From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ig0-f171.google.com (mail-ig0-f171.google.com [209.85.213.171]) by kanga.kvack.org (Postfix) with ESMTP id 679006B0005 for ; Mon, 18 Jan 2016 03:17:40 -0500 (EST) Received: by mail-ig0-f171.google.com with SMTP id z14so50917522igp.0 for ; Mon, 18 Jan 2016 00:17:40 -0800 (PST) Received: from lgeamrelo12.lge.com (LGEAMRELO12.lge.com. [156.147.23.52]) by mx.google.com with ESMTPS id fs8si24869518igb.27.2016.01.18.00.17.39 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Mon, 18 Jan 2016 00:17:39 -0800 (PST) Date: Mon, 18 Jan 2016 17:20:00 +0900 From: Minchan Kim Subject: Re: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition Message-ID: <20160118082000.GA20244@bbox> References: <1453095596-44055-1-git-send-email-junil0814.lee@lge.com> <20160118063611.GC7453@bbox> <20160118065434.GB459@swordfish> <20160118071157.GD7453@bbox> <20160118073939.GA30668@swordfish> <569C9A1F.2020303@suse.cz> MIME-Version: 1.0 In-Reply-To: <569C9A1F.2020303@suse.cz> Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline Sender: owner-linux-mm@kvack.org List-ID: To: Vlastimil Babka Cc: Sergey Senozhatsky , Junil Lee , ngupta@vflare.org, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On Mon, Jan 18, 2016 at 08:54:07AM +0100, Vlastimil Babka wrote: > 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. WRITE_ONCE in record_obj would add more memory operations in obj_malloc but I don't feel it's too heavy in this phase so, How about this? Junil, Could you resend patch if others agree this? Thanks. +/* + * record_obj updates handle's value to free_obj and it shouldn't + * invalidate lock bit(ie, HANDLE_PIN_BIT) of handle, otherwise + * it breaks synchronization using pin_tag(e,g, zs_free) so let's + * keep the lock bit. + */ static void record_obj(unsigned long handle, unsigned long obj) { - *(unsigned long *)handle = obj; + int locked = (*(unsigned long *)handle) & (1< > 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