 Feb 2014 10:13:49 -0500") Message-ID: <87mwhe26kn.fsf@xmission.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-AID: U2FsdGVkX19M7RSGxyplI+A0Kv08TCl2FWOtdftHgx0= X-SA-Exim-Connect-IP: 98.207.154.105 X-SA-Exim-Mail-From: ebiederm@xmission.com X-Spam-Report: * -1.0 ALL_TRUSTED Passed through trusted hosts only via SMTP * 1.0 KHOP_BIG_TO_CC Sent to 10+ recipients instaed of Bcc or a list * 0.5 XMGappySubj_01 Very gappy subject * 1.5 XMNoVowels Alpha-numberic number with no vowels * 0.7 XMSubLong Long Subject * 0.0 T_TM2_M_HEADER_IN_MSG BODY: T_TM2_M_HEADER_IN_MSG * -0.5 BAYES_05 BODY: Bayes spam probability is 1 to 5% * [score: 0.0165] * -0.0 DCC_CHECK_NEGATIVE Not listed in DCC * [sa05 1397; Body=1 Fuz1=1 Fuz2=1] * 0.0 T_TooManySym_01 4+ unique symbols in subject * 1.0 T_XMDrugObfuBody_08 obfuscated drug references * 0.1 XMSolicitRefs_0 Weightloss drug X-Spam-DCC: XMission; sa05 1397; Body=1 Fuz1=1 Fuz2=1 X-Spam-Combo: ***;"J. Bruce Fields" X-Spam-Relay-Country: Subject: Re: [PATCH 08/11] vfs: Merge check_submounts_and_drop and d_invalidate X-Spam-Flag: No X-SA-Exim-Version: 4.2.1 (built Wed, 14 Nov 2012 14:26:46 -0700) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org "J. Bruce Fields" writes: > On Mon, Feb 24, 2014 at 04:01:29PM -0800, Eric W. Biederman wrote: >> Miklos Szeredi writes: >> >> > >> > You can optimize this by including the negative check within the above d_locked >> > region and calling __d_drop() instead. >> >> For this patch just moving the code and not changing it is the corret >> thing to do because it helps with review and understanding the code. >> >> There are two ways I could see going with optimizing the preamble. >> Simply dropping the d_lock from around the d_unhashed test as a pointer >> dereference should be atomic, and the test is racy against >> d_materialise_unique. > > Could you explain? What's the race, and what are the consequences? >> (We don't always hold the parent directories inode mutex when d_invalidate is called). d_unhashed is not a permanent condition because of d_materialise_unique, and d_splice_alias. d_invalidate can be called on an unhashed dentry in one of two ways (either d_revalidate dropped the dentry or another routine that drops the dentry beat the current invocation of d_invalidate to the job). There are 3 places d_revalidate is called. Once on the rcu path with with the appropriate flag set. Once without out the parent i_mutex held, just off of the rcu path, on that path d_invalidate is when d_revalidate fails. Once during lookup with the parent directory i_mutex held. Because the parent direcories i_mutex is not always held accross d_revalidate and the following d_invalidate it happens that d_invalidate is not always an atomic operation. At worst the race results in a dentry that is dropped when it could be hashed, that we will resurrect next time someone attempts to look it up and d_materialise_unique/d_splice_alias is called. None of that really matters for optimizing d_invalidate, but it is part of the background in which d_invalidate lives. All that is significant in d_invalidate is knowing that d_materialise_unique, and possibly d_splice_alias may run concurrently with d_invalidate. It is unlikely and essentially harmless. After my patchset (because I removed all of the d_drop's from .d_revalidate) the only race that should remain is between two parallel calls of d_invalidate. Which probably means we can remove the test for d_unhashed altogether. Right now I just want to make this first big step and make certain the code is solid. After that optimization is easy. Eric