From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f198.google.com (mail-qt0-f198.google.com [209.85.216.198]) by kanga.kvack.org (Postfix) with ESMTP id 425316B0033 for ; Thu, 14 Sep 2017 04:58:31 -0400 (EDT) Received: by mail-qt0-f198.google.com with SMTP id u48so2904887qtc.3 for ; Thu, 14 Sep 2017 01:58:31 -0700 (PDT) Received: from mx0a-001b2d01.pphosted.com (mx0b-001b2d01.pphosted.com. [148.163.158.5]) by mx.google.com with ESMTPS id 13si6908046qtr.283.2017.09.14.01.58.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 14 Sep 2017 01:58:30 -0700 (PDT) Received: from pps.filterd (m0098421.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id v8E8s6oV139754 for ; Thu, 14 Sep 2017 04:58:29 -0400 Received: from e06smtp14.uk.ibm.com (e06smtp14.uk.ibm.com [195.75.94.110]) by mx0a-001b2d01.pphosted.com with ESMTP id 2cyp6rsuq2-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Thu, 14 Sep 2017 04:58:29 -0400 Received: from localhost by e06smtp14.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Thu, 14 Sep 2017 09:58:27 +0100 Subject: Re: [PATCH v3 04/20] mm: VMA sequence count References: <1504894024-2750-1-git-send-email-ldufour@linux.vnet.ibm.com> <1504894024-2750-5-git-send-email-ldufour@linux.vnet.ibm.com> <20170913115354.GA7756@jagdpanzerIV.localdomain> <44849c10-bc67-b55e-5788-d3c6bb5e7ad1@linux.vnet.ibm.com> <20170914003116.GA599@jagdpanzerIV.localdomain> <441ff1c6-72a7-5d96-02c8-063578affb62@linux.vnet.ibm.com> <20170914081358.GG599@jagdpanzerIV.localdomain> From: Laurent Dufour Date: Thu, 14 Sep 2017 10:58:16 +0200 MIME-Version: 1.0 In-Reply-To: <20170914081358.GG599@jagdpanzerIV.localdomain> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Message-Id: <26fa0b71-4053-5af7-baa0-e5fff9babf41@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: Sergey Senozhatsky Cc: paulmck@linux.vnet.ibm.com, peterz@infradead.org, akpm@linux-foundation.org, kirill@shutemov.name, ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net, jack@suse.cz, Matthew Wilcox , benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org, Thomas Gleixner , Ingo Molnar , hpa@zytor.com, Will Deacon , Sergey Senozhatsky , linux-kernel@vger.kernel.org, linux-mm@kvack.org, haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com, npiggin@gmail.com, bsingharora@gmail.com, Tim Chen , linuxppc-dev@lists.ozlabs.org, x86@kernel.org On 14/09/2017 10:13, Sergey Senozhatsky wrote: > Hi, > > On (09/14/17 09:55), Laurent Dufour wrote: > [..] >>> so if there are two CPUs, one doing write_seqcount() and the other one >>> doing read_seqcount() then what can happen is something like this >>> >>> CPU0 CPU1 >>> >>> fs_reclaim_acquire() >>> write_seqcount_begin() >>> fs_reclaim_acquire() read_seqcount_begin() >>> write_seqcount_end() >>> >>> CPU0 can't write_seqcount_end() because of fs_reclaim_acquire() from >>> CPU1, CPU1 can't read_seqcount_begin() because CPU0 did write_seqcount_begin() >>> and now waits for fs_reclaim_acquire(). makes sense? >> >> Yes, this makes sense. >> >> But in the case of this series, there is no call to >> __read_seqcount_begin(), and the reader (the speculative page fault >> handler), is just checking for (vm_seq & 1) and if this is true, simply >> exit the speculative path without waiting. >> So there is no deadlock possibility. > > probably lockdep just knows that those locks interleave at some > point. > > > by the way, I think there is one path that can spin > > find_vma_srcu() > read_seqbegin() > read_seqcount_begin() > raw_read_seqcount_begin() > __read_seqcount_begin() That's right, but here this is the sequence counter mm->mm_seq, not the vm_seq one. This one is held to protect walking the VMA list "locklessly"... Cheers, Laurent. -- 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