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

On Fri, Jun 2, 2017 at 1:18 PM, Yan, Zheng <ukernel(a)gmail.com&gt; wrote: > On Fri, Jun 2, 2017 at 6:51 PM, Arnd Bergmann <arnd(a)arndb.de&gt; wrote: >> On Fri, Jun 2, 2017 at 12:10 PM, Yan, Zheng <ukernel(a)gmail.com&gt; wrote: >>> On Fri, Jun 2, 2017 at 5:45 PM, Arnd Bergmann <arnd(a)arndb.de&gt; wrote: >>>> On Fri, Jun 2, 2017 at 4:09 AM, Yan, Zheng <ukernel(a)gmail.com&gt; wrote: >>>>> On Fri, Jun 2, 2017 at 8:57 AM, Deepa Dinamani <deepa.kernel(a)gmail.com&gt; wrote: >>>>>> On Thu, Jun 1, 2017 at 5:36 PM, John Stultz <john.stultz(a)linaro.org&gt; wrote: >>>>>>> On Thu, Jun 1, 2017 at 5:26 PM, Yan, Zheng <ukernel(a)gmail.com&gt; 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/24f54cd18e195a002ee3d2ab50dbc9... >> >> Looks good to me. In case anyone cares: >> Acked-by: Arnd Bergmann <arnd(a)arndb.de&gt; >> >>>> 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. But the __ceph_setattr() problem should be fixed by your patch, right? What I meant is another related problem in ceph_mkdir() where the i_ctime field of the parent inode is different between the persistent representation in the mds and the in-memory representation.

Arnd >> 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?