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 11:10:08 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jiri Olsa <jolsa@...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 08/07, Jiri Olsa wrote:
>
> On Mon, Aug 06, 2018 at 06:34:36PM +0200, Oleg Nesterov wrote:
> > > --- 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

Hmm. So modify_user_hw_breakpoint() does perf_event_disable(bp) first and afaics
this doesn't set bp->attr.disabled = 1.

If modify_user_hw_breakpoint_check() fails after that we do not update bp->attr,
so bp->attr.disabled is still zero while this bp is actually disabled.

Again, afaics the core perf code doesn't actually use bp->attr.disabled after
perf_event__state_init(). But this can confuse ptrace_write_dr7/ptrace_get_dr7.

No?

I am on Linus's tree, but I see the same logic in
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/kernel/events/hw_breakpoint.c?h=perf/core

Oleg.