Re: call_rcu from trace_preempt

Daniel Wagner
2015-06-16T06:06:19Z

On 06/16/2015 07:45 AM, Alexei Starovoitov wrote:
> On 6/15/15 7:14 PM, Paul E. McKenney wrote:
>>
>> Why do you believe that it is better to fix it within call_rcu()?
>
> found it:
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8cf7304b2867..a3be09d482ae 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -935,9 +935,9 @@ bool notrace rcu_is_watching(void)
> {
> bool ret;
>
> - preempt_disable();
> + preempt_disable_notrace();
> ret = __rcu_is_watching();
> - preempt_enable();
> + preempt_enable_notrace();
> return ret;
> }
>
> the rcu_is_watching() and __rcu_is_watching() are already marked
> notrace, so imo it's a good 'fix'.
> What was happening is that the above preempt_enable was triggering
> recursive call_rcu that was indeed messing 'rdp' that was
> prepared by __call_rcu and before __call_rcu_core could use that.
>
> btw, also noticed that local_irq_save done by note_gp_changes
> is partially redundant. In __call_rcu_core path the irqs are
> already disabled.
>
>> Perhaps you are self-deadlocking within __call_rcu_core(). If you have
>> not already done so, please try running with CONFIG_PROVE_LOCKING=y.
>
> yes, I had CONFIG_PROVE_LOCKING on.
>
>> I suspect that your problem may range quite a bit further than just
>> call_rcu(). For example, in your stack trace, you have a recursive
>> call to debug_object_activate(), which might not be such good thing.
>
> nope :) recursive debug_object_activate() is fine.
> with the above 'fix' the trace.patch is now passing.

It still crashes for me with the original test program

[ 145.908013] [<ffffffff810d1da1>] ? __rcu_reclaim+0x101/0x3d0
[ 145.908013] [<ffffffff810d1ca0>] ? rcu_barrier_func+0x250/0x250
[ 145.908013] [<ffffffff810abc03>] ? trace_hardirqs_on_caller+0xf3/0x240
[ 145.908013] [<ffffffff810d9afa>] rcu_do_batch+0x2ea/0x6b0
[ 145.908013] [<ffffffff8151a803>] ? __this_cpu_preempt_check+0x13/0x20
[ 145.908013] [<ffffffff810abc03>] ? trace_hardirqs_on_caller+0xf3/0x240
[ 145.921092] [<ffffffff81b6f072>] ? _raw_spin_unlock_irqrestore+0x42/0x80
[ 145.921092] [<ffffffff810d2794>] ? rcu_report_qs_rnp+0x1b4/0x3f0
[ 145.921092] [<ffffffff8151a803>] ? __this_cpu_preempt_check+0x13/0x20
[ 145.921092] [<ffffffff810d9f96>] rcu_process_callbacks+0xd6/0x6a0
[ 145.921092] [<ffffffff81060042>] __do_softirq+0xe2/0x670
[ 145.921092] [<ffffffff810605ef>] run_ksoftirqd+0x1f/0x60
[ 145.921092] [<ffffffff81081843>] smpboot_thread_fn+0x193/0x2a0
[ 145.921092] [<ffffffff810816b0>] ? sort_range+0x30/0x30
[ 145.921092] [<ffffffff8107da12>] kthread+0xf2/0x110
[ 145.921092] [<ffffffff81b6a523>] ? wait_for_completion+0xc3/0x120
[ 145.921092] [<ffffffff8108a77b>] ? preempt_count_sub+0xab/0xf0
[ 145.921092] [<ffffffff8107d920>] ? kthread_create_on_node+0x240/0x240
[ 145.921092] [<ffffffff81b6ff02>] ret_from_fork+0x42/0x70
[ 145.921092] [<ffffffff8107d920>] ? kthread_create_on_node+0x240/0x240

> Why I'm digging into all of these? Well, to find out when
> it's safe to finally do call_rcu. If I will use deferred kfree
> approach in bpf maps, I need to know when it's safe to finally
> call_rcu and it's not an easy answer.
> kprobes potentially can be placed in any part of call_rcu stack,
> so things can go messy quickly. So it helps to understand the call_rcu
> logic well enough to come up with good solution.