On Sat, Jun 1, 2013 at 6:19 AM, Bob Copeland <me@xxxxxxxxxxxxxxx> wrote:
> As of "cfg80211/mac80211: use cfg80211 wdev mutex in mac80211",
> mac80211 expects to be able to take the wdev mutex around sdata
> accesses. This causes a recursive deadlock since
> __cfg80211_leave_mesh() already holds the wdev mutex. Removing
> the sdata_lock() calls in ieee80211_stop_mesh() alone won't fix
> this, as the cancel_work_sync() in mesh runs the iface work,
> and various work items also want to take the wdev lock (not
> just in mesh, see e.g. ieee80211_sta_rx_queued_mgmt().)
>
> We don't seem to need the wdev lock held over rdev_leave_mesh()
> anyway, so drop it before calling.
>
> Fixes:
> [ 75.571222] =============================================
> [ 75.571222] [ INFO: possible recursive locking detected ]
> [ 75.571222] 3.10.0-rc1-a8cd57b+ #113 Not tainted
> [ 75.571222] ---------------------------------------------
> [ 75.571222] iw/2597 is trying to acquire lock:
> [ 75.571222] (&wdev->mtx){+.+.+.}, at: [<ffffffffa0115180>] ieee80211_stop_mesh+0x60/0x160 [mac80211]
> [ 75.571222]
> [ 75.571222] but task is already holding lock:
> [ 75.571222] (&wdev->mtx){+.+.+.}, at: [<ffffffffa0035fa5>] cfg80211_leave_mesh+0x35/0x370 [cfg80211]
> [ 75.571222]
> [ 75.571222] other info that might help us debug this:
> [ 75.571222] Possible unsafe locking scenario:
> [ 75.571222]
> [ 75.571222] CPU0
> [ 75.571222] ----
> [ 75.571222] lock(&wdev->mtx);
> [ 75.571222] lock(&wdev->mtx);
> [ 75.571222]
> [ 75.571222] *** DEADLOCK ***
> [ 75.571222]
> [ 75.571222] May be due to missing lock nesting notation
> [ 75.571222]
> [ 75.571222] 4 locks held by iw/2597:
> [ 75.571222] #0: (cb_lock){++++++}, at: [<ffffffff8165a4cd>] genl_rcv+0x1d/0x40
> [ 75.571222] #1: (genl_mutex){+.+.+.}, at: [<ffffffff8165a997>] genl_lock+0x17/0x20
> [ 75.571222] #2: (rtnl_mutex){+.+.+.}, at: [<ffffffff81638e47>] rtnl_lock+0x17/0x20
> [ 75.571222] #3: (&wdev->mtx){+.+.+.}, at: [<ffffffffa0035fa5>] cfg80211_leave_mesh+0x35/0x370 [cfg80211]
> [ 75.571222]
> [ 75.571222] stack backtrace:
> [ 75.571222] CPU: 1 PID: 2597 Comm: iw Not tainted 3.10.0-rc1-a8cd57b+ #113
> [ 75.571222] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
> [ 75.571222] ffffffff82155790 ffff8800035c5768 ffffffff817b8335 ffff8800035c5868
> [ 75.571222] ffffffff810a3da2 0000000000000000 0000000000000000 0000000000000002
> [ 75.571222] 0000000000000046 00000000035c5818 ffffffff82155790 ffff8800057e2360
> [ 75.571222] Call Trace:
> [ 75.571222] [<ffffffff817b8335>] dump_stack+0x19/0x1b
> [ 75.571222] [<ffffffff810a3da2>] __lock_acquire+0xba2/0x1d30
> [ 75.571222] [<ffffffff817c0bed>] ? _raw_spin_unlock_irqrestore+0x4d/0x70
> [ 75.571222] [<ffffffff810a615d>] ? trace_hardirqs_on_caller+0x16d/0x200
> [ 75.571222] [<ffffffff810a568a>] lock_acquire+0x17a/0x200
> [ 75.571222] [<ffffffffa0115180>] ? ieee80211_stop_mesh+0x60/0x160 [mac80211]
> [ 75.571222] [<ffffffff817bd4fe>] mutex_lock_nested+0x6e/0x380
