linux-kernel - Re: [PATCH v2] ftrace: Introduce PERMANENT ftrace

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Message-ID: <alpine.LSU.2.21.1910151244550.30206@pobox.suse.cz>
Date:   Tue, 15 Oct 2019 12:50:59 +0200 (CEST)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Petr Mladek <pmladek@...e.com>
cc:     Steven Rostedt <rostedt@...dmis.org>, jikos@...nel.org,
        joe.lawrence@...hat.com, jpoimboe@...hat.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH v2] ftrace: Introduce PERMANENT ftrace_ops flag

On Tue, 15 Oct 2019, Petr Mladek wrote:

> On Mon 2019-10-14 11:17:19, Steven Rostedt wrote:
> > On Mon, 14 Oct 2019 12:59:23 +0200
> > Miroslav Benes <mbenes@...e.cz> wrote:
> > 
> > >  int
> > >  ftrace_enable_sysctl(struct ctl_table *table, int write,
> > >  		     void __user *buffer, size_t *lenp,
> > > @@ -6740,8 +6754,6 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
> > >  	if (ret || !write || (last_ftrace_enabled == !!ftrace_enabled))
> > >  		goto out;
> > >  
> > > -	last_ftrace_enabled = !!ftrace_enabled;
> > > -
> > >  	if (ftrace_enabled) {
> > >  
> > >  		/* we are starting ftrace again */
> > > @@ -6752,12 +6764,19 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
> > >  		ftrace_startup_sysctl();
> > >  
> > >  	} else {
> > > +		if (is_permanent_ops_registered()) {
> > > +			ftrace_enabled = last_ftrace_enabled;
> > 
> > Although this is not incorrect, but may be somewhat confusing.
> > 
> > At this location, last_ftrace_enabled is always true.
> > 
> > I'm thinking this would be better to simply set it to false here.
> 
> IMHO, we want to set ftrace_enabled = true here.
> 
> It was set to "false" by writing to the sysfs file. But the change
> gets rejected. ftrace will stay enabled. So, we should set
> the value back to "true".

That's correct.

I can make it explicit as proposed. I just thought that 'ftrace_enabled = 
last_ftrace_enabled' was clear enough given Petr's explanation.

> > > +			ret = -EBUSY;
> > > +			goto out;
> > > +		}
> > > +
> > >  		/* stopping ftrace calls (just send to ftrace_stub) */
> > >  		ftrace_trace_function = ftrace_stub;
> > >  
> > >  		ftrace_shutdown_sysctl();
> > >  	}
> > >  
> > > +	last_ftrace_enabled = !!ftrace_enabled;
> > >   out:
> > 
> > And move the assignment of last_ftrace_enabled to after the "out:"
> > label.
> 
> This change might make sense anyway. But it is not strictly necessary
> from my POV.

If it stays before "out:" label, last_ftrace_enabled is set if and only if 
it has to be set. I think it is better, but I can, of course, move it in 
v3 if Steven prefers it.

Thanks
Miroslav