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Subject: Re: [PATCH v2 1/2] Bluetooth: ISO: Fix circular locking dependency
 warning
From: Pauli Virtanen <pav@iki.fi>
To: Iulia Tanasescu <iulia.tanasescu@nxp.com>
Cc: linux-bluetooth@vger.kernel.org
Date: Wed, 06 Mar 2024 18:11:52 +0200
In-Reply-To: <20240305140939.5090-2-iulia.tanasescu@nxp.com>
References: <20240305140939.5090-1-iulia.tanasescu@nxp.com>
	 <20240305140939.5090-2-iulia.tanasescu@nxp.com>
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Hi,

ti, 2024-03-05 kello 16:09 +0200, Iulia Tanasescu kirjoitti:
> This fixes the circular locking dependency warning caused
> by iso_listen_bis acquiring the hdev lock while the socket
> has been locked in the caller function.

I think the commit message and the comment should explain why there is
no deadlock, and that the lockdep warning is a false positive.

E.g. fuzzer calling connect() and listen() at the same time seems it
would be dangerous here?

>=20
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D
> WARNING: possible circular locking dependency detected
> 6.8.0-rc5+ #1 Not tainted
> ------------------------------------------------------
> iso-tester/2950 is trying to acquire lock:
> ffff88817a048080 (&hdev->lock){+.+.}-{3:3}, at: iso_sock_listen+0x305/0x8=
d0
>=20
>                but task is already holding lock:
> ffff888197c39278 (sk_lock-AF_BLUETOOTH-BTPROTO_ISO){+.+.}-{0:0},
>                at: iso_sock_listen+0x91/0x8d0
>=20
>                which lock already depends on the new lock.
>=20
>                the existing dependency chain (in reverse order) is:
>=20
>  -> #1 (sk_lock-AF_BLUETOOTH-BTPROTO_ISO){+.+.}-{0:0}:
>         lock_sock_nested+0x3b/0xb0
>         iso_connect_cis+0x185/0x540
>         iso_sock_connect+0x445/0x7e0
>         __sys_connect_file+0xd5/0x100
>         __sys_connect+0x11e/0x150
>         __x64_sys_connect+0x42/0x60
>         do_syscall_64+0x8d/0x150
>         entry_SYSCALL_64_after_hwframe+0x6e/0x76
>=20
> -> #0 (&hdev->lock){+.+.}-{3:3}:
>         __lock_acquire+0x208f/0x3720
>         lock_acquire+0x16d/0x3f0
>         __mutex_lock+0x155/0x1310
>         mutex_lock_nested+0x1b/0x30
>         iso_sock_listen+0x305/0x8d0
>         __sys_listen+0x106/0x190
>         __x64_sys_listen+0x30/0x40
>         do_syscall_64+0x8d/0x150
>         entry_SYSCALL_64_after_hwframe+0x6e/0x76
>=20
>         other info that might help us debug this:
>=20
> Possible unsafe locking scenario:
>=20
>         CPU0                    CPU1
>         ----                    ----
>    lock(sk_lock-AF_BLUETOOTH-BTPROTO_ISO);
>                                 lock(&hdev->lock);
>                                 lock(sk_lock-AF_BLUETOOTH-BTPROTO_ISO);
>    lock(&hdev->lock);
>=20
>                 *** DEADLOCK ***
>=20
> 1 lock held by iso-tester/2950:
> 0: ffff888197c39278 (sk_lock-AF_BLUETOOTH-BTPROTO_ISO){+.+.}-{0:0},
>                 at: iso_sock_listen+0x91/0x8d0
>=20
> Signed-off-by: Iulia Tanasescu <iulia.tanasescu@nxp.com>
> ---
>  net/bluetooth/iso.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>=20
> diff --git a/net/bluetooth/iso.c b/net/bluetooth/iso.c
> index 8af75d37b14c..5ca7bb0806b0 100644
> --- a/net/bluetooth/iso.c
> +++ b/net/bluetooth/iso.c
> @@ -1095,7 +1095,10 @@ static int iso_listen_bis(struct sock *sk)
>  	if (!hdev)
>  		return -EHOSTUNREACH;
> =20
> -	hci_dev_lock(hdev);
> +	/* To avoid circular locking dependency warnings,
> +	 * use nested lock for hdev.
> +	 */
> +	mutex_lock_nested(&hdev->lock, SINGLE_DEPTH_NESTING);
> =20
>  	/* Fail if user set invalid QoS */
>  	if (iso_pi(sk)->qos_user_set && !check_bcast_qos(&iso_pi(sk)->qos)) {

--=20
Pauli Virtanen