From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1757151AbcLOBrC (ORCPT <rfc822;w@1wt.eu>);
        Wed, 14 Dec 2016 20:47:02 -0500
Received: from mail-oi0-f66.google.com ([209.85.218.66]:35380 "EHLO
        mail-oi0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1753056AbcLOBq7 (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 14 Dec 2016 20:46:59 -0500
Subject: Re: [PATCH] ocfs2: fix crash caused by stale lvb with fsdlm plugin
To: Eric Ren <zren@suse.com>, ocfs2-devel@oss.oracle.com
References: <1481275846-6604-1-git-send-email-zren@suse.com>
Cc: akpm@linux-foundation.org, mfasheh@versity.com, jlbec@evilplan.org,
        teigland@redhat.com, ghe@suse.com, junxiao.bi@oracle.com,
        linux-kernel@vger.kernel.org
From: Joseph Qi <jiangqi903@gmail.com>
Message-ID: <7dbdc1d8-f1c5-c6bf-0614-1d8f03f0e3f0@gmail.com>
Date: Thu, 15 Dec 2016 09:46:40 +0800
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:45.0)
 Gecko/20100101 Thunderbird/45.5.1
MIME-Version: 1.0
In-Reply-To: <1481275846-6604-1-git-send-email-zren@suse.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

In you description, this issue can only happen in case of stack user +

fsdlm.

So I feel we'd better to make stack user and o2cb behaves the same,

other than treat it as a special case.


