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Date:   Fri, 18 Dec 2020 10:21:41 -0800
From:   Yonghong Song <yhs@...com>
To:     Jonathan Lemon <jonathan.lemon@...il.com>
CC:     <netdev@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <bpf@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH 1/1 v3 bpf-next] bpf: increment and use correct thread
 iterator



On 12/18/20 10:06 AM, Jonathan Lemon wrote:
> On Fri, Dec 18, 2020 at 08:53:22AM -0800, Yonghong Song wrote:
>>
>>
>> On 12/11/20 9:11 AM, Jonathan Lemon wrote:
>>> From: Jonathan Lemon <bsd@...com>
>>>
>>> On some systems, some variant of the following splat is
>>> repeatedly seen.  The common factor in all traces seems
>>> to be the entry point to task_file_seq_next().  With the
>>> patch, all warnings go away.
>>>
>>>       rcu: INFO: rcu_sched self-detected stall on CPU
>>>       rcu: \x0926-....: (20992 ticks this GP) idle=d7e/1/0x4000000000000002 softirq=81556231/81556231 fqs=4876
>>>       \x09(t=21033 jiffies g=159148529 q=223125)
>>>       NMI backtrace for cpu 26
>>>       CPU: 26 PID: 2015853 Comm: bpftool Kdump: loaded Not tainted 5.6.13-0_fbk4_3876_gd8d1f9bf80bb #1
>>>       Hardware name: Quanta Twin Lakes MP/Twin Lakes Passive MP, BIOS F09_3A12 10/08/2018
>>>       Call Trace:
>>>        <IRQ>
>>>        dump_stack+0x50/0x70
>>>        nmi_cpu_backtrace.cold.6+0x13/0x50
>>>        ? lapic_can_unplug_cpu.cold.30+0x40/0x40
>>>        nmi_trigger_cpumask_backtrace+0xba/0xca
>>>        rcu_dump_cpu_stacks+0x99/0xc7
>>>        rcu_sched_clock_irq.cold.90+0x1b4/0x3aa
>>>        ? tick_sched_do_timer+0x60/0x60
>>>        update_process_times+0x24/0x50
>>>        tick_sched_timer+0x37/0x70
>>>        __hrtimer_run_queues+0xfe/0x270
>>>        hrtimer_interrupt+0xf4/0x210
>>>        smp_apic_timer_interrupt+0x5e/0x120
>>>        apic_timer_interrupt+0xf/0x20
>>>        </IRQ>
>>>       RIP: 0010:get_pid_task+0x38/0x80
>>>       Code: 89 f6 48 8d 44 f7 08 48 8b 00 48 85 c0 74 2b 48 83 c6 55 48 c1 e6 04 48 29 f0 74 19 48 8d 78 20 ba 01 00 00 00 f0 0f c1 50 20 <85> d2 74 27 78 11 83 c2 01 78 0c 48 83 c4 08 c3 31 c0 48 83 c4 08
>>>       RSP: 0018:ffffc9000d293dc8 EFLAGS: 00000202 ORIG_RAX: ffffffffffffff13
>>>       RAX: ffff888637c05600 RBX: ffffc9000d293e0c RCX: 0000000000000000
>>>       RDX: 0000000000000001 RSI: 0000000000000550 RDI: ffff888637c05620
>>>       RBP: ffffffff8284eb80 R08: ffff88831341d300 R09: ffff88822ffd8248
>>>       R10: ffff88822ffd82d0 R11: 00000000003a93c0 R12: 0000000000000001
>>>       R13: 00000000ffffffff R14: ffff88831341d300 R15: 0000000000000000
>>>        ? find_ge_pid+0x1b/0x20
>>>        task_seq_get_next+0x52/0xc0
>>>        task_file_seq_get_next+0x159/0x220
>>>        task_file_seq_next+0x4f/0xa0
>>>        bpf_seq_read+0x159/0x390
>>>        vfs_read+0x8a/0x140
>>>        ksys_read+0x59/0xd0
>>>        do_syscall_64+0x42/0x110
>>>        entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>>       RIP: 0033:0x7f95ae73e76e
>>>       Code: Bad RIP value.
>>>       RSP: 002b:00007ffc02c1dbf8 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
>>>       RAX: ffffffffffffffda RBX: 000000000170faa0 RCX: 00007f95ae73e76e
>>>       RDX: 0000000000001000 RSI: 00007ffc02c1dc30 RDI: 0000000000000007
>>>       RBP: 00007ffc02c1ec70 R08: 0000000000000005 R09: 0000000000000006
>>>       R10: fffffffffffff20b R11: 0000000000000246 R12: 00000000019112a0
>>>       R13: 0000000000000000 R14: 0000000000000007 R15: 00000000004283c0
>>>
>>> The attached patch does 3 things:
>>>
>>> 1) If unable to obtain the file structure for the current task,
>>>      proceed to the next task number after the one returned from
>>>      task_seq_get_next(), instead of the next task number from the
>>>      original iterator.
>>
>> Looks like this fix is the real fix for the above warnings.
>> Basically, say we have
>>     info->tid = 10 and returned curr_tid = 3000 and tid 3000 has no files.
>> the current logic will go through
>>     - set curr_tid = 11 (info->tid++) and returned curr_tid = 3000
>>     - set curr_tid = 12 and returned curr_tid = 3000
>>     ...
>>     - set curr_tid = 3000 and returned curr_tid = 3000
>>     - set curr_tid = 3001 and return curr_tid >= 3001
>>
>> All the above works are redundant work, and it may cause issues
>> for non preemptable kernel.
>>
>> I suggest you factor out this change plus the following change
>> which suggested by Andrii early to a separate patch carried with
>> the below Fixes tag.
>>
>> diff --git a/kernel/bpf/task_iter.c b/kernel/bpf/task_iter.c
>> index 0458a40edf10..56bcaef72e36 100644
>> --- a/kernel/bpf/task_iter.c
>> +++ b/kernel/bpf/task_iter.c
>> @@ -158,6 +158,7 @@ task_file_seq_get_next(struct
>> bpf_iter_seq_task_file_info *info)
>>                  if (!curr_task) {
>>                          info->task = NULL;
>>                          info->files = NULL;
>> +                       info->tid = curr_tid + 1;
>>                          return NULL;
>>                  }
> 
> Sure this isn't supposed to be 'curr_tid'?  task_seq_get_next() stops
> when there are no more threads found.  This increments the thread id
> past the search point, and would seem to introduce a potential off-by-one
> error.

This is for the case where read() syscall return length 0, but user
space still keep read(). You are right, we may skip one newly created 
one indeed.

Although such a usecase is not common, but info->tid = curr_tid
certainly more correct than info->tid = curr_tid + 1.

So thanks for suggestion, LGTM.

> 
> That is:
>     curr_tid = 3000.
>     call task_seq_get_next() --> return NULL, curr_tid = 3000.
>        (so there is no tid >= 3000)
>     set curr_tid = 3001.
> 
>     next restart (if there is one) skips a newly created 3000.
> 

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