On Mon, 2024-09-30 at 21:16 +0200, Thomas Gleixner wrote:
> On Mon, Sep 16 2024 at 06:57, Jeff Layton wrote:
> > On Mon, 2024-09-16 at 12:32 +0200, Thomas Gleixner wrote:
> > > > 'Something has changed' is a truly understandable technical
> > > > explanation.
> > >
> > > old = mg_floor
> > > mono = T1;
> > > mg_floor = mono
> > > preemption
> > >
> > > do {
> > > mono = T2;
> > > }
> > >
> > > cmpxchg fails and the function returns a value based on T1
> > >
> > > No?
> > >
> > >
> >
> > Packing for LPC, so I can't respond to all of these just now, but I
> > will later. You're correct, but either outcome is OK.
> >
> > The requirement is that we don't hand out any values that were below
> > the floor at the time that the task entered the kernel. Since the time
> > changed while the task was already inside the kernel, either T1 or T2
> > would be valid timestamps.
>
> That really needs to be documented. A similar scenario exists
> vs. ktime_get_coarse_real_ts64_mg().
>
Yes.
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:
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.
--
Jeff Layton <[email protected]>
