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Date:   Fri, 18 Dec 2020 10:06:28 -0800
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     Yonghong Song <yhs@...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 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.

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.

-- 
Jonathan

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