linux-kernel - Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real

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Message-ID: <CAK8P3a0D3EXuwBj+sSVL9cmpysdvQWXnkE0zEctBJS8XOqhX7A@mail.gmail.com>
Date:   Fri, 2 Jun 2017 12:51:16 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     "Yan, Zheng" <ukernel@...il.com>
Cc:     Deepa Dinamani <deepa.kernel@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        gregkh <gregkh@...uxfoundation.org>,
        "Dilger, Andreas" <andreas.dilger@...el.com>,
        "J. Bruce Fields" <bfields@...ldses.org>, Chris Mason <clm@...com>,
        David Miller <davem@...emloft.net>,
        David Sterba <dsterba@...e.com>,
        Evgeniy Dushistov <dushistov@...l.ru>,
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        Paul Moore <paul@...l-moore.com>,
        Steven Rostedt <rostedt@...dmis.org>, yuchao0@...wei.com,
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Subject: Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

On Fri, Jun 2, 2017 at 12:10 PM, Yan, Zheng <ukernel@...il.com> wrote:
> On Fri, Jun 2, 2017 at 5:45 PM, Arnd Bergmann <arnd@...db.de> wrote:
>> On Fri, Jun 2, 2017 at 4:09 AM, Yan, Zheng <ukernel@...il.com> wrote:
>>> On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@...il.com> wrote:
>>>> On Thu, Jun 1, 2017 at 5:36 PM, John Stultz <john.stultz@...aro.org> wrote:
>>>>> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel@...il.com> wrote:
>>
>> I believe the bug you see is the result of the two timestamps
>> currently being almost guaranteed to be different in the latest
>> kernels.
>> Changing r_stamp to use current_kernel_time() will make it the
>> same value most of the time (as it was before Deepa's patch),
>> but when the timer interrupt happens between the timestamps,
>> the two are still different, it's just much harder to hit.
>>
>> I think the proper solution should be to change __ceph_setattr()
>> in a way that has req->r_stamp always synchronized with i_ctime.
>> If we copy i_ctime to r_stamp, that will also take care of the
>> future issues with the planned changes to current_time().
>>
> I already have a patch
> https://github.com/ceph/ceph-client/commit/24f54cd18e195a002ee3d2ab50dbc952fd9f82af

Looks good to me. In case anyone cares:
Acked-by: Arnd Bergmann <arnd@...db.de>

>> The part I don't understand is what else r_stamp (i.e. the time
>> stamp in ceph_msg_data with type==
>> CEPH_MSG_CLIENT_REQUEST) is used for, other than setting
>> ctime in CEPH_MDS_OP_SETATTR.
>>
>> Will this be used to update the stored i_ctime for other operations
>> too? If so, we would need to synchronize it with the in-memory
>> i_ctime for all operations that do this.
>>
>
> yes,  mds uses it to update ctime of modified inodes. For example,
> when handling mkdir, mds set ctime of both parent inode and new inode
> to r_stamp.

I see, so we may have a variation of that problem there as well: From
my reading of the code, the child inode is not in memory yet, so
that seems fine, but I could not find where the parent in-memory inode
i_ctime is updated in ceph, but it is most likely not the same as
req->r_stamp (assuming it gets updated at all).

Would it make sense require all callers of ceph_mdsc_do_request()
to update r_stamp at the same time as i_ctime to keep them in sync?

        Arnd