Hi Marcel, Gustavo and Johan,
My original E-mail is below for completeness so that you understand
the context of this new E-mail.
We have been successful in demonstrating a L2CAP ERTM shutdown
deadlock by using the Bluez l2test utility. We think the latest 4.x
kernel can also deadlock (not tested).
Details of the simple test case are as follows:
Apparatus
======
Raspberry Pi (RPi) original B variant (single ARM core) running Linux
Raspian with kernel 3.18.11+ and a USB BT v4.0 dongle. bluez is v5.15
(locally built)
Dell laptop running Linux Mageia 4 with kernel 3.14.32 and an on-board
USB BT dongle. bluez is v5.11
Method
=====
INITIAL SETTINGS:
Ensure bluetoothd is running on the laptop and RPi so the D-Bus
services are available for the Bluez utilities to use
Run bluetoothctl on the laptop and RPi and ensure that an adaptor is
shown
Type in the following commands on both systems:
agent on
default-agent
power on
discoverable on
scan on
Both systems should now detect each other
Make a note of each device's BT address. Call them <lap_BT_ADDR> and
<RPi_BT_ADDR>
On the RPi pair with the laptop (visa versa will also work):
On the RPi do command
pair <lap_BT_ADDR>
Accept the pin code
On the laptop do
Accept the pin code
Pairing should now be successful. bluetoothd must continue to run so
that the linkkeys can be serviced by the agent.
bluetoothctl can be quit on both systems
USING L2TEST
l2test can be used in various modes of operation controlled via
command line options. l2test can be used in a client-server
arrangement whereby 1 BT device listens for a connection (server) and
the other BT device connects (client) to the listening device.
For this testcase:
RPi is the l2test client (unit under test)
laptop is the l2test server
On the laptop use the command:
l2test -i hci0 -X ertm -d
This command listens for connections with optional L2CAP ERTM
supported and dumps out the received data.
TEST 1: NON-ERTM CONNECTION
On the RPi use the command:
l2test -s -b 1024 -B /dev/urandom -P 4113 <lap_BT_ADDR>
This command sends 1024 bytes to the laptop via PSM 4113 used by
l2test over a L2CAP basic mode channel.
TEST 2: ERTM CONNECTION
On the RPi use the command:
l2test -s -b 1024 -B /dev/urandom -X ertm -P 4113 <lap_BT_ADDR>
This command sends 1024 bytes to the laptop via PSM 4113 used by
l2test over an ERTM channel
Results
=====
TEST 1
root@raspberrypi:~# l2test -s -b 1024 -B /dev/urandom -P 4113
5C:F3:70:68:BA:DD
l2test[10346]: Connected to 5C:F3:70:68:BA:DD (bredr, psm 4113, scid 64)
l2test[10346]: Local device 5C:F3:70:68:BA:DA (bredr, psm 4113, scid 64)
l2test[10346]: Options [imtu 672, omtu 672, flush_to 65535, mode 0,
handle 11, class 0x000000, priority 0, rcvbuf 163840]
l2test[10346]: Sending ...
l2test[10346]: Closing channel ...
l2test[10346]: Done
l2test[10346]: Exit
Using basic L2CAP mode, the channel sends and closes OK.
l2test terminates as expected.
TEST 2
root@raspberrypi:~# l2test -s -b 1024 -B /dev/urandom -X ertm -P 4113
5C:F3:70:68:BA:DD
l2test[10366]: Connected to 5C:F3:70:68:BA:DD (bredr, psm 4113, scid 64)
l2test[10366]: Local device 5C:F3:70:68:BA:DA (bredr, psm 4113, scid 64)
l2test[10366]: Options [imtu 672, omtu 672, flush_to 65535, mode 3,
handle 11, class 0x000000, priority 0, rcvbuf 163840]
l2test[10366]: Sending ...
l2test[10366]: Closing channel ...
Using L2CAP ERTM, the channel is instructed to close but hangs.
l2test fails to naturally terminate.
From Bluez 5.15 tools/l2test.c:
static void send_mode(int sk)
{
do_send(sk);
syslog(LOG_INFO, "Closing channel ...");
if (shutdown(sk, SHUT_RDWR) < 0)
syslog(LOG_INFO, "Close failed: %m");
else
syslog(LOG_INFO, "Done");
}
The -s send l2test command line option calls send_mode() to send the
data via do_send() and then closes the channel with the call to
shutdown().
do_send() is probably not checking that the data was successfully sent
before returning meaning potentially not all the I frames were ACK'ed.
The results shows that shutdown() fails to return as no "Done" textual
message is seen.
When the RPi is left in this condition for up to 10 minutes (need to
wait longer than 2 times the hung task detector period plus L2CAP
timer expiry time), dmesg shows:
[ 601.116119] INFO: task kworker/0:1:19 blocked for more than 120
seconds.
