Re: [RFC] metadata updates vs. fetches (was Re: [PATCH v4] fs: Fix data race in inode_set_ctime_to_ts)

[Al Viro]

On Sun, Nov 24, 2024 at 02:10:30PM -0800, Linus Torvalds wrote:

> I *do* think that we could perhaps extend (and rename) the
> inode->i_size_seqcount to just cover all of the core inode metadata
> stuff.

That would bring ->i_size_seqcount to 64bit architectures and
potentially extend the time it's being held quite a bit even
on 32bit...

> And then - exactly like we already do in practice with
> inode->i_size_seqcount - some places might just choose to ignore it
> anyway.
> 
> But at least using a sequence count shouldn't make things like stat()
> any worse in practice.
>
> That said, I don't think this is a real problem in practice. The race
> window is too small, and the race effects are too insignificant.
> 
> Yes, getting the nanoseconds out of sync with the seconds is a bug,
> but when it effectively never happens, and when it *does* happen it
> likely has no real downsides, I suspect it's also not something we
> should worry over-much about.
> 
> So I mention the "rename and extend i_size_seqcount" as a solution
> that I suspect might be acceptable if somebody has the motivation and
> energy, but honestly I also think "nobody can be bothered" is
> acceptable in practice.

*nod*