Thanks,

Joseph

On 16/12/9 17:30, Eric Ren wrote:
> The crash happens rather often when we reset some cluster
> nodes while nodes contend fiercely to do truncate and append.
>
> The crash backtrace is below:
> "
> [  245.197849] dlm: C21CBDA5E0774F4BA5A9D4F317717495: dlm_recover_grant 1 locks on 971 resources
> [  245.197859] dlm: C21CBDA5E0774F4BA5A9D4F317717495: dlm_recover 9 generation 5 done: 4 ms
> [  245.198379] ocfs2: Begin replay journal (node 318952601, slot 2) on device (253,18)
> [  247.272338] ocfs2: End replay journal (node 318952601, slot 2) on device (253,18)
> [  247.547084] ocfs2: Beginning quota recovery on device (253,18) for slot 2
> [  247.683263] ocfs2: Finishing quota recovery on device (253,18) for slot 2
> [  247.833022] (truncate,30154,1):ocfs2_truncate_file:470 ERROR: bug expression: le64_to_cpu(fe->i_size) != i_size_read(inode)
> [  247.833029] (truncate,30154,1):ocfs2_truncate_file:470 ERROR: Inode 290321, inode i_size = 732 != di i_size = 937, i_flags = 0x1
> [  247.833074] ------------[ cut here ]------------
> [  247.833077] kernel BUG at /usr/src/linux/fs/ocfs2/file.c:470!
> [  247.833079] invalid opcode: 0000 [#1] SMP
> [  247.833081] Modules linked in: ocfs2_stack_user(OEN) ocfs2(OEN) ocfs2_nodemanager ocfs2_stackglue(OEN) quota_tree dlm(OEN) configfs fuse sd_mod    iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi af_packet iscsi_ibft iscsi_boot_sysfs softdog xfs libcrc32c ppdev parport_pc pcspkr parport      joydev virtio_balloon virtio_net i2c_piix4 acpi_cpufreq button processor ext4 crc16 jbd2 mbcache ata_generic cirrus virtio_blk ata_piix               drm_kms_helper ahci syscopyarea libahci sysfillrect sysimgblt fb_sys_fops ttm floppy libata drm virtio_pci virtio_ring uhci_hcd virtio ehci_hcd       usbcore serio_raw usb_common sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua scsi_mod autofs4
> [  247.833107] Supported: No, Unsupported modules are loaded
> [  247.833110] CPU: 1 PID: 30154 Comm: truncate Tainted: G           OE   N  4.4.21-69-default #1
> [  247.833111] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.1-0-g4adadbd-20151112_172657-sheep25 04/01/2014
> [  247.833112] task: ffff88004ff6d240 ti: ffff880074e68000 task.ti: ffff880074e68000
> [  247.833113] RIP: 0010:[<ffffffffa05c8c30>]  [<ffffffffa05c8c30>] ocfs2_truncate_file+0x640/0x6c0 [ocfs2]
> [  247.833151] RSP: 0018:ffff880074e6bd50  EFLAGS: 00010282
> [  247.833152] RAX: 0000000000000074 RBX: 000000000000029e RCX: 0000000000000000
> [  247.833153] RDX: 0000000000000001 RSI: 0000000000000246 RDI: 0000000000000246
> [  247.833154] RBP: ffff880074e6bda8 R08: 000000003675dc7a R09: ffffffff82013414
> [  247.833155] R10: 0000000000034c50 R11: 0000000000000000 R12: ffff88003aab3448
> [  247.833156] R13: 00000000000002dc R14: 0000000000046e11 R15: 0000000000000020
> [  247.833157] FS:  00007f839f965700(0000) GS:ffff88007fc80000(0000) knlGS:0000000000000000
> [  247.833158] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [  247.833159] CR2: 00007f839f97e000 CR3: 0000000036723000 CR4: 00000000000006e0
> [  247.833164] Stack:
> [  247.833165]  00000000000003a9 0000000000000001 ffff880060554000 ffff88004fcaf000
> [  247.833167]  ffff88003aa7b090 1000000000000000 ffff88003aab3448 ffff880074e6beb0
> [  247.833169]  0000000000000001 0000000000002068 0000000000000020 0000000000000000
> [  247.833171] Call Trace:
> [  247.833208]  [<ffffffffa05cb1a8>] ocfs2_setattr+0x698/0xa90 [ocfs2]
> [  247.833225]  [<ffffffff8121587e>] notify_change+0x1ae/0x380
> [  247.833242]  [<ffffffff811f8bfe>] do_truncate+0x5e/0x90
> [  247.833246]  [<ffffffff811f8f58>] do_sys_ftruncate.constprop.11+0x108/0x160
> [  247.833257]  [<ffffffff815e142e>] entry_SYSCALL_64_fastpath+0x12/0x6d
> [  247.834724] DWARF2 unwinder stuck at entry_SYSCALL_64_fastpath+0x12/0x6d
> [  247.834725]
> [  247.834726] Leftover inexact backtrace:
>
> [  247.834728] Code: 24 28 ba d6 01 00 00 48 c7 c6 30 43 62 a0 8b 41 2c 89 44 24 08 48 8b 41 20 48 c7 c1 78 a3 62 a0 48 89 04 24 31 c0 e8 a0 97 f9 ff <0f> 0b 3d 00 fe ff ff 0f 84 ab fd ff ff 83 f8 fc 0f 84 a2 fd ff
> [  247.834748] RIP  [<ffffffffa05c8c30>] ocfs2_truncate_file+0x640/0x6c0 [ocfs2]
> [  247.834774]  RSP <ffff880074e6bd50>
> "
>
> It's because ocfs2_inode_lock() get us stale LVB in which the i_size is not
> equal to the disk i_size. We mistakenly trust the LVB because the underlaying
