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: <Z0OmaPQ4HAdgfJyK@casper.infradead.org>
Date: Sun, 24 Nov 2024 22:19:20 +0000
From: Matthew Wilcox <willy@...radead.org>
To: "Dr. David Alan Gilbert" <linux@...blig.org>
Cc: Al Viro <viro@...iv.linux.org.uk>, 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,
	Linus Torvalds <torvalds@...ux-foundation.org>
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 10:10:31PM +0000, Dr. David Alan Gilbert wrote:
> * Al Viro (viro@...iv.linux.org.uk) wrote:
> > [Linus Cc'd]
> > On Sun, Nov 24, 2024 at 06:56:57PM +0100, Mateusz Guzik wrote:
> > 
> > > However, since both sec and nsec are updated separately and there is no
> > > synchro, reading *both* can still result in values from 2 different
> > > updates which is a bug not addressed by any of the above. To my
> > > underestanding of the vfs folk take on it this is considered tolerable.
> > 
> > Well...   You have a timestamp changing.  A reader might get the value
> > before change, the value after change *or* one of those with nanoseconds
> > from another.  It's really hard to see the scenario where that would
> > be a problem - theoretically something might get confused seeing something
> > like
> > 	Jan 14 1995 12:34:49.214 ->
> > 	Jan 14 1995 12:34:49.137 ->
> > 	Nov 23 2024 14:09:17.137
> > but... what would that something be?
> 
> make?
> i.e. if the change was from:
>  a) mmm dd yyyy hh::MM::00:950 ->
>  b) mmm dd yyyy hh::MM::01:950 ->
>  c) mmm dd yyyy hh::MM::01:200 ->
>    
> If you read (b) then you'd think that the file was 750ms newer
> than it really was; which is a long time these days.

... and file fs/inode.c might have a timestamp of :01:717 so inode.o
doesn't get rebuilt when it ought to have been?