From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753839AbcARHjE (ORCPT ); Mon, 18 Jan 2016 02:39:04 -0500 Received: from mail-pa0-f68.google.com ([209.85.220.68]:35424 "EHLO mail-pa0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753858AbcARHi2 (ORCPT ); Mon, 18 Jan 2016 02:38:28 -0500 Date: Mon, 18 Jan 2016 16:39:39 +0900 From: Sergey Senozhatsky To: Minchan Kim Cc: Sergey Senozhatsky , Junil Lee , ngupta@vflare.org, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, vbabka@suse.cz Subject: Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition Message-ID: <20160118073939.GA30668@swordfish> References: <1453095596-44055-1-git-send-email-junil0814.lee@lge.com> <20160118063611.GC7453@bbox> <20160118065434.GB459@swordfish> <20160118071157.GD7453@bbox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160118071157.GD7453@bbox> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On (01/18/16 16:11), Minchan Kim wrote: [..] > > so, even if clear_bit_unlock/test_and_set_bit_lock do smp_mb or > > barrier(), there is no corresponding barrier from record_obj()->WRITE_ONCE(). > > so I don't think WRITE_ONCE() will help the compiler, or am I missing > > something? > > We need two things thanks. > 1. compiler barrier um... probably gcc can reorder that sequence to something like this *handle = obj_malloc() /* unpin the object */ zs_object_copy(*handle, used_obj, class) /* now use it*/ ok. > 2. memory barrier. > > As compiler barrier, WRITE_ONCE works to prevent store tearing here > by compiler. > However, if we omit unpin_tag here, we lose memory barrier(e,g, smp_mb) > so another CPU could see stale data caused CPU memory reordering. oh... good find! lost release semantic of unpin_tag()... -ss