On Thu, 2020-10-29 at 11:19 -0400, Jeff Layton wrote: > I'm working on a F_SETLEASE implementation for kcephfs, and am hitting a > deadlock of sorts, due to a truncate triggering a cap revoke at an > inopportune time. > > The issue is that truncates to a smaller size are always done via > synchronous call to the MDS, whereas a truncate larger does not if Fx > caps are held. That synchronous call causes the MDS to issue the client > a cap revoke for caps that the lease holds references on (Frw, in > particular). > > The client code has been this way since the inception and I haven't been > able to locate any rationale for it. Some questions about this: > > 1) Why doesn't the client ever buffer a truncate to smaller size? It > seems like that is something that could be done without a synchronous > MDS call if we hold Fx caps. > > 2) The client setattr implementations set inode_drop values in the > MetaRequest, but as far as I can tell, those values end up being ignored > by the MDS. What purpose does inode_drop actually serve? Is this field > vestigial? I think I answered the second question myself. It _is_ potentially used to encoded a cap release into the call. That's not happening here because of the extra references held by the lease. I think I see a couple of potential fixes for that problem. I think the first question is still valid though. -- Jeff Layton <jlayton@xxxxxxxxxx>
