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

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Date: Wed, 26 Jun 2024 01:05:02 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: 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>, Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] bpf: defer printk() inside __bpf_prog_run()

On 2024/06/26 0:47, John Ogness wrote:
> On 2024-06-26, Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:
>> On 2024/06/25 23:17, John Ogness wrote:
>>> On 2024-06-25, Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp> wrote:
>>>> syzbot is reporting circular locking dependency inside __bpf_prog_run(),
>>>> for fault injection calls printk() despite rq lock is already held.
>>>>
>>>> Guard __bpf_prog_run() using printk_deferred_{enter,exit}() (and
>>>> preempt_{disable,enable}() if CONFIG_PREEMPT_RT=n) in order to defer any
>>>> printk() messages.
>>>
>>> Why is the reason for disabling preemption?
>>
>> Because since kernel/printk/printk_safe.c uses a percpu counter for deferring
>> printk(), printk_safe_enter() and printk_safe_exit() have to be called from
>> the same CPU. preempt_disable() before printk_safe_enter() and preempt_enable()
>> after printk_safe_exit() guarantees that printk_safe_enter() and
>> printk_safe_exit() are called from the same CPU.
> 
> Yes, but we already have cant_migrate(). Are you suggesting there are
> configurations where cant_migrate() is true but the context can be
> migrated anyway?

No, I'm not aware of such configuration.

Does migrate_disable() imply preempt_disable() ?
If yes, we don't need to also call preempt_disable().
My understanding is that migration is about "on which CPU a process runs"
and preemption is about "whether a different process runs on this CPU".
That is, disabling migration and disabling preemption are independent.

Is migrate_disable() alone sufficient for managing a percpu counter?
If yes, we don't need to also call preempt_disable() in order to manage
a percpu counter.