From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f53.google.com (mail-wm0-f53.google.com [74.125.82.53]) by kanga.kvack.org (Postfix) with ESMTP id 278CA6B0005 for ; Mon, 18 Jan 2016 02:54:03 -0500 (EST) Received: by mail-wm0-f53.google.com with SMTP id n5so49868429wmn.0 for ; Sun, 17 Jan 2016 23:54:03 -0800 (PST) Received: from mx2.suse.de (mx2.suse.de. [195.135.220.15]) by mx.google.com with ESMTPS id ld8si36658905wjc.77.2016.01.17.23.54.01 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Sun, 17 Jan 2016 23:54:01 -0800 (PST) Subject: Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition References: <1453095596-44055-1-git-send-email-junil0814.lee@lge.com> <20160118063611.GC7453@bbox> <20160118065434.GB459@swordfish> <20160118071157.GD7453@bbox> <20160118073939.GA30668@swordfish> From: Vlastimil Babka Message-ID: <569C9A1F.2020303@suse.cz> Date: Mon, 18 Jan 2016 08:54:07 +0100 MIME-Version: 1.0 In-Reply-To: <20160118073939.GA30668@swordfish> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Sergey Senozhatsky , Minchan Kim Cc: Junil Lee , ngupta@vflare.org, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 18.1.2016 8:39, Sergey Senozhatsky wrote: > 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 >> 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()... Ah, release semantic, good point indeed. OK then we need the v2 approach again, with WRITE_ONCE() in record_obj(). Or some kind of record_obj_release() with release semantic, which would be a bit more effective, but I guess migration is not that critical path to be worth introducing it. Thanks, Vlastimil > > -ss > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org