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

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, Sep 30 2024 at 16:53, Jeff Layton wrote:
> On Mon, 2024-09-30 at 22:19 +0200, Thomas Gleixner wrote:
>> On Mon, Sep 30 2024 at 15:37, Jeff Layton wrote:
>> > 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.

The reason I'm asking is that if it's always syscall context,
i.e. write() or io_uring()/RPC request etc., then you can avoid the
whole global floor value dance and make it strictly per thread, which
simplifies the exercise significantly.

But even if it's not syscall/thread context then the worker or io_uring
state machine might just require to serialize against itself and not
coordinate with something else. But what do I know.

Thanks,

        tglx