> fsdlm dlm_lock() doesn't set lkb_sbflags with DLM_SBF_VALNOTVALID properly for
> us. But, why?
>
> The current code tries to downconvert lock without DLM_LKF_VALBLK
> flag to tell o2cb don't update RSB's LVB if it's a PR->NULL conversion,
> even if the lock resource type needs LVB. This is not the right way for fsdlm.
>
> The fsdlm plugin behaves different on DLM_LKF_VALBLK, it depends on
> DLM_LKF_VALBLK to decide if we care about the LVB in the LKB. If DLM_LKF_VALBLK
> is not set, fsdlm will skip recovering RSB's LVB from this lkb and set the right
> DLM_SBF_VALNOTVALID appropriately when node failure happens.
>
> The following diagram briefly illustrates how this crash happens:
>
> RSB1 is inode metadata lock resource with LOCK_TYPE_USES_LVB;
>
> The 1st round:
>
>               Node1                                    Node2
> RSB1: PR
>                                                    RSB1(master): NULL->EX
> ocfs2_downconvert_lock(PR->NULL, set_lvb==0)
>    ocfs2_dlm_lock(no DLM_LKF_VALBLK)
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>
> dlm_lock(no DLM_LKF_VALBLK)
>    convert_lock(overwrite lkb->lkb_exflags
>                 with no DLM_LKF_VALBLK)
>
> RSB1: NULL                                        RSB1: EX
>                                                    reset Node2
> dlm_recover_rsbs()
>    recover_lvb()
>
> /* The LVB is not trustable if the node with EX fails and
>   * no lock >= PR is left. We should set RSB_VALNOTVALID for RSB1.
>   */
>
>   if(!(kb_exflags & DLM_LKF_VALBLK)) /* This means we miss the chance to
>             return;                   * to invalid the LVB here.
>                                       */
>
> The 2nd round:
>
>           Node 1                                Node2
> RSB1(become master from recovery)
>
> ocfs2_setattr()
>    ocfs2_inode_lock(NULL->EX)
>      /* dlm_lock() return the stale lvb without setting DLM_SBF_VALNOTVALID */
>      ocfs2_meta_lvb_is_trustable() return 1 /* so we don't refresh inode from disk */
>    ocfs2_truncate_file()
>        mlog_bug_on_msg(disk isize != i_size_read(inode))  /* crash! */
>
> The fix is quite straightforward. We keep to set DLM_LKF_VALBLK flag for dlm_lock()
> if the lock resource type needs LVB and the fsdlm plugin is uesed.
>
> Signed-off-by: Eric Ren <zren@suse.com>
> ---
>   fs/ocfs2/dlmglue.c   | 10 ++++++++++
>   fs/ocfs2/stackglue.c |  6 ++++++
>   fs/ocfs2/stackglue.h |  3 +++
>   3 files changed, 19 insertions(+)
>
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index 83d576f..77d1632 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -3303,6 +3303,16 @@ static int ocfs2_downconvert_lock(struct ocfs2_super *osb,
>   	mlog(ML_BASTS, "lockres %s, level %d => %d\n", lockres->l_name,
>   	     lockres->l_level, new_level);
>   
> +	/*
> +	 * On DLM_LKF_VALBLK, fsdlm behaves differently with o2cb. It always
> +	 * expects DLM_LKF_VALBLK being set if the LKB has LVB, so that
> +	 * we can recover correctly from node failure. Otherwise, we may get
> +	 * invalid LVB in LKB, but without DLM_SBF_VALNOTVALID being set.
> +	 */
> +	if (!ocfs2_is_o2cb_active() &&
> +	    lockres->l_ops->flags & LOCK_TYPE_USES_LVB)
> +		lvb = 1;
> +
>   	if (lvb)
>   		dlm_flags |= DLM_LKF_VALBLK;
>   
> diff --git a/fs/ocfs2/stackglue.c b/fs/ocfs2/stackglue.c
> index 52c07346b..8203590 100644
> --- a/fs/ocfs2/stackglue.c
> +++ b/fs/ocfs2/stackglue.c
> @@ -48,6 +48,12 @@ static char ocfs2_hb_ctl_path[OCFS2_MAX_HB_CTL_PATH] = "/sbin/ocfs2_hb_ctl";
>    */
>   static struct ocfs2_stack_plugin *active_stack;
>   
> +inline int ocfs2_is_o2cb_active(void)
> +{
> +	return !strcmp(active_stack->sp_name, OCFS2_STACK_PLUGIN_O2CB);
> +}
> +EXPORT_SYMBOL_GPL(ocfs2_is_o2cb_active);
> +
>   static struct ocfs2_stack_plugin *ocfs2_stack_lookup(const char *name)
>   {
>   	struct ocfs2_stack_plugin *p;
> diff --git a/fs/ocfs2/stackglue.h b/fs/ocfs2/stackglue.h
> index f2dce10..e3036e1 100644
> --- a/fs/ocfs2/stackglue.h
> +++ b/fs/ocfs2/stackglue.h
> @@ -298,6 +298,9 @@ void ocfs2_stack_glue_set_max_proto_version(struct ocfs2_protocol_version *max_p
>   int ocfs2_stack_glue_register(struct ocfs2_stack_plugin *plugin);
>   void ocfs2_stack_glue_unregister(struct ocfs2_stack_plugin *plugin);
>   
> +/* In ocfs2_downconvert_lock(), we need to know which stack we are using */
> +int ocfs2_is_o2cb_active(void);
> +
>   extern struct kset *ocfs2_kset;
>   
>   #endif  /* STACKGLUE_H */

