Re: linux-next fsnotify mod breaks tail -f

[Hugh Dickins]

On Fri, 11 Dec 2020, Amir Goldstein wrote:
> On Fri, Dec 11, 2020 at 1:45 AM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> >
> > Hi Jan, Amir,
> >
> > There's something wrong with linux-next commit ca7fbf0d29ab
> > ("fsnotify: fix events reported to watching parent and child").
> >
> > If I revert that commit, no problem;
> > but here's a one-line script "tailed":
> >
> > for i in 1 2 3 4 5; do date; sleep 1; done &
> >
> > Then if I run that (same result doing ./tailed after chmod a+x):
> >
> > sh tailed >log; tail -f log
> >
> > the "tail -f log" behaves in one of three ways:
> >
> > 1) On a console, before graphical screen, no problem,
> >    it shows the five lines coming from "date" as you would expect.
> > 2) From xterm or another tty, shows just the first line from date,
> >    but after I wait and Ctrl-C out, "cat log" shows all five lines.
> > 3) From xterm or another tty, doesn't even show that first line.
> >
> > The before/after graphical screen thing seems particularly weird:
> > I expect you'll end up with a simpler explanation for what's
> > causing that difference.
> >
> > tailed and log are on ext4, if that's relevant;
> > ah, I just tried on tmpfs, and saw no problem there.
> 
> Nice riddle Hugh :)
> Thanks for this early testing!
> 
> I was able to reproduce this.

Oh good!

> The outcome does not depend on the type of terminal or filesystem
> it depends on the existence of a watch on the parent dir of the log file.
> Running ' inotifywait -m . &' will stop tail from getting notifications:
> 
> echo > log
> tail -f log &
> sleep 1
> echo "can you see this?" >> log
> inotifywait -m . &
> sleep 1
> echo "how about this?" >> log
> kill $(jobs -p)
> 
> I suppose with a graphical screen you have systemd or other services
> in the system watching the logs/home dir in your test env.

And great answer to the riddle, Amir: yes, I was testing in
my home directory all the time, except when I tried tmpfs.

> 
> Attached fix patch. I suppose Jan will want to sqhash it.

Yes, I confirm that "tail -f" is back to normal for me
with this fix. Thank you both.

Hugh

> 
> We missed a subtle logic change in the switch from inode/child marks
> to parent/inode marks terminology.
> 
> Before the change (!inode_mark && child_mark) meant that name
> was not NULL and should be discarded (which the old code did).
> After the change (!parent_mark && inode_mark) is not enough to
> determine if name should be discarded (it should be discarded only
> for "events on child"), so another check is needed.
> 
> Thanks,
> Amir.