linux-kernel - Re: [RFC] metadata updates vs. fetches (was Re: [PATCH v4] fs: Fix data race in inode_set_ctime_to

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Message-ID: <20241124222450.GB3387508@ZenIV>
Date: Sun, 24 Nov 2024 22:24:50 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mateusz Guzik <mjguzik@...il.com>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Hao-ran Zheng <zhenghaoran@...a.edu.cn>, brauner@...nel.org,
	jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, baijiaju1990@...il.com,
	21371365@...a.edu.cn
Subject: Re: [RFC] metadata updates vs. fetches (was Re: [PATCH v4] fs: Fix
 data race in inode_set_ctime_to_ts)

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*