From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joseph Qi <jiangqi903@gmail.com>
Date: Thu, 15 Dec 2016 09:46:40 +0800
Subject: [Ocfs2-devel] [PATCH] ocfs2: fix crash caused by stale lvb with
	fsdlm plugin
In-Reply-To: <1481275846-6604-1-git-send-email-zren@suse.com>
References: <1481275846-6604-1-git-send-email-zren@suse.com>
Message-ID: <7dbdc1d8-f1c5-c6bf-0614-1d8f03f0e3f0@gmail.com>
List-Id: <ocfs2-devel.oss.oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
To: Eric Ren <zren@suse.com>, ocfs2-devel@oss.oracle.com
Cc: akpm@linux-foundation.org, mfasheh@versity.com, jlbec@evilplan.org, teigland@redhat.com, ghe@suse.com, junxiao.bi@oracle.com, linux-kernel@vger.kernel.org

In you description, this issue can only happen in case of stack user +

fsdlm.

So I feel we'd better to make stack user and o2cb behaves the same,

other than treat it as a special case.


Thanks,

Joseph

On 16/12/9 17:30, Eric Ren wrote:
> The crash happens rather often when we reset some cluster
> nodes while nodes contend fiercely to do truncate and append.
>
> The crash backtrace is below:
> "
> [  245.197849] dlm: C21CBDA5E0774F4BA5A9D4F317717495: dlm_recover_grant 1 locks on 971 resources
> [  245.197859] dlm: C21CBDA5E0774F4BA5A9D4F317717495: dlm_recover 9 generation 5 done: 4 ms
> [  245.198379] ocfs2: Begin replay journal (node 318952601, slot 2) on device (253,18)
> [  247.272338] ocfs2: End replay journal (node 318952601, slot 2) on device (253,18)
> [  247.547084] ocfs2: Beginning quota recovery on device (253,18) for slot 2
> [  247.683263] ocfs2: Finishing quota recovery on device (253,18) for slot 2
> [  247.833022] (truncate,30154,1):ocfs2_truncate_file:470 ERROR: bug expression: le64_to_cpu(fe->i_size) != i_size_read(inode)
> [  247.833029] (truncate,30154,1):ocfs2_truncate_file:470 ERROR: Inode 290321, inode i_size = 732 != di i_size = 937, i_flags = 0x1
> [  247.833074] ------------[ cut here ]------------
> [  247.833077] kernel BUG at /usr/src/linux/fs/ocfs2/file.c:470!
> [  247.833079] invalid opcode: 0000 [#1] SMP
> [  247.833081] Modules linked in: ocfs2_stack_user(OEN) ocfs2(OEN) ocfs2_nodemanager ocfs2_stackglue(OEN) quota_tree dlm(OEN) configfs fuse sd_mod    iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi af_packet iscsi_ibft iscsi_boot_sysfs softdog xfs libcrc32c ppdev parport_pc pcspkr parport      joydev virtio_balloon virtio_net i2c_piix4 acpi_cpufreq button processor ext4 crc16 jbd2 mbcache ata_generic cirrus virtio_blk ata_piix               drm_kms_helper ahci syscopyarea libahci sysfillrect sysimgblt fb_sys_fops ttm floppy libata drm virtio_pci virtio_ring uhci_hcd virtio ehci_hcd       usbcore serio_raw usb_common sg dm_multipath dm_mod scsi_dh_rdac scsi_dh_emc scsi_dh_alua scsi_mod autofs4
> [  247.833107] Supported: No, Unsupported modules are loaded
> [  247.833110] CPU: 1 PID: 30154 Comm: truncate Tainted: G           OE   N  4.4.21-69-default #1
> [  247.833111] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.1-0-g4adadbd-20151112_172657-sheep25 04/01/2014
> [  247.833112] task: ffff88004ff6d240 ti: ffff880074e68000 task.ti: ffff880074e68000
> [  247.833113] RIP: 0010:[<ffffffffa05c8c30>]  [<ffffffffa05c8c30>] ocfs2_truncate_file+0x640/0x6c0 [ocfs2]
