Re: [PATCH] RISC-V: insn: Use a raw spinlock to protect TEXT_POKE*

From: Steven Rostedt <rostedt@goodmis.org>
To: Palmer Dabbelt <palmer@dabbelt.com>
Cc: linux-riscv@lists.infradead.org,
	Paul Walmsley <paul.walmsley@sifive.com>,
	aou@eecs.berkeley.edu, peterz@infradead.org, jpoimboe@redhat.com,
	jbaron@akamai.com, ardb@kernel.org,
	Atish Patra <Atish.Patra@wdc.com>,
	Anup Patel <Anup.Patel@wdc.com>,
	akpm@linux-foundation.org, rppt@kernel.org, mhiramat@kernel.org,
	zong.li@sifive.com, guoren@linux.alibaba.com,
	wangkefeng.wang@huawei.com, 0x7f454c46@gmail.com,
	chenhuang5@huawei.com, linux-kernel@vger.kernel.org,
	kernel-team@android.com,
	Palmer Dabbelt <palmerdabbelt@google.com>,
	Changbin Du <changbin.du@gmail.com>
Subject: Re: [PATCH] RISC-V: insn: Use a raw spinlock to protect TEXT_POKE*
Date: Thu, 29 Apr 2021 12:30:07 -0400	[thread overview]
Message-ID: <20210429123007.5144fc0d@gandalf.local.home> (raw)
In-Reply-To: <20210429061713.783628-1-palmer@dabbelt.com>

On Wed, 28 Apr 2021 23:17:13 -0700
Palmer Dabbelt <palmer@dabbelt.com> wrote:

> From: Palmer Dabbelt <palmerdabbelt@google.com>
> 
> We currently use text_mutex to protect the fixmap sections from
> concurrent callers.  This is convienent for kprobes as the generic code
> already holds text_mutex, but ftrace doesn't which triggers a lockdep
> assertion.  We could take text_mutex for ftrace, but the jump label
> implementation (which is currently taking text_mutex) isn't explicitly
> listed as being sleepable and it's called from enough places it seems
> safer to just avoid sleeping.
> 
> arm64 and parisc, the other two TEXT_POKE-style patching
> implemnetations, already use raw spinlocks.  abffa6f3b157 ("arm64:
> convert patch_lock to raw lock") lays out the case for a raw spinlock as
> opposed to a regular spinlock, and while I don't know of anyone using rt
> on RISC-V I'm sure it'll eventually show up and I don't see any reason
> to wait.

On x86 we use text_mutex for jump label and ftrace. I don't understand the
issue here. The arm64 update was already using spin locks in the
insn_write() function itself. riscv just makes sure that text_mutex is held.

It also looks like ftrace on riscv should also have text_mutex held
whenever it modifies the code. Because I see this in
arch/riscv/kernel/ftrace.c:

int ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
{
        mutex_lock(&text_mutex);
        return 0;
}

int ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
{
        mutex_unlock(&text_mutex);
        return 0;
}

Which should be getting called before and after respectively from when
ftrace does its updates.

Can you show me the back trace of that lockdep splat?

-- Steve