[ 601.116156] Not tainted 3.18.11+ #781
[ 601.116169] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[ 601.116183] kworker/0:1 D c054cbcc 0 19 2 0x00000000
[ 601.116537] Workqueue: events l2cap_retrans_timeout [bluetooth]
[ 601.116685] [<c054cbcc>] (__schedule) from [<c054cfa4>]
(schedule+0x40/0x8c)
[ 601.116727] [<c054cfa4>] (schedule) from [<c054d3c8>]
(schedule_preempt_disabled+0x30/0x40)
[ 601.116812] [<c054d3c8>] (schedule_preempt_disabled) from
[<c054ee14>] (__mutex_lock_slowpath+0xb8/0x18c)
[ 601.116856] [<c054ee14>] (__mutex_lock_slowpath) from [<c054ef20>]
(mutex_lock+0x38/0x3c)
[ 601.117085] [<c054ef20>] (mutex_lock) from [<bf05173c>]
(l2cap_retrans_timeout+0x24/0x5c [bluetooth])
[ 601.117371] [<bf05173c>] (l2cap_retrans_timeout [bluetooth]) from
[<c003aa68>] (process_one_work+0x13c/0x444)
[ 601.117410] [<c003aa68>] (process_one_work) from [<c003b954>]
(worker_thread+0x190/0x580)
[ 601.117445] [<c003b954>] (worker_thread) from [<c003fdb4>]
(kthread+0xd8/0xf4)
[ 601.117488] [<c003fdb4>] (kthread) from [<c000e8c8>]
(ret_from_fork+0x14/0x20)
[ 601.117569] INFO: task kworker/u3:2:327 blocked for more than 120
seconds.
[ 601.117588] Not tainted 3.18.11+ #781
[ 601.117601] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[ 601.117614] kworker/u3:2 D c054cbcc 0 327 2 0x00000000
[ 601.117806] Workqueue: hci0 hci_rx_work [bluetooth]
[ 601.117941] [<c054cbcc>] (__schedule) from [<c054cfa4>]
(schedule+0x40/0x8c)
[ 601.117981] [<c054cfa4>] (schedule) from [<c054d3c8>]
(schedule_preempt_disabled+0x30/0x40)
[ 601.118056] [<c054d3c8>] (schedule_preempt_disabled) from
[<c054ee14>] (__mutex_lock_slowpath+0xb8/0x18c)
[ 601.118092] [<c054ee14>] (__mutex_lock_slowpath) from [<c054ef20>]
(mutex_lock+0x38/0x3c)
[ 601.118307] [<c054ef20>] (mutex_lock) from [<bf04d2c8>]
(l2cap_get_chan_by_scid+0x28/0x98 [bluetooth])
[ 601.118696] [<bf04d2c8>] (l2cap_get_chan_by_scid [bluetooth]) from
[<bf054590>] (l2cap_recv_frame+0xa4/0x2cec [bluetooth])
[ 601.119146] [<bf054590>] (l2cap_recv_frame [bluetooth]) from
[<bf058194>] (l2cap_recv_acldata+0x214/0x22c [bluetooth])
[ 601.119559] [<bf058194>] (l2cap_recv_acldata [bluetooth]) from
[<bf02bf88>] (hci_rx_work+0x180/0x288 [bluetooth])
[ 601.119801] [<bf02bf88>] (hci_rx_work [bluetooth]) from
[<c003aa68>] (process_one_work+0x13c/0x444)
[ 601.119839] [<c003aa68>] (process_one_work) from [<c003b954>]
(worker_thread+0x190/0x580)
[ 601.119883] [<c003b954>] (worker_thread) from [<c003fdb4>]
(kthread+0xd8/0xf4)
[ 601.119919] [<c003fdb4>] (kthread) from [<c000e8c8>]
(ret_from_fork+0x14/0x20)
Conclusions
========
The hung task failure is easy to reproduce with the l2test utility. It
could be argued that the do_send() function should only return when
all the data has been successfully acknowledged but on the other hand,
it is ideal to generate the right conditions for the failure to be
triggered.
Probably, the low resources on the RPi help to make the failure more
likely than using a PC for example.
hcidump shows that the S RR frame from the laptop comes 320ms after
the last I frame from the RPi which generates a window of opportunity
for the shutdown() call to execute l2cap_sock_shutdown() before the S
RR frame has been processed.
It can be seen that the root cause function __l2cap_wait_ack() is not
reported by the hung task detector because the function is running in
an infinite loop but periodically waiting (sleeping) for 200ms so is
not detectable by the hung task detector. The hung task detector is
triggered when the L2CAP recovery timers expire
(l2cap_retrans_timeout) and gets blocked, or when a new frame is
received (l2cap_get_chan_by_scid) and processing gets blocked.
