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Date:   Thu, 10 Oct 2019 17:19:01 +0000
From:   Alexei Starovoitov <ast@...com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Song Liu <songliubraving@...com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 2/2] bpf/stackmap: fix A-A deadlock in
 bpf_get_stack()

On 10/10/19 12:36 AM, Peter Zijlstra wrote:
> On Wed, Oct 09, 2019 at 11:19:16PM -0700, Song Liu wrote:
>> bpf stackmap with build-id lookup (BPF_F_STACK_BUILD_ID) can trigger A-A
>> deadlock on rq_lock():
>>
>> rcu: INFO: rcu_sched detected stalls on CPUs/tasks:
>> [...]
>> Call Trace:
>>   try_to_wake_up+0x1ad/0x590
>>   wake_up_q+0x54/0x80
>>   rwsem_wake+0x8a/0xb0
>>   bpf_get_stack+0x13c/0x150
>>   bpf_prog_fbdaf42eded9fe46_on_event+0x5e3/0x1000
>>   bpf_overflow_handler+0x60/0x100
>>   __perf_event_overflow+0x4f/0xf0
>>   perf_swevent_overflow+0x99/0xc0
>>   ___perf_sw_event+0xe7/0x120
>>   __schedule+0x47d/0x620
>>   schedule+0x29/0x90
>>   futex_wait_queue_me+0xb9/0x110
>>   futex_wait+0x139/0x230
>>   do_futex+0x2ac/0xa50
>>   __x64_sys_futex+0x13c/0x180
>>   do_syscall_64+0x42/0x100
>>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
> 
>> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
>> index 052580c33d26..3b278f6b0c3e 100644
>> --- a/kernel/bpf/stackmap.c
>> +++ b/kernel/bpf/stackmap.c
>> @@ -287,7 +287,7 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
>>   	bool irq_work_busy = false;
>>   	struct stack_map_irq_work *work = NULL;
>>
>> -	if (in_nmi()) {
>> +	if (in_nmi() || this_rq_is_locked()) {
>>   		work = this_cpu_ptr(&up_read_work);
>>   		if (work->irq_work.flags & IRQ_WORK_BUSY)
>>   			/* cannot queue more up_read, fallback */
> 
> This is horrific crap. Just say no to that get_build_id_offset()
> trainwreck.

this is not a helpful comment.
What issues do you see with this approach?



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