Messages in this thread | | | Date | Sun, 03 Sep 2006 14:26:50 -0500 | From | Larry Finger <> | Subject | Re: 2.6.18-rc5 with GRE, iptables and Speedtouch ADSL, PPP over ATM |
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Herbert Xu wrote: > [NET]: Drop tx lock in dev_watchdog_up > > On Sat, Sep 02, 2006 at 08:39:28PM +0000, Krzysztof Halasa wrote: >> ======================================================= >> [ INFO: possible circular locking dependency detected ] >> ------------------------------------------------------- >> swapper/0 is trying to acquire lock: >> (&dev->queue_lock){-+..}, at: [<c02c8c46>] dev_queue_xmit+0x56/0x290 >> >> but task is already holding lock: >> (&dev->_xmit_lock){-+..}, at: [<c02c8e14>] dev_queue_xmit+0x224/0x290 >> >> which lock already depends on the new lock. > > This turns out to be a genuine bug. The queue lock and xmit lock are > intentionally taken out of order. Two things are supposed to prevent > dead-locks from occuring: > > 1) When we hold the queue_lock we're supposed to only do try_lock on the > tx_lock. > > 2) We always drop the queue_lock after taking the tx_lock and before doing > anything else. > >> the existing dependency chain (in reverse order) is: >> >> -> #1 (&dev->_xmit_lock){-+..}: >> [<c012e7b6>] lock_acquire+0x76/0xa0 >> [<c0336241>] _spin_lock_bh+0x31/0x40 >> [<c02d25a9>] dev_activate+0x69/0x120 > > This path obviously breaks assumption 1) and therefore can lead to ABBA > dead-locks. > > I've looked at the history and there seems to be no reason for the lock > to be held at all in dev_watchdog_up. The lock appeared in day one and > even there it was unnecessary. In fact, people added __dev_watchdog_up > precisely in order to get around the tx lock there. > > The function dev_watchdog_up is already serialised by rtnl_lock since > its only caller dev_activate is always called under it. > > So here is a simple patch to remove the tx lock from dev_watchdog_up. > In 2.6.19 we can eliminate the unnecessary __dev_watchdog_up and > replace it with dev_watchdog_up. > > Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This patch also fixes a circular locking problem that I found on Linville's wireless-dev tree involving ieee80211 and wpa_supplicant.
Larry Finger
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