From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753859Ab3IINtr (ORCPT ); Mon, 9 Sep 2013 09:49:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:28657 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753565Ab3IINtp (ORCPT ); Mon, 9 Sep 2013 09:49:45 -0400 Message-ID: <522DD125.1030607@redhat.com> Date: Mon, 09 Sep 2013 15:46:13 +0200 From: Jerome Marchand User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130110 Thunderbird/17.0.2 MIME-Version: 1.0 To: Dan Carpenter CC: Sergey Senozhatsky , Greg Kroah-Hartman , devel@driverdev.osuosl.org, Minchan Kim , linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] staging: zram: minimize `slot_free_lock' usage (v2) References: <20130906151255.GE2238@swordfish.minsk.epam.com> <20130909123329.GZ19256@mwanda> <20130909124942.GA2221@swordfish.minsk.epam.com> <20130909132124.GY6329@mwanda> In-Reply-To: <20130909132124.GY6329@mwanda> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/09/2013 03:21 PM, Dan Carpenter wrote: > On Mon, Sep 09, 2013 at 03:49:42PM +0300, Sergey Senozhatsky wrote: >>>> Calling handle_pending_slot_free() for every RW operation may >>>> cause unneccessary slot_free_lock locking, because most likely >>>> process will see NULL slot_free_rq. handle_pending_slot_free() >>>> only when current detects that slot_free_rq is not NULL. >>>> >>>> v2: protect handle_pending_slot_free() with zram rw_lock. >>>> >>> >>> zram->slot_free_lock protects zram->slot_free_rq but shouldn't the zram >>> rw_lock be wrapped around the whole operation like the original code >>> does? I don't know the zram code, but the original looks like it makes >>> sense but in this one it looks like the locks are duplicative. >>> >>> Is the down_read() in the original code be changed to down_write()? >>> >> >> I'm not touching locking around existing READ/WRITE commands. >> > > Your patch does change the locking because now instead of taking the > zram lock once it takes it and then drops it and then retakes it. This > looks potentially racy to me but I don't know the code so I will defer > to any zram maintainer. You're right. Nothing prevents zram_slot_free_notify() to repopulate the free slot queue while we drop the lock. Actually, the original code is already racy. handle_pending_slot_free() modifies zram->table while holding only a read lock. It needs to hold a write lock to do that. Using down_write for all requests would obviously fix that, but at the cost of read performance. > > 1) You haven't given us any performance numbers so it's not clear if the > locking is even a problem. > > 2) The v2 patch introduces an obvious deadlock in zram_slot_free() > because now we take the rw_lock twice. Fix your testing to catch > this kind of bug next time. > > 3) Explain why it is safe to test zram->slot_free_rq when we are not > holding the lock. I think it is unsafe. I don't want to even think > about it without the numbers. > > regards, > dan carpenter >