linux-kernel - Re: [PATCH] ftrace/direct: Fix lockup in modify_ftrace_direct

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Date:   Tue, 9 Nov 2021 19:04:59 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>, lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ftrace/direct: Fix lockup in modify_ftrace_direct_multi

On Tue, Nov 09, 2021 at 11:02:37AM -0500, Steven Rostedt wrote:
> On Tue,  9 Nov 2021 12:42:17 +0100
> Jiri Olsa <jolsa@...hat.com> wrote:
> 
> > We can't call unregister_ftrace_function under ftrace_lock.
> 
> Ouch. 
> 
> Do you have a sample module that triggers this?
> 
> I thought we had something that tested this code, but I don't see it in my
> tree.

I triggered that with my bpf batch changes

there's not test for modify at the moment,
just register/unregister I'll add one

jirka

> 
> -- Steve
> 
> > 
> > Fixes: ed29271894aa ("ftrace/direct: Do not disable when switching direct callers")
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >  kernel/trace/ftrace.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index b4ed1a301232..fc49e8809a56 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -5602,10 +5602,11 @@ int modify_ftrace_direct_multi(struct ftrace_ops *ops, unsigned long addr)
> >  		}
> >  	}
> >  
> > +	mutex_unlock(&ftrace_lock);
> > +
> >  	/* Removing the tmp_ops will add the updated direct callers to the functions */
> >  	unregister_ftrace_function(&tmp_ops);
> >  
> > -	mutex_unlock(&ftrace_lock);
> >   out_direct:
> >  	mutex_unlock(&direct_mutex);
> >  	return err;
>