We looked at a range of kernels from 3.8 to 4.0 which we think can
suffer the deadlock.
We managed to get a deadlock on Fedora using kernel 4.0.4 but it was a
different test case with an iPhone.
Please can we work together to find a solution to the deadlock ?
Note that our interest is in fixing kernels 3.8 and 3.14 so hopefully
any fix to the latest 4.x kernel can be back-ported to the kernels
that we are using.
Many thanks for your time.
Regards,
Dean
On 07/05/15 16:26, Dean Jenkins wrote:
Hi Marcel, Gustavo and Johan,
Please see the end of the E-mail for a workaround, do you agree with
the workaround, any comments ?
Description of L2CAP ERTM deadlock
======================
Using kernel 3.8 on an ARM i.MX6 based platform, we have identified
some deadlock situations involving L2CAP ERTM. We believe kernel
v4.0.1 can deadlock as well (not tested it) as the affected code is
similar to kernel 3.8.
With a L2CAP ERTM channel connected, and l2cap_sock_shutdown() is
called, then __l2cap_wait_ack() is called which is supposed to wait
for all sent I frames to be acknowledged.
l2cap_sock_shutdown() takes all 3 locks namely:
mutex_lock(&conn->chan_lock);
l2cap_chan_lock(chan);
lock_sock(sk);
__l2cap_wait_ack() is implemented as follows:
int __l2cap_wait_ack(struct sock *sk)
{
struct l2cap_chan *chan = l2cap_pi(sk)->chan;
DECLARE_WAITQUEUE(wait, current);
int err = 0;
int timeo = HZ/5;
add_wait_queue(sk_sleep(sk), &wait);
set_current_state(TASK_INTERRUPTIBLE);
while (chan->unacked_frames > 0 && chan->conn) {
if (!timeo)
timeo = HZ/5;
if (signal_pending(current)) {
err = sock_intr_errno(timeo);
break;
}
release_sock(sk);
timeo = schedule_timeout(timeo);
lock_sock(sk);
set_current_state(TASK_INTERRUPTIBLE);
err = sock_error(sk);
if (err)
break;
}
set_current_state(TASK_RUNNING);
remove_wait_queue(sk_sleep(sk), &wait);
return err;
}
Analysis shows that the while loop checks chan->unacked_frames > 0
and chan->conn is !NULL. So when both are true the loop waits
(sleeps) for 200ms using schedule_timeout() and then rechecks the
while loop conditions before waiting again.
Unfortunately, during this waiting (sleeping) period the following
locks are still held:
mutex_lock(&conn->chan_lock);
l2cap_chan_lock(chan);
These held locks prevent l2cap_recv_frame() from processing the
received ACKs for the sent I frames in ERTM. This means
chan->unacked_frames > 0 remains true.
chan->conn cannot be set to NULL by the channel being closed by other
means as the held locks prevent the state being updated.
Therefore a deadlock occurs because __l2cap_wait_ack() gets stuck in
an infinite loop which periodically waits (sleeps). The kernel's hung
task detector is not able to detect this "hung" thread because the
thread periodically runs. This means l2cap_sock_shutdown() does not
return. With debug, we have proven that the infinite loop occurs.
Here is an example hung task log and LOCKDEP output of a L2CAP ERTM
deadlock scenario: (we do have some other different traces as well so
more than 1 failure scenario exists)
[ 360.497912] INFO: task kworker/u:0H:7 blocked for more than 180
seconds.
