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

[Changbin Du]

On Fri, Feb 24, 2023 at 01:46:38PM +0000, Conor Dooley wrote:
> 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.
> 
Understood your concern. Then how abount below instead of discrete changes?

int patch_text(void *addr, u32 insn)
 {
+       int ret;
        struct patch_insn patch = {
                .addr = addr,
                .insn = insn,
                .cpu_count = ATOMIC_INIT(0),
        };

-       return stop_machine_cpuslocked(patch_text_cb,
+       lockdep_assert_held(&text_mutex);
+       riscv_patch_in_stop_machine = true
+       ret = stop_machine_cpuslocked(patch_text_cb,
                                       &patch, cpu_online_mask);
+       riscv_patch_in_stop_machine = false;
+       return ret;
 }

> I'm probably just erring on the paranoid/conservative side of things!
-- 
Cheers,
Changbin Du