linux-kernel - Re: linux-next fsnotify mod breaks tail -f

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Date:   Fri, 11 Dec 2020 10:42:16 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Jan Kara <jack@...e.cz>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: linux-next fsnotify mod breaks tail -f

On Fri, Dec 11, 2020 at 1:45 AM Hugh Dickins <hughd@...gle.com> 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.
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.

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

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.

View attachment "fsnotify-fix-for-fix-events-reported-to-watching-parent-and-child.patch.txt" of type "text/plain" (1276 bytes)