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

[Dr. David Alan Gilbert]

* Al Viro (viro@xxxxxxxxxxxxxxxxxx) 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.

Dave

> We could add a seqcount, but stat(2) and friends already cost more than
> they should, IMO...
> 
> Linus, do you see any good reasons to bother with that kind of stuff?
> It's not the first time such metadata update vs. read atomicity comes
> up, maybe we ought to settle that for good and document the decision
> and reasons for it.
> 
> This time it's about timestamp (seconds vs. nanoseconds), but there'd
> been mode vs. uid vs. gid mentioned in earlier threads.
> 
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
\        dave @ treblig.org |                               | In Hex /
 \ _________________________|_____ http://www.treblig.org   |_______/