Re: [PATCH v3] RISC-V: Don't check text_mutex during stop_machine

From: Conor Dooley <conor.dooley@microchip.com>
To: Changbin Du <changbin.du@huawei.com>
Cc: Conor Dooley <conor@kernel.org>, <palmer@dabbelt.com>,
	Palmer Dabbelt <palmerdabbelt@google.com>,
	<linux-riscv@lists.infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Changbin Du <changbin.du@gmail.com>,
	Palmer Dabbelt <palmer@rivosinc.com>,
	Hui Wang <hw.huiwang@huawei.com>
Subject: Re: [PATCH v3] RISC-V: Don't check text_mutex during stop_machine
Date: Fri, 24 Feb 2023 13:46:38 +0000	[thread overview]
Message-ID: <Y/i/vkcX5AEQ6qAm@wendy> (raw)
In-Reply-To: <20230224125857.dbsaj7mv2by66wm3@M910t>

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On Fri, Feb 24, 2023 at 08:58:57PM +0800, Changbin Du wrote:
> On Fri, Feb 24, 2023 at 11:07:42AM +0000, Conor Dooley wrote:
> > > > -	lockdep_assert_held(&text_mutex);
> > > > +	if (!riscv_ftrace_in_stop_machine)
> > > > +		lockdep_assert_held(&text_mutex);
> > > >  
> > > >  	if (across_pages)
> > > >  		patch_map(addr + len, FIX_TEXT_POKE1);
> > > This misses this function.
> > > 
> > > int patch_text(void *addr, u32 insn)
> > 
> > So, with a corresponding rename to the symbol, does the following look
> > okay to you?
> > 
> > diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
> > index f21592d20306..433b454e693f 100644
> > --- a/arch/riscv/kernel/probes/kprobes.c
> > +++ b/arch/riscv/kernel/probes/kprobes.c
> > @@ -27,9 +27,15 @@ static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
> >  
> >  	p->ainsn.api.restore = (unsigned long)p->addr + offset;
> >  
> > +	/*
> > +	 * kprobes takes text_mutex, but patch_text() calls stop_machine and
> > +	 * lockdep gets confused by the context in which the lock is taken.
> > +	 */
> > +	riscv_patch_in_stop_machine = true;
> >  	patch_text(p->ainsn.api.insn, p->opcode);
> >  	patch_text((void *)((unsigned long)(p->ainsn.api.insn) + offset),
> >  		   __BUG_INSN_32);
> > +	riscv_patch_in_stop_machine = false;
> >  }
> hmm, why not just put 'riscv_patch_in_stop_machine' into patch_text()? Then you
> just need to modify that function.

Right, I intentionally didn't do that as `riscv_patch_in_stop_machine`
skips the lockdep check, which we only want to do for codepaths we know
the lock will be held for.
I didn't want to put it in patch_text() so if users of patch_text() that
do not take the lock are added, they will be caught.

I'm probably just erring on the paranoid/conservative side of things!

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