linux-kernel - Re: [PATCH 2/2] perf/hw_breakpoint: Modify breakpoint even if the new attr has disabled set

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Date:   Mon, 6 Aug 2018 15:23:53 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Milind Chabbi <chabbi.milind@...il.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        David Ahern <dsahern@...il.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 2/2] perf/hw_breakpoint: Modify breakpoint even if the
 new attr has disabled set

On Mon, Aug 06, 2018 at 02:48:40PM +0200, Oleg Nesterov wrote:
> On 08/06, Jiri Olsa wrote:
> >
> > We need to change the breakpoint even if the attr with
> > new fields has disabled set to true.
> 
> Agreed... The patch looks fine to me, but I have a question
> 
> >  int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *attr)
> >  {
> > +	int err;
> > +
> >  	/*
> >  	 * modify_user_hw_breakpoint can be invoked with IRQs disabled and hence it
> >  	 * will not be possible to raise IPIs that invoke __perf_event_disable.
> > @@ -520,11 +522,11 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
> >  	else
> >  		perf_event_disable(bp);
> >  
> > -	if (!attr->disabled) {
> > -		int err = modify_user_hw_breakpoint_check(bp, attr, false);
> > +	err = modify_user_hw_breakpoint_check(bp, attr, false);
> > +	if (err)
> > +		return err;
> >  
> > -		if (err)
> > -			return err;
> > +	if (!attr->disabled) {
> >  		perf_event_enable(bp);
> >  		bp->attr.disabled = 0;
> 
> Afaics you do not need to clear attr.disabled, modify_user_hw_breakpoint_check()
> updates it if err = 0. So I think
> 
> 	if (!bp->attr.disabled)
> 		perf_event_enable(bp);
> 
> will look a bit better.
> 
> 
> But, with or without this fix, shouldn't we set .disabled = 1 if modify_() fails?
> IIUC this doesn't matter, bp->attr.disabled is not really used anyway, but looks a
> bit confusing.
> 

yea, I was looking on that, but as u said it makes no difference
and I wanted to keep the patch as simple as possible ;-)

I'll send something on top of this patch

jirka