> [  247.833151] RSP: 0018:ffff880074e6bd50  EFLAGS: 00010282
> [  247.833152] RAX: 0000000000000074 RBX: 000000000000029e RCX: 0000000000000000
> [  247.833153] RDX: 0000000000000001 RSI: 0000000000000246 RDI: 0000000000000246
> [  247.833154] RBP: ffff880074e6bda8 R08: 000000003675dc7a R09: ffffffff82013414
> [  247.833155] R10: 0000000000034c50 R11: 0000000000000000 R12: ffff88003aab3448
> [  247.833156] R13: 00000000000002dc R14: 0000000000046e11 R15: 0000000000000020
> [  247.833157] FS:  00007f839f965700(0000) GS:ffff88007fc80000(0000) knlGS:0000000000000000
> [  247.833158] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
> [  247.833159] CR2: 00007f839f97e000 CR3: 0000000036723000 CR4: 00000000000006e0
> [  247.833164] Stack:
> [  247.833165]  00000000000003a9 0000000000000001 ffff880060554000 ffff88004fcaf000
> [  247.833167]  ffff88003aa7b090 1000000000000000 ffff88003aab3448 ffff880074e6beb0
> [  247.833169]  0000000000000001 0000000000002068 0000000000000020 0000000000000000
> [  247.833171] Call Trace:
> [  247.833208]  [<ffffffffa05cb1a8>] ocfs2_setattr+0x698/0xa90 [ocfs2]
> [  247.833225]  [<ffffffff8121587e>] notify_change+0x1ae/0x380
> [  247.833242]  [<ffffffff811f8bfe>] do_truncate+0x5e/0x90
> [  247.833246]  [<ffffffff811f8f58>] do_sys_ftruncate.constprop.11+0x108/0x160
> [  247.833257]  [<ffffffff815e142e>] entry_SYSCALL_64_fastpath+0x12/0x6d
> [  247.834724] DWARF2 unwinder stuck at entry_SYSCALL_64_fastpath+0x12/0x6d
> [  247.834725]
> [  247.834726] Leftover inexact backtrace:
>
> [  247.834728] Code: 24 28 ba d6 01 00 00 48 c7 c6 30 43 62 a0 8b 41 2c 89 44 24 08 48 8b 41 20 48 c7 c1 78 a3 62 a0 48 89 04 24 31 c0 e8 a0 97 f9 ff <0f> 0b 3d 00 fe ff ff 0f 84 ab fd ff ff 83 f8 fc 0f 84 a2 fd ff
> [  247.834748] RIP  [<ffffffffa05c8c30>] ocfs2_truncate_file+0x640/0x6c0 [ocfs2]
> [  247.834774]  RSP <ffff880074e6bd50>
> "
>
> It's because ocfs2_inode_lock() get us stale LVB in which the i_size is not
> equal to the disk i_size. We mistakenly trust the LVB because the underlaying
> fsdlm dlm_lock() doesn't set lkb_sbflags with DLM_SBF_VALNOTVALID properly for
> us. But, why?
>
> The current code tries to downconvert lock without DLM_LKF_VALBLK
> flag to tell o2cb don't update RSB's LVB if it's a PR->NULL conversion,
> even if the lock resource type needs LVB. This is not the right way for fsdlm.
>
> The fsdlm plugin behaves different on DLM_LKF_VALBLK, it depends on
> DLM_LKF_VALBLK to decide if we care about the LVB in the LKB. If DLM_LKF_VALBLK
> is not set, fsdlm will skip recovering RSB's LVB from this lkb and set the right
> DLM_SBF_VALNOTVALID appropriately when node failure happens.
>
> The following diagram briefly illustrates how this crash happens:
>
> RSB1 is inode metadata lock resource with LOCK_TYPE_USES_LVB;
>
> The 1st round:
>
>               Node1                                    Node2
> RSB1: PR
>                                                    RSB1(master): NULL->EX
> ocfs2_downconvert_lock(PR->NULL, set_lvb==0)
>    ocfs2_dlm_lock(no DLM_LKF_VALBLK)
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>
> dlm_lock(no DLM_LKF_VALBLK)
>    convert_lock(overwrite lkb->lkb_exflags
>                 with no DLM_LKF_VALBLK)
>
