On 12/11/20 9:11 AM, Jonathan Lemon wrote:

quoted

From: Jonathan Lemon <redacted>

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;
                }