Re: [PATCH bpf-next 2/2] bpf/stackmap: fix A-A deadlock in bpf_get_stack()

[Peter Zijlstra]

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.