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

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Message-ID: <CAAM7YAk8JNU=XV=sOCczagWjBimO4JSPU08th7E4u6FTJN--9A@mail.gmail.com>
Date:   Fri, 2 Jun 2017 19:18:53 +0800
From:   "Yan, Zheng" <ukernel@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
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>,
        Eric Paris <eparis@...hat.com>,
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        Jeff Layton <jlayton@...chiereds.net>,
        James Simmons <jsimmons@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Drokin, Oleg" <oleg.drokin@...el.com>,
        Paul Moore <paul@...l-moore.com>,
        Steven Rostedt <rostedt@...dmis.org>, yuchao0@...wei.com,
        ceph-devel <ceph-devel@...r.kernel.org>,
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Subject: Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

On Fri, Jun 2, 2017 at 6:51 PM, Arnd Bergmann <arnd@...db.de> wrote:
> 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).

i_ctime is updated when handling request reply, by ceph_fill_file_time().
__ceph_setattr() can update the in-memory inode's ctime after request
reply is received. The difference between ktime_get_real_ts() and
current_time() can be larger than round-trip time of request. So it's
still possible that __ceph_setattr() make ctime go back.

Regards
Yan, Zheng


>
> 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