> RSB1: NULL                                        RSB1: EX
>                                                    reset Node2
> dlm_recover_rsbs()
>    recover_lvb()
>
> /* The LVB is not trustable if the node with EX fails and
>   * no lock >= PR is left. We should set RSB_VALNOTVALID for RSB1.
>   */
>
>   if(!(kb_exflags & DLM_LKF_VALBLK)) /* This means we miss the chance to
>             return;                   * to invalid the LVB here.
>                                       */
>
> The 2nd round:
>
>           Node 1                                Node2
> RSB1(become master from recovery)
>
> ocfs2_setattr()
>    ocfs2_inode_lock(NULL->EX)
>      /* dlm_lock() return the stale lvb without setting DLM_SBF_VALNOTVALID */
>      ocfs2_meta_lvb_is_trustable() return 1 /* so we don't refresh inode from disk */
>    ocfs2_truncate_file()
>        mlog_bug_on_msg(disk isize != i_size_read(inode))  /* crash! */
>
> The fix is quite straightforward. We keep to set DLM_LKF_VALBLK flag for dlm_lock()
> if the lock resource type needs LVB and the fsdlm plugin is uesed.
>
> Signed-off-by: Eric Ren <zren@suse.com>
> ---
>   fs/ocfs2/dlmglue.c   | 10 ++++++++++
>   fs/ocfs2/stackglue.c |  6 ++++++
>   fs/ocfs2/stackglue.h |  3 +++
>   3 files changed, 19 insertions(+)
>
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index 83d576f..77d1632 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -3303,6 +3303,16 @@ static int ocfs2_downconvert_lock(struct ocfs2_super *osb,
>   	mlog(ML_BASTS, "lockres %s, level %d => %d\n", lockres->l_name,
>   	     lockres->l_level, new_level);
>   
> +	/*
> +	 * On DLM_LKF_VALBLK, fsdlm behaves differently with o2cb. It always
> +	 * expects DLM_LKF_VALBLK being set if the LKB has LVB, so that
> +	 * we can recover correctly from node failure. Otherwise, we may get
> +	 * invalid LVB in LKB, but without DLM_SBF_VALNOTVALID being set.
> +	 */
> +	if (!ocfs2_is_o2cb_active() &&
> +	    lockres->l_ops->flags & LOCK_TYPE_USES_LVB)
> +		lvb = 1;
> +
>   	if (lvb)
>   		dlm_flags |= DLM_LKF_VALBLK;
>   
> diff --git a/fs/ocfs2/stackglue.c b/fs/ocfs2/stackglue.c
> index 52c07346b..8203590 100644
> --- a/fs/ocfs2/stackglue.c
> +++ b/fs/ocfs2/stackglue.c
> @@ -48,6 +48,12 @@ static char ocfs2_hb_ctl_path[OCFS2_MAX_HB_CTL_PATH] = "/sbin/ocfs2_hb_ctl";
>    */
>   static struct ocfs2_stack_plugin *active_stack;
>   
> +inline int ocfs2_is_o2cb_active(void)
> +{
> +	return !strcmp(active_stack->sp_name, OCFS2_STACK_PLUGIN_O2CB);
> +}
> +EXPORT_SYMBOL_GPL(ocfs2_is_o2cb_active);
> +
>   static struct ocfs2_stack_plugin *ocfs2_stack_lookup(const char *name)
>   {
>   	struct ocfs2_stack_plugin *p;
> diff --git a/fs/ocfs2/stackglue.h b/fs/ocfs2/stackglue.h
> index f2dce10..e3036e1 100644
> --- a/fs/ocfs2/stackglue.h
> +++ b/fs/ocfs2/stackglue.h
> @@ -298,6 +298,9 @@ void ocfs2_stack_glue_set_max_proto_version(struct ocfs2_protocol_version *max_p
>   int ocfs2_stack_glue_register(struct ocfs2_stack_plugin *plugin);
>   void ocfs2_stack_glue_unregister(struct ocfs2_stack_plugin *plugin);
>   
> +/* In ocfs2_downconvert_lock(), we need to know which stack we are using */
> +int ocfs2_is_o2cb_active(void);
> +
>   extern struct kset *ocfs2_kset;
>   
>   #endif  /* STACKGLUE_H */