Re: [PATCH v8 01/11] timekeeping: move multigrain timestamp floor handling into timekeeper

[Thomas Gleixner]

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.

> 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?

Thanks,

        tglx