Messages in this thread |  | | Subject | Re: RCU stall in af_unix.c, should use spin_lock_irqsave? | From | Eric Dumazet <> | Date | Tue, 21 Oct 2014 03:04:34 -0700 |
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On Tue, 2014-10-21 at 10:03 +0200, Thomas Petazzoni wrote: > 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
Locks in af_unix do not need to mask irqs. Ever.
skb_queue_tail() uses an irqsave variant because its a generic function, and _some_ skb list might be manipulated from hard irq handlers in pre NAPI drivers. But af_unix does not have an interrupt handler that could potentially try to lock sk_receive_queue.lock
mvpp2 is seriously brain damaged : on_each_cpu() cannot be used from a bottom half handler.
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