linux-kernel - Re: [PATCH] rfcomm deadlock fix

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Date:	Sat, 31 May 2008 17:16:28 +0800
From:	"Dave Young" <hidave.darkstar@...il.com>
To:	davem@...emloft.net
Cc:	arjan@...ux.intel.com, marcel@...tmann.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rfcomm deadlock fix

Sorry, I sent a wrong version, I will sent again after a while.

On Sat, May 31, 2008 at 5:09 PM, Dave Young <hidave.darkstar@...il.com> wrote:
> There's logic in __rfcomm_dlc_close:
>        rfcomm_dlc_lock(d);
>        d->state = BT_CLOSED;
>        d->state_changed(d, err);
>        rfcomm_dlc_unlock(d);
>
> In rfcomm_dev_state_change, it's possible that rfcomm_dev_put try to take the
> dlc lock, then we will deadlock.
>
> Here fixed it by unlock dlc before rfcomm_dev_get in rfcomm_dev_state_change.
>
> why not unlock just before rfcomm_dev_put? it's because there's another problem.
> rfcomm_dev_get/rfcomm_dev_del will take rfcomm_dev_lock, but in rfcomm_dev_add
> the lock order is : rfcomm_dev_lock --> dlc lock
>
> so I unlock dlc before the taken of rfcomm_dev_lock.
>
> Actually it's a regression caused by commit
> 1905f6c736cb618e07eca0c96e60e3c024023428, the dlc state_change could be two
> callbacks : rfcomm_sk_state_change and rfcomm_dev_state_change. I missed the rfcomm_sk_state_change that time.
>
> Thanks Arjan van de Ven <arjan@...ux.intel.com> for the effort in commit
> 4c8411f8c115def968820a4df6658ccfd55d7f1a
> but he missed the rfcomm_dev_state_change lock issue.
>
> Signed-off-by: Dave Young <hidave.darkstar@...il.com>
>
> ---
> net/bluetooth/rfcomm/tty.c  |    9 +++++++++
> 1 files changed, 9 insertions(+)
>
> diff -upr linux/net/bluetooth/rfcomm/tty.c linux.new/net/bluetooth/rfcomm/tty.c
> --- linux/net/bluetooth/rfcomm/tty.c    2008-05-30 15:46:33.000000000 +0800
> +++ linux.new/net/bluetooth/rfcomm/tty.c        2008-05-30 16:02:38.000000000 +0800
> @@ -566,11 +566,20 @@ static void rfcomm_dev_state_change(stru
>        if (dlc->state == BT_CLOSED) {
>                if (!dev->tty) {
>                        if (test_bit(RFCOMM_RELEASE_ONHUP, &dev->flags)) {
> +                               /* Drop DLC lock here to avoid deadlock
> +                                * 1. rfcomm_dev_get will take rfcomm_dev_lock
> +                                *    but in rfcomm_dev_add there's lock order:
> +                                *    rfcomm_dev_lock -> dlc lock
> +                                * 2. rfcomm_dev_put will deaklock if it's
> +                                *    the last reference
> +                                */
> +                               rfcomm_dlc_unlock(dlc);
>                                if (rfcomm_dev_get(dev->id) == NULL)
>                                        return;
>
>                                rfcomm_dev_del(dev);
>                                rfcomm_dev_put(dev);
> +                               rfcomm_dlc_lock(dlc);
>                        }
>                } else
>                        tty_hangup(dev->tty);
>



-- 
---
Regards
dave
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