> [ 75.571222] [<ffffffffa0115180>] ? ieee80211_stop_mesh+0x60/0x160 [mac80211]
> [ 75.571222] [<ffffffffa00b26b2>] ? ieee80211_bss_info_change_notify+0x202/0x3d0 [mac80211]
> [ 75.571222] [<ffffffffa0115180>] ieee80211_stop_mesh+0x60/0x160 [mac80211]
> [ 75.571222] [<ffffffffa00d195d>] ieee80211_leave_mesh+0x1d/0x30 [mac80211]
> [ 75.571222] [<ffffffffa00360f8>] cfg80211_leave_mesh+0x188/0x370 [cfg80211]
> [ 75.571222] [<ffffffffa000dfb9>] nl80211_leave_mesh+0x19/0x20 [cfg80211]
> [ 75.571222] [<ffffffff8165a756>] genl_family_rcv_msg+0x266/0x2e0
> [ 75.571222] [<ffffffff8165a9a0>] ? genl_lock+0x20/0x20
> [ 75.571222] [<ffffffff8165aa2e>] genl_rcv_msg+0x8e/0xc0
> [ 75.571222] [<ffffffff8165a1ce>] netlink_rcv_skb+0x6e/0xd0
> [ 75.571222] [<ffffffff8165a4cd>] ? genl_rcv+0x1d/0x40
> [ 75.571222] [<ffffffff8165a4dc>] genl_rcv+0x2c/0x40
> [ 75.571222] [<ffffffff81659b6a>] netlink_unicast+0x11a/0x1f0
> [ 75.571222] [<ffffffff812a11b5>] ? sock_has_perm+0x5/0x1f0
> [ 75.571222] [<ffffffff81659f4d>] netlink_sendmsg+0x30d/0x360
> [ 75.571222] [<ffffffff8160f196>] sock_sendmsg+0xa6/0xd0
> [ 75.571222] [<ffffffff810a4fde>] ? lock_release_non_nested+0xae/0x2e0
> [ 75.571222] [<ffffffff8160f529>] __sys_sendmsg+0x369/0x390
> [ 75.571222] [<ffffffff81071723>] ? up_read+0x23/0x40
> [ 75.571222] [<ffffffff817c4e64>] ? __do_page_fault+0x4b4/0x570
> [ 75.571222] [<ffffffff8117d27d>] ? dput+0x13d/0x1d0
> [ 75.571222] [<ffffffff81184c6c>] ? fget_light+0x12c/0x430
> [ 75.571222] [<ffffffff8161084d>] SyS_sendmsg+0x4d/0x80
> [ 75.571222] [<ffffffff817c9e02>] system_call_fastpath+0x16/0x1b
>
> Signed-off-by: Bob Copeland <me@xxxxxxxxxxxxxxx>
>
> ---
> I don't like dropping locks, but I can't see any races added by
> this -- join/leave are already serialized by rtnl and I didn't see
> leave_mesh using the wdev fields in a way that would matter -- but
> let me know if I missed something. We could also split out the
> cancel_work_sync into a separate (unlocked) op but this approach
> seems simpler. Thoughts?
>
> net/wireless/mesh.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/net/wireless/mesh.c b/net/wireless/mesh.c
> index 5dfb289..6344a81 100644
> --- a/net/wireless/mesh.c
> +++ b/net/wireless/mesh.c
> @@ -250,7 +250,9 @@ static int __cfg80211_leave_mesh(struct cfg80211_registered_device *rdev,
> if (!wdev->mesh_id_len)
> return -ENOTCONN;
>
> + wdev_unlock(wdev);
> err = rdev_leave_mesh(rdev, dev);
> + wdev_lock(wdev);
Since the sdata_lock() will be taken later, I don't think there should
be any races here. Other calls to mbss_info_change_notify() either
take the lock from the mac80211 workqueue or not at all (since
wdev->mtx is already held). But it does look weird for cfg80211 to
drop a lock on behalf of the driver. I'm also a little worried this
special case will make it harder to predict future deadlocks in the
mesh code. Maybe we should punt the beacon update to the workqueue? :)
--
Thomas
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