Re: [PATCH 04/12] fs: ceph: CURRENT_TIME with ktime_get_real_ts()

[Arnd Bergmann]

On Fri, Jun 2, 2017 at 12:10 PM, Yan, Zheng <ukernel@xxxxxxxxx> wrote:
> On Fri, Jun 2, 2017 at 5:45 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Fri, Jun 2, 2017 at 4:09 AM, Yan, Zheng <ukernel@xxxxxxxxx> wrote:
>>> On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel@xxxxxxxxx> wrote:
>>>> On Thu, Jun 1, 2017 at 5:36 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
>>>>> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel@xxxxxxxxx> 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@xxxxxxxx>

>> 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