Subject: Re: [PATCH v3 bpf-next 0/4] cgroup bpf auto-detachment

From: Roman Gushchin <[email-masked]>
Date: Fri, 24 May 2019 21:40:39 +0000

On Fri, May 24, 2019 at 02:03:23PM -0700, Alexei Starovoitov wrote:
> On Thu, May 23, 2019 at 12:45:28PM -0700, Roman Gushchin wrote:
> > This patchset implements a cgroup bpf auto-detachment functionality:
> > bpf programs are detached as soon as possible after removal of the
> > cgroup, without waiting for the release of all associated resources.
> 
> The idea looks great, but doesn't quite work:
> 
> $ ./test_cgroup_attach
> #override:PASS
> [   66.475219] BUG: sleeping function called from invalid context at ../include/linux/percpu-rwsem.h:34
> [   66.476095] in_atomic(): 1, irqs_disabled(): 0, pid: 21, name: ksoftirqd/2
> [   66.476706] CPU: 2 PID: 21 Comm: ksoftirqd/2 Not tainted 5.2.0-rc1-00211-g1861420d0162 #1564
> [   66.477595] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014
> [   66.478360] Call Trace:
> [   66.478591]  dump_stack+0x5b/0x8b
> [   66.478892]  ___might_sleep+0x22f/0x290
> [   66.479230]  cpus_read_lock+0x18/0x50
> [   66.479550]  static_key_slow_dec+0x41/0x70
> [   66.479914]  cgroup_bpf_release+0x1a6/0x400
> [   66.480285]  percpu_ref_switch_to_atomic_rcu+0x203/0x330
> [   66.480754]  rcu_core+0x475/0xcc0
> [   66.481047]  ? switch_mm_irqs_off+0x684/0xa40
> [   66.481422]  ? rcu_note_context_switch+0x260/0x260
> [   66.481842]  __do_softirq+0x1cf/0x5ff
> [   66.482174]  ? takeover_tasklets+0x5f0/0x5f0
> [   66.482542]  ? smpboot_thread_fn+0xab/0x780
> [   66.482911]  run_ksoftirqd+0x1a/0x40
> [   66.483225]  smpboot_thread_fn+0x3ad/0x780
> [   66.483583]  ? sort_range+0x20/0x20
> [   66.483894]  ? __kthread_parkme+0xb0/0x190
> [   66.484253]  ? sort_range+0x20/0x20
> [   66.484562]  ? sort_range+0x20/0x20
> [   66.484878]  kthread+0x2e2/0x3e0
> [   66.485166]  ? kthread_create_worker_on_cpu+0xb0/0xb0
> [   66.485620]  ret_from_fork+0x1f/0x30
> 
> Same test runs fine before the patches.
> 

Ouch, static_branch_dec() might block, so it's not possible to call it from
percpu ref counter release callback. It's not what I expected, tbh.

Good catch, thanks!