RCU stall in af_unix.c, should use spin_lock_irqsave?

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Hello,

I stumbled across a reproducible RCU stall related to the AF_UNIX code
(on 3.17, on an ARM SMP system), and I'm not sure whether the problem
is caused by:

 * The af_unix.c code using spin_lock() on sk->sk_receive_queue.lock
   while it should be using spin_lock_irqsave().

OR

 * The mvpp2 Ethernet driver using on_each_cpu() in a softirq context.

At least, switching to use spin_lock_irqsave() instead of spin_lock()
has proven to fix the issue (see patch below). The spinlock validator
complains that a lockup has been detected, which might indicate that
something is indeed wrong with the spinlock handling.

Now, to the gory details. When stress-testing a lighttpd web server
that does a lot of CGI calls and therefore a lot of Unix socket
traffic, I get typically after couple of minutes the following RCU
stall:

INFO: rcu_sched self-detected stall on CPU { 0}  (t=2100 jiffies g=14665 c=14664 q=1352)
Task dump for CPU 0:
lighttpd        R running      0  1549      1 0x00000002
[<c0014f94>] (unwind_backtrace) from [<c001130c>] (show_stack+0x10/0x14)
[<c001130c>] (show_stack) from [<c0059688>] (rcu_dump_cpu_stacks+0x98/0xd4)
[<c0059688>] (rcu_dump_cpu_stacks) from [<c005c3ec>] (rcu_check_callbacks+0x424/0x740)
[<c005c3ec>] (rcu_check_callbacks) from [<c005e7c8>] (update_process_times+0x40/0x60)
[<c005e7c8>] (update_process_times) from [<c006ce70>] (tick_sched_timer+0x70/0x210)
[<c006ce70>] (tick_sched_timer) from [<c005efc4>] (__run_hrtimer.isra.35+0x6c/0x128)
[<c005efc4>] (__run_hrtimer.isra.35) from [<c005f600>] (hrtimer_interrupt+0x11c/0x2d0)
[<c005f600>] (hrtimer_interrupt) from [<c00148f8>] (twd_handler+0x34/0x44)
[<c00148f8>] (twd_handler) from [<c00557ec>] (handle_percpu_devid_irq+0x6c/0x84)
[<c00557ec>] (handle_percpu_devid_irq) from [<c0051c80>] (generic_handle_irq+0x2c/0x3c)
[<c0051c80>] (generic_handle_irq) from [<c000eafc>] (handle_IRQ+0x40/0x90)
[<c000eafc>] (handle_IRQ) from [<c00086d0>] (gic_handle_irq+0x2c/0x5c)
[<c00086d0>] (gic_handle_irq) from [<c0011e40>] (__irq_svc+0x40/0x54)
Exception stack(0xde0c1ce8 to 0xde0c1d30)
1ce0:                   c073a684 20010113 c06e30fc 00000003 de0c1d30 00000001
1d00: 00000001 0000012c dfbdcc40 ffffe70c dfbdcc48 df5bd800 00000002 de0c1d30
1d20: c00712d4 c00712bc 20010113 ffffffff
[<c0011e40>] (__irq_svc) from [<c00712bc>] (generic_exec_single+0x10c/0x180)
[<c00712bc>] (generic_exec_single) from [<c00713d0>] (smp_call_function_single+0xa0/0xcc)
[<c00713d0>] (smp_call_function_single) from [<c0071818>] (on_each_cpu+0x2c/0x48)
[<c0071818>] (on_each_cpu) from [<c03312dc>] (mvpp2_poll+0x30/0x594)
[<c03312dc>] (mvpp2_poll) from [<c041d024>] (net_rx_action+0xb0/0x170)
[<c041d024>] (net_rx_action) from [<c00220c4>] (__do_softirq+0x120/0x274)
[<c00220c4>] (__do_softirq) from [<c0022468>] (irq_exit+0x78/0xb0)
[<c0022468>] (irq_exit) from [<c000eb00>] (handle_IRQ+0x44/0x90)
[<c000eb00>] (handle_IRQ) from [<c00086d0>] (gic_handle_irq+0x2c/0x5c)
[<c00086d0>] (gic_handle_irq) from [<c0011e40>] (__irq_svc+0x40/0x54)
Exception stack(0xde0c1eb8 to 0xde0c1f00)
1ea0:                                                       de1b986c 00000000
1ec0: 00000420 de1b986c de1b9800 d761c080 be913a34 be913a34 00000007 de0c0000
1ee0: d761c0a0 be913a34 00000010 de0c1f00 c0491898 c0491918 60010013 ffffffff
[<c0011e40>] (__irq_svc) from [<c0491918>] (unix_inq_len+0x9c/0xa8)
[<c0491918>] (unix_inq_len) from [<c049194c>] (unix_ioctl+0x28/0x88)
[<c049194c>] (unix_ioctl) from [<c0407ccc>] (sock_ioctl+0x124/0x280)
[<c0407ccc>] (sock_ioctl) from [<c00c11bc>] (do_vfs_ioctl+0x3fc/0x5c0)
[<c00c11bc>] (do_vfs_ioctl) from [<c00c13b4>] (SyS_ioctl+0x34/0x5c)
[<c00c13b4>] (SyS_ioctl) from [<c000e220>] (ret_fast_syscall+0x0/0x30)
Task dump for CPU 1:
kiplink_admin.f R running      0  1932   1549 0x00000000
[<c0513a04>] (__schedule) from [<00000007>] (0x7)

