Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754155AbbHLIvM (ORCPT ); Wed, 12 Aug 2015 04:51:12 -0400 Received: from mail-wi0-f172.google.com ([209.85.212.172]:34065 "EHLO mail-wi0-f172.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752510AbbHLIvJ (ORCPT ); Wed, 12 Aug 2015 04:51:09 -0400 Message-ID: <55CB08F9.6030901@plexistor.com> Date: Wed, 12 Aug 2015 11:51:05 +0300 From: Boaz Harrosh User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 MIME-Version: 1.0 To: Dave Chinner , "Wilcox, Matthew R" CC: Boaz Harrosh , Jan Kara , "Kirill A. Shutemov" , Andrew Morton , "linux-mm@kvack.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Davidlohr Bueso Subject: Re: [PATCH, RFC 2/2] dax: use range_lock instead of i_mmap_lock References: <1439219664-88088-1-git-send-email-kirill.shutemov@linux.intel.com> <1439219664-88088-3-git-send-email-kirill.shutemov@linux.intel.com> <20150811081909.GD2650@quack.suse.cz> <20150811093708.GB906@dastard> <20150811135004.GC2659@quack.suse.cz> <55CA0728.7060001@plexistor.com> <100D68C7BA14664A8938383216E40DE040914C3E@FMSMSX114.amr.corp.intel.com> <20150811214822.GA20596@dastard> In-Reply-To: <20150811214822.GA20596@dastard> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2103 Lines: 47 On 08/12/2015 12:48 AM, Dave Chinner wrote: > On Tue, Aug 11, 2015 at 04:51:22PM +0000, Wilcox, Matthew R wrote: >> The race that you're not seeing is page fault vs page fault. Two >> threads each attempt to store a byte to different locations on the >> same page. With a read-mutex to exclude truncates, each thread >> calls ->get_block. One of the threads gets back a buffer marked >> as BH_New and calls memset() to clear the page. The other thread >> gets back a buffer which isn't marked as BH_New and simply inserts >> the mapping, returning to userspace, which stores the byte ... >> just in time for the other thread's memset() to write a zero over >> the top of it. > > So, this is not a truncate race that the XFS MMAPLOCK solves. > > However, that doesn't mean that the DAX code needs to add locking to > solve it. The race here is caused by block initialisation being > unserialised after a ->get_block call allocates the block (which the > filesystem serialises via internal locking). Hence two simultaneous > ->get_block calls to the same block is guaranteed to have the DAX > block initialisation race with the second ->get_block call that says > the block is already allocated. > > IOWs, the way to handle this is to have the ->get_block call handle > the block zeroing for new blocks instead of doing it after the fact > in the generic DAX code where there is no fine-grained serialisation > object available. By calling dax_clear_blocks() in the ->get_block > callback, the filesystem can ensure that the second racing call will > only make progress once the block has been fully initialised by the > first call. > > IMO the fix is - again - to move the functionality into the > filesystem where we already have the necessary exclusion in place to > avoid this race condition entirely. > Exactly, thanks > Cheers, > > Dave. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/