linux-kernel - Re: [PATCH 3/2] perf/hw_breakpoint: Remove superfluous bp->attr.disabled = 0 new attr has disabled set

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Date:   Tue, 7 Aug 2018 10:16:16 +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 3/2] perf/hw_breakpoint: Remove superfluous
 bp->attr.disabled = 0 new attr has disabled set

On Mon, Aug 06, 2018 at 06:34:36PM +0200, Oleg Nesterov wrote:
> On 08/06, Jiri Olsa wrote:
> >
> > Once the breakpoint was succesfully modified, the attr->disabled
> > value is in bp->attr.disabled. So there's no reason to set it
> > again, removing that.
> > 
> > Link: http://lkml.kernel.org/n/tip-v5oaellzsmyszv3rfucuxkp0@git.kernel.org
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >  kernel/events/hw_breakpoint.c | 5 ++---
> >  1 file changed, 2 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> > index fb229d9c7f3c..3e560d7609fd 100644
> > --- a/kernel/events/hw_breakpoint.c
> > +++ b/kernel/events/hw_breakpoint.c
> > @@ -526,10 +526,9 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
> >  	if (err)
> >  		return err;
> >  
> > -	if (!attr->disabled) {
> > +	if (!attr->disabled)
> >  		perf_event_enable(bp);
> > -		bp->attr.disabled = 0;
> > -	}
> > +
> 
> Yes, but again, this still looks confusing.
> 
> IMO, we should either remove "bp->attr.disabled = attr->disabled" in
> modify_user_hw_breakpoint_check() because bp->attr.disabled  is not really
> used, or we should set bp->attr.disabled = 1 on failure just for consistency.
> 
> 
> Hmm... actually ptrace_get_dr7() checks ->attr.disabled, so we can hit
> WARN_ON(second_pass) in ptrace_write_dr7() in case when attr.disabled is
> falsely 0 because modify_user_hw_breakpoint_check() failed before?

hum, I can't see how modify_user_hw_breakpoint_check could falsely set disabled
new attr stuff is copied once all checks passed

jirka