linux-kernel - Re: [PATCH] bpf: defer printk() inside __bpf_prog

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Date: Thu, 27 Jun 2024 07:15:25 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Ogness <john.ogness@...utronix.de>,
        Alexei Starovoitov
 <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau
 <martin.lau@...ux.dev>,
        Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
        Yonghong Song <yonghong.song@...ux.dev>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: defer printk() inside __bpf_prog_run()

On 2024/06/27 1:27, Steven Rostedt wrote:
> On Wed, 26 Jun 2024 09:02:22 +0900
> Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:
> 
>> On 2024/06/26 8:56, Alexei Starovoitov wrote:
>>> You are missing the point. The bug has nothing to do with bpf.  
>>
>> The bug is caused by calling tracing hooks with rq lock held.
>> If tracing hooks do not exist, this bug does not exist.
> 
> Could you expand on this. What tracing hooks are called with rq lock
> held? You mean the scheduling events?

Yes, trace_sched_switch().
__schedule() calls trace_sched_switch() hook with rq lock held.

 #2: ffff8880b943e798 (&rq->__lock){-.-.}-{2:2}, at: raw_spin_rq_lock_nested+0x2a/0x140 kernel/sched/core.c:559

 __bpf_prog_run include/linux/filter.h:691 [inline]
 bpf_prog_run include/linux/filter.h:698 [inline]
 __bpf_trace_run kernel/trace/bpf_trace.c:2403 [inline]
 bpf_trace_run4+0x334/0x590 kernel/trace/bpf_trace.c:2446
 __traceiter_sched_switch+0x98/0xd0 include/trace/events/sched.h:222
 trace_sched_switch include/trace/events/sched.h:222 [inline]
 __schedule+0x2587/0x4a20 kernel/sched/core.c:6742
 preempt_schedule_notrace+0x100/0x140 kernel/sched/core.c:7017