[ 360.497931] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[ 360.497944] kworker/u:0H D 8046bb28 0 7 2 0x00000000
[ 360.498051] Backtrace:
[ 360.498145] [<8046b624>] (__schedule+0x0/0x790) from [<8046be3c>]
(schedule+0x88/0x8c)
[ 360.498231] [<8046bdb4>] (schedule+0x0/0x8c) from [<8046c1c8>]
(schedule_preempt_disabled+0x18/0x24)
[ 360.498318] [<8046c1b0>] (schedule_preempt_disabled+0x0/0x24) from
[<80469b70>] (mutex_lock_nested+0x250/0x3e8)
[ 360.498495] [<80469920>] (mutex_lock_nested+0x0/0x3e8) from
[<7f44d2a8>] (l2cap_get_chan_by_scid+0x24/0x54 [bluetooth])
[ 360.498696] [<7f44d284>] (l2cap_get_chan_by_scid+0x0/0x54
[bluetooth]) from [<7f453f68>] (l2cap_recv_frame+0x1918/0x1cc8
[bluetooth])
[ 360.498779] r6:0000006b r5:9773ef00 r4:96bfdc00 r3:0000003f
[ 360.498919] [<7f452650>] (l2cap_recv_frame+0x0/0x1cc8 [bluetooth])
from [<7f454e3c>] (l2cap_recv_acldata+0xc0/0x1f8 [bluetooth])
[ 360.499088] [<7f454d7c>] (l2cap_recv_acldata+0x0/0x1f8
[bluetooth]) from [<7f437c74>] (hci_rx_work+0x1b4/0x2ec [bluetooth])
[ 360.499176] r6:9759a000 r5:9759a77c r4:9773ef00 r3:00000000
[ 360.499259] [<7f437ac0>] (hci_rx_work+0x0/0x2ec [bluetooth]) from
[<8003ee1c>] (process_one_work+0x344/0x540)
[ 360.499352] [<8003ead8>] (process_one_work+0x0/0x540) from
[<8003f2e4>] (worker_thread+0x298/0x3ec)
[ 360.499449] [<8003f04c>] (worker_thread+0x0/0x3ec) from
[<80044a40>] (kthread+0xb4/0xc0)
[ 360.499543] [<8004498c>] (kthread+0x0/0xc0) from [<8000da30>]
(ret_from_fork+0x14/0x24)
[ 360.499622] r7:00000000 r6:00000000 r5:8004498c r4:a40a3e34
[ 360.499880]
Showing all locks held in the system:
[ 360.499909] 3 locks held by kworker/u:0H/7:
[ 360.499921] #0: (hdev->name){++++..}, at: [<8003ed28>]
process_one_work+0x250/0x540
[ 360.499968] #1: ((&hdev->rx_work)){+.+...}, at: [<8003ed28>]
process_one_work+0x250/0x540
[ 360.500009] #2: (&conn->chan_lock){+.+...}, at: [<7f44d2a8>]
l2cap_get_chan_by_scid+0x24/0x54 [bluetooth]
<snip>
[ 360.500256] 3 locks held by kworker/1:2/64:
[ 360.500266] #0: (events){.+.+..}, at: [<8003ed28>]
process_one_work+0x250/0x540
[ 360.500307] #1: ((&(&chan->retrans_timer)->work)){+.+...}, at:
[<8003ed28>] process_one_work+0x250/0x540
[ 360.500346] #2: (&chan->lock){+.+...}, at: [<7f450414>]
l2cap_retrans_timeout+0x24/0x58 [bluetooth]
<snip>
[ 360.502066] 2 locks held by <userland BT daemon>/1433:
[ 360.502076] #0: (&conn->chan_lock){+.+...}, at: [<7f455378>]
l2cap_sock_shutdown+0x34/0x130 [bluetooth]
[ 360.502179] #1: (&chan->lock){+.+...}, at: [<7f455388>]
l2cap_sock_shutdown+0x44/0x130 [bluetooth]
<snip>
It can be seen that l2cap_get_chan_by_scid() is blocked so that the
ACK cannot be processed and l2cap_retrans_timeout() has timed out in
an attempt to retransmit the I frame but is also blocked. The
Bluetooth stack is now paralysed.
This scenario will fail every time. The locking seems to be flawed in
l2cap_sock_shutdown() for L2CAP ERTM. Did it ever work ?
Workaround
========
Here is our workaround patch (based on kernel 3.8) which releases all
the locks before waiting (sleeping):
diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index a53f47a..bf49066 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -1766,7 +1766,13 @@ int __l2cap_wait_ack(struct sock *sk)
}
release_sock(sk);
+ l2cap_chan_unlock(chan);
+ if (chan->conn)
+ mutex_unlock(&chan->conn->chan_lock);
timeo = schedule_timeout(timeo);
+ if (chan->conn)
+ mutex_lock(&chan->conn->chan_lock);
+ l2cap_chan_lock(chan);
lock_sock(sk);
set_current_state(TASK_INTERRUPTIBLE);
diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index 657b3a4..bd0d1ae 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -867,8 +867,10 @@ static int l2cap_sock_shutdown(struct socket
*sock, int how)
lock_sock(sk);
if (!sk->sk_shutdown) {
- if (chan->mode == L2CAP_MODE_ERTM)
+ if (chan->mode == L2CAP_MODE_ERTM) {
err = __l2cap_wait_ack(sk);
+ conn = chan->conn;
+ }
sk->sk_shutdown = SHUTDOWN_MASK;
If you think this is the right approach then we will formally create
a patch and send it to the mailing list for review.
Our initial tests show that the deadlock no longer occurs with the
patch applied, but we are worried about side effects.
We have some doubts:
a) What are the locks protecting ? Do all 3 locks need to be taken ?
b) Can a better architectural solution be created that avoids the
locks ?
Perhaps arm the channel to disconnect, then after all ACKs have been
received, call a callback to wake up l2cap_sock_shutdown ?
c) What stops new I frames being sent during the waiting period so
chan->unacked_frames increases ?
Thanks for your time.
Regards,
Dean
