From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751447AbcAPKHm (ORCPT ); Sat, 16 Jan 2016 05:07:42 -0500 Received: from mail-pa0-f68.google.com ([209.85.220.68]:33370 "EHLO mail-pa0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751097AbcAPKHj (ORCPT ); Sat, 16 Jan 2016 05:07:39 -0500 Date: Sat, 16 Jan 2016 19:05:57 +0900 From: Sergey Senozhatsky To: Vlastimil Babka Cc: Sergey Senozhatsky , Minchan Kim , Junil Lee , ngupta@vflare.org, sergey.senozhatsky.work@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] zsmalloc: fix migrate_zspage-zs_free race condition Message-ID: <20160116100557.GC566@swordfish> References: <1452843551-4464-1-git-send-email-junil0814.lee@lge.com> <20160115143434.GA25332@blaptop.local> <56991514.9000609@suse.cz> <20160116040913.GA566@swordfish> <5699F4C9.1070902@suse.cz> <20160116080650.GB566@swordfish> <5699FC69.4010000@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5699FC69.4010000@suse.cz> 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 (01/16/16 09:16), Vlastimil Babka wrote: [..] > BTW, couldn't the correct fix also just look like this? > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > index 9f15bdd9163c..43f743175ede 100644 > --- a/mm/zsmalloc.c > +++ b/mm/zsmalloc.c > @@ -1635,8 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct > size_class *class, > free_obj = obj_malloc(d_page, class, handle); > zs_object_copy(free_obj, used_obj, class); > index++; > + /* This also effectively unpins the handle */ > record_obj(handle, free_obj); > - unpin_tag(handle); > obj_free(pool, class, used_obj); > } I think this will work. > But I'd still recommend WRITE_ONCE in record_obj(). And I'm not even sure it's > safe on all architectures to do a simple overwrite of a word against somebody > else trying to lock a bit there? hm... for example, generic bitops from include/asm-generic/bitops/atomic.h use _atomic_spin_lock_irqsave() #define test_and_set_bit_lock(nr, addr) test_and_set_bit(nr, addr) static inline int test_and_set_bit(int nr, volatile unsigned long *addr) { unsigned long mask = BIT_MASK(nr); unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr); unsigned long old; unsigned long flags; _atomic_spin_lock_irqsave(p, flags); old = *p; *p = old | mask; _atomic_spin_unlock_irqrestore(p, flags); return (old & mask) != 0; } so overwriting it from the outside world (w/o taking _atomic_spin_lock_irqsave(p)) can theoretically be tricky in some cases. -ss