On Mon, 2024-09-30 at 22:19 +0200, Thomas Gleixner wrote: > On Mon, Sep 30 2024 at 15:37, Jeff Layton wrote: > > On Mon, 2024-09-30 at 21:16 +0200, Thomas Gleixner wrote: > > I have the following section in the multigrain-ts.rst file that gets > > added in patch 7 of this series. I'll also plan to add some extra > > wording about how backward realtime clock jumps can affect ordering: > > Please also add comments into the code / interface. > Will do. > > Inode Timestamp Ordering > > ======================== > > > > In addition to providing info about changes to individual files, file > > timestamps also serve an important purpose in applications like "make". These > > programs measure timestamps in order to determine whether source files might be > > newer than cached objects. > > > > Userland applications like make can only determine ordering based on > > operational boundaries. For a syscall those are the syscall entry and exit > > points. For io_uring or nfsd operations, that's the request submission and > > response. In the case of concurrent operations, userland can make no > > determination about the order in which things will occur. > > > > For instance, if a single thread modifies one file, and then another file in > > sequence, the second file must show an equal or later mtime than the first. The > > same is true if two threads are issuing similar operations that do not overlap > > in time. > > > > If however, two threads have racing syscalls that overlap in time, then there > > is no such guarantee, and the second file may appear to have been modified > > before, after or at the same time as the first, regardless of which one was > > submitted first. > > That makes me ask a question. Are the timestamps always taken in thread > (syscall) context or can they be taken in other contexts (worker, > [soft]interrupt, etc.) too? > That's a good question. The main place we do this is inode_set_ctime_current(). That is mostly called in the context of a syscall or similar sort of operation (io_uring, nfsd RPC request, etc.). I certainly wouldn't rule out a workqueue job calling that function, but this is something we do while dirtying an inode, and that's not typically done in interrupt context. -- Jeff Layton <jlayton@xxxxxxxxxx>
