Received-SPF: pass (google.com: domain of linux-wireless+bounces-4095-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Message-ID: <4a3edd404b84da53b56bb0a391cb42d00f42f4c5.camel@sipsolutions.net>
Subject: Re: Kernel deadlock in 6.7.5 + hacks, maybe debugfs related.
From: Johannes Berg <johannes@sipsolutions.net>
To: Ben Greear <greearb@candelatech.com>, linux-wireless
	 <linux-wireless@vger.kernel.org>
Date: Tue, 27 Feb 2024 15:32:39 +0100
In-Reply-To: <33cc22b8-5852-42ae-9e80-8c3c58c36b6d@candelatech.com>
References: <1c9fa9e5-09f1-0522-fdbc-dbcef4d255ca@candelatech.com>
	 <bdc7c51e3b6afaa7ac79efa15c6d1d41e9e918b5.camel@sipsolutions.net>
	 <95d56b9a-4b91-476f-bab3-7af0f28ff469@candelatech.com>
	 <0f8a0451e547f318febcbe36823e1f9914cc684b.camel@sipsolutions.net>
	 <33cc22b8-5852-42ae-9e80-8c3c58c36b6d@candelatech.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.50.4 (3.50.4-1.fc39) 
Precedence: bulk
MIME-Version: 1.0

On Tue, 2024-02-27 at 06:29 -0800, Ben Greear wrote:
> > --- a/fs/debugfs/inode.c
> > +++ b/fs/debugfs/inode.c
> > @@ -751,13 +751,19 @@ static void __debugfs_file_removed(struct dentry =
*dentry)
> >   	if ((unsigned long)fsd & DEBUGFS_FSDATA_IS_REAL_FOPS_BIT)
> >   		return;
> >  =20
> > -	/* if we hit zero, just wait for all to finish */
> > -	if (!refcount_dec_and_test(&fsd->active_users)) {
> > -		wait_for_completion(&fsd->active_users_drained);
> > -		return;
> > -	}
> > +	/*
> > +	 * Now that debugfs_file_get() no longer sees a valid entry,
> > +	 * decrement the refcount to remove the initial reference.
> > +	 */
> > +	refcount_dec(&fsd->active_users);

>=20
> [   94.576688] ------------[ cut here ]------------
> [   94.576699] refcount_t: decrement hit 0; leaking memory.
>=20

Ah ... right, refcount_dec() doesn't like to hit 0, it's not meant for
this path.

I guess we can

if (refcount_dec_and_test(...))
   return;

while (refcount_read(...)) { ... }

johannes