If my analysis is correct, what happens on CPU0 is that:

 * lighttpd does an ioctl() on a socket, which ends up calling
   unix_inq_len(), which tries to get a spinlock using spin_lock(). The
   lock is probably taken.

 * while waiting for this lock, we get a network RX interrupt, which
   schedules the network RX softirq, which ends up calling the ->poll()
   function of the network driver, in our case mvpp2_poll().

 * since the network hardware has some per-CPU registers that we need
   to read on all CPUs, the network driver does a on_each_cpu() call.
   This apparently leads nowhere, as after a while, the timer interrupt
   kicks in and decides we're not making progress anymore.

After enabling spinlock debugging, I get the following right before the
RCU stall (note how the RCU stall happens on CPU0, while the spinlock
lockup suspected happens on CPU1) :

BUG: spinlock lockup suspected on CPU#1, kiplink_admin.f/1938
 lock: 0xde4998c0, .magic: dead4ead, .owner: lighttpd/1910, .owner_cpu: 0
CPU: 1 PID: 1938 Comm: kiplink_admin.f Tainted: G        W  O   3.17.0-00017-g53fa061 #2
[<c00154d8>] (unwind_backtrace) from [<c001183c>] (show_stack+0x10/0x14)
[<c001183c>] (show_stack) from [<c053f560>] (dump_stack+0x9c/0xbc)
[<c053f560>] (dump_stack) from [<c0057338>] (do_raw_spin_lock+0x118/0x18c)
[<c0057338>] (do_raw_spin_lock) from [<c05466fc>] (_raw_spin_lock_irqsave+0x60/0x6c)
[<c05466fc>] (_raw_spin_lock_irqsave) from [<c042a7d4>] (skb_queue_tail+0x18/0x48)
[<c042a7d4>] (skb_queue_tail) from [<c04b9f58>] (unix_stream_sendmsg+0x1c8/0x36c)
[<c04b9f58>] (unix_stream_sendmsg) from [<c0422eb8>] (sock_aio_write+0xcc/0xec)
[<c0422eb8>] (sock_aio_write) from [<c00bf414>] (do_sync_write+0x80/0xa8)
[<c00bf414>] (do_sync_write) from [<c00bfe60>] (vfs_write+0x108/0x1b0)
[<c00bfe60>] (vfs_write) from [<c00c0418>] (SyS_write+0x40/0x94)
[<c00c0418>] (SyS_write) from [<c000e3a0>] (ret_fast_syscall+0x0/0x48)

And interestingly, skb_queue_tail() is also taking the same spinlock as
unix_inq_len(), except that it does so with spin_lock_irqsave(). And
this is causing the issue: since this spin_lock_irqsave() takes place
on CPU1, the interupts are disabled, and therefore we're not getting
the IPI that allows the on_each_cpu() call coming from CPU0 to make
progress, causing the lockup.

The patch below has proven to fix the issue: I was able to reproduce
the issue in maximum 5 to 10 minutes, and with the patch the system has
survived an entire night of testing.

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index e968843..c60205a 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2124,11 +2124,12 @@ long unix_inq_len(struct sock *sk)
 {
        struct sk_buff *skb;
        long amount = 0;
+       unsigned long flags;
 
        if (sk->sk_state == TCP_LISTEN)
                return -EINVAL;
 
-       spin_lock(&sk->sk_receive_queue.lock);
+       spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
        if (sk->sk_type == SOCK_STREAM ||
            sk->sk_type == SOCK_SEQPACKET) {
                skb_queue_walk(&sk->sk_receive_queue, skb)
@@ -2138,7 +2139,7 @@ long unix_inq_len(struct sock *sk)
                if (skb)
                        amount = skb->len;
        }
-       spin_unlock(&sk->sk_receive_queue.lock);
+       spin_unlock_irqrestore(&sk->sk_receive_queue.lock, flags);
 
        return amount;
 }

So, the question is: is this patch the correct solution (but then other
usage of spin_lock in af_unix.c might also need fixing) ? Or is the
network driver at fault?

Thanks for your input,

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com