linux-kernel - Re: [PATCH] RISC-V: insn: Use a raw spinlock to protect TEXT

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Date:   Fri, 30 Apr 2021 12:44:37 -0700 (PDT)
From:   Palmer Dabbelt <palmerdabbelt@...gle.com>
To:     rostedt@...dmis.org
CC:     Anup Patel <Anup.Patel@....com>, changbin.du@...il.com,
        linux-riscv@...ts.infradead.org,
        Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, peterz@...radead.org, jpoimboe@...hat.com,
        jbaron@...mai.com, ardb@...nel.org,
        Atish Patra <Atish.Patra@....com>, akpm@...ux-foundation.org,
        rppt@...nel.org, mhiramat@...nel.org, zong.li@...ive.com,
        guoren@...ux.alibaba.com, wangkefeng.wang@...wei.com,
        0x7f454c46@...il.com, chenhuang5@...wei.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject:     Re: [PATCH] RISC-V: insn: Use a raw spinlock to protect TEXT_POKE*

On Fri, 30 Apr 2021 04:34:31 PDT (-0700), rostedt@...dmis.org wrote:
> On Fri, 30 Apr 2021 04:06:35 +0000
> Anup Patel <Anup.Patel@....com> wrote:
>
>> This patch only takes care of ftrace path.
>>
>> The RISC-V instruction patching is used by RISC-V jump label implementation
>> as well and will called from various critical parts of core kernel.
>>
>> The RAW spinlock approach allows same instruction patching to be used
>> for kprobes, ftrace, and jump label.
>
> So what path hits this outside of stop machine?

I didn't actually dig through all the usages of jump_label, I just saw a 
handful in places where it's generally not sane to assume that sleeping 
is safe -- for example, thoughout kernel/sched.  If you think it's OK to 
rely on users of the static branch stuff (IIUC the only jump_label user 
in the kernel?) to know that it can sleep then I'm fine keeping the 
text_mutex call in jump_label and adding one to ftrace (I'm fine with 
something generic, but it's simple to do in arch/riscv).

IMO if the static branch stuff can be expected to sleep it'd be good to 
call that out in the documentation, and I'd like to audit the uses 
before committing to that.  I'm happy to do that, we can just take the 
lock in arch/riscv's frace code for now to get around the lockdep 
assertion failure -- IIUC that's indicating a real bug, as nothing in 
ftrace avoids concurrency with jump_label and kprobes.