From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751344AbdFHKlC (ORCPT ); Thu, 8 Jun 2017 06:41:02 -0400 Received: from mga01.intel.com ([192.55.52.88]:18912 "EHLO mga01.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750752AbdFHKlA (ORCPT ); Thu, 8 Jun 2017 06:41:00 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.39,314,1493708400"; d="scan'208";a="97027872" Date: Thu, 8 Jun 2017 13:40:56 +0300 From: "Kirill A. Shutemov" To: Vlastimil Babka Cc: Will Deacon , linux-mm@kvack.org, linux-kernel@vger.kernel.org, mark.rutland@arm.com, akpm@linux-foundation.org, Punit.Agrawal@arm.com, mgorman@suse.de, steve.capper@arm.com Subject: Re: [PATCH 2/3] mm/page_ref: Ensure page_ref_unfreeze is ordered against prior accesses Message-ID: <20170608104056.ujuytybmwumuty64@black.fi.intel.com> References: <1496771916-28203-1-git-send-email-will.deacon@arm.com> <1496771916-28203-3-git-send-email-will.deacon@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20161126 (1.7.0) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 08, 2017 at 11:38:21AM +0200, Vlastimil Babka wrote: > On 06/06/2017 07:58 PM, Will Deacon wrote: > > page_ref_freeze and page_ref_unfreeze are designed to be used as a pair, > > wrapping a critical section where struct pages can be modified without > > having to worry about consistency for a concurrent fast-GUP. > > > > Whilst page_ref_freeze has full barrier semantics due to its use of > > atomic_cmpxchg, page_ref_unfreeze is implemented using atomic_set, which > > doesn't provide any barrier semantics and allows the operation to be > > reordered with respect to page modifications in the critical section. > > > > This patch ensures that page_ref_unfreeze is ordered after any critical > > section updates, by invoking smp_mb__before_atomic() prior to the > > atomic_set. > > > > Cc: "Kirill A. Shutemov" > > Acked-by: Steve Capper > > Signed-off-by: Will Deacon > > Undecided if it's really needed. This is IMHO not the classical case > from Documentation/core-api/atomic_ops.rst where we have to make > modifications visible before we let others see them? Here the one who is > freezing is doing it so others can't get their page pin and interfere > with the freezer's work. Hm.. I'm not sure I'm getting what you are talking about. What would guarantee others to see changes to page before seeing page unfreezed? > > include/linux/page_ref.h | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h > > index 610e13271918..74d32d7905cb 100644 > > --- a/include/linux/page_ref.h > > +++ b/include/linux/page_ref.h > > @@ -174,6 +174,7 @@ static inline void page_ref_unfreeze(struct page *page, int count) > > VM_BUG_ON_PAGE(page_count(page) != 0, page); > > VM_BUG_ON(count == 0); > > > > + smp_mb__before_atomic(); > > atomic_set(&page->_refcount, count); I *think* it should be smp_mb(), not __before_atomic(). atomic_set() is not really atomic. For instance on x86 it's plain WRITE_ONCE() which CPU would happily reorder. > > if (page_ref_tracepoint_active(__tracepoint_page_ref_unfreeze)) > > __page_ref_unfreeze(page, count); > > > -- Kirill A. Shutemov