linux-kernel - Re: [PATCH v2 4/7] tracefs: dentry lookup crapectomy

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Date: Wed, 31 Jan 2024 22:21:27 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 linux-fsdevel@...r.kernel.org, Linus Torvalds
 <torvalds@...ux-foundation.org>, Masami Hiramatsu <mhiramat@...nel.org>,
 Mark Rutland <mark.rutland@....com>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Christian Brauner <brauner@...nel.org>,
 Ajay Kaher <ajay.kaher@...adcom.com>, Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>, stable@...r.kernel.org
Subject: Re: [PATCH v2 4/7] tracefs: dentry lookup crapectomy

On Thu, 1 Feb 2024 03:02:05 +0000
Al Viro <viro@...iv.linux.org.uk> wrote:

> > We had a problem here with just returning NULL. It leaves the negative
> > dentry around and doesn't get refreshed.  
> 
> Why would that dentry stick around?  And how would anyone find
> it, anyway, when it's not hashed?

We (Linus and I) got it wrong. It originally had:

	d_add(dentry, NULL);
	[..]
	return NULL;

and it caused the:


  # ls events/kprobes/sched/
ls: cannot access 'events/kprobes/sched/': No such file or directory

  # echo 'p:sched schedule' >> /sys/kernel/tracing/kprobe_events 
  # ls events/kprobes/sched/
ls: cannot access 'events/kprobes/sched/': No such file or directory

I just changed the code to simply return NULL, and it had no issues:

  # ls events/kprobes/sched/
ls: cannot access 'events/kprobes/sched/': No such file or directory

  # echo 'p:sched schedule' >> /sys/kernel/tracing/kprobe_events 
  # ls events/kprobes/sched/
enable  filter  format  hist  hist_debug  id  inject  trigger

But then I added the: d_add(dentry, NULL); that we originally had, and then
it caused the issue again.

So it wasn't the returning NULL that was causing a problem, it was calling
the d_add(dentry, NULL); that was.

I'll update the patch.

-- Steve