Re: [RFC PATCH 2/6] debugfs: annotate debugfs handlers vs. removal with lockdep

From: Johannes Berg <johannes@sipsolutions.net>
To: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: linux-wireless@vger.kernel.org, linux-kernel@vger.kernel.org,
	 linux-fsdevel@vger.kernel.org,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	 "Rafael J. Wysocki" <rafael@kernel.org>,
	Nicolai Stange <nicstange@gmail.com>,
	Ben Greear <greearb@candelatech.com>,
	Minchan Kim <minchan@kernel.org>
Subject: Re: [RFC PATCH 2/6] debugfs: annotate debugfs handlers vs. removal with lockdep
Date: Sat, 02 Dec 2023 11:40:43 +0100	[thread overview]
Message-ID: <9ffaef32a70d3ba5cd015ec22cf8437cd7c80e79.camel@sipsolutions.net> (raw)
In-Reply-To: <20231202063735.GD404241@google.com>

On Sat, 2023-12-02 at 15:37 +0900, Sergey Senozhatsky wrote:
> On (23/11/09 22:22), Johannes Berg wrote:
> > From: Johannes Berg <johannes.berg@intel.com>
> > 
> > When you take a lock in a debugfs handler but also try
> > to remove the debugfs file under that lock, things can
> > deadlock since the removal has to wait for all users
> > to finish.
> > 
> > Add lockdep annotations in debugfs_file_get()/_put()
> > to catch such issues.
> 
> So this triggers when I reset zram device (zsmalloc compiled with
> CONFIG_ZSMALLOC_STAT).

I shouldn't have put that into the rc, that was more or less an
accident. I think I'll do a revert.

> debugfs_create_file() and debugfs_remove_recursive() are called
> under zram->init_lock, and zsmalloc never calls into zram code.
> What I don't really get is where does the
> 	`debugfs::classes -> zram->init_lock`
> dependency come from?

"debugfs:classes" means a file is being accessed and "classes" is the
name, so that's 

static int zs_stats_size_show(struct seq_file *s, void *v)

which uses seq_file, so there we have a seq_file lock.

I think eventually the issue is that lockdep isn't telling the various
seq_file instances apart, and you have one that's removed under lock
(classes) and another one that's taking the lock (reset).

Anyway, I'll send a revert, don't think this is ready yet. I was fixing
the wireless debugfs lockdep and had used that to debug it.

johannes