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

From: Oleg Nesterov <oleg@redhat.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Milind Chabbi <chabbi.milind@gmail.com>,
	lkml <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Namhyung Kim <namhyung@kernel.org>,
	David Ahern <dsahern@gmail.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH 3/2] perf/hw_breakpoint: Remove superfluous bp->attr.disabled = 0 new attr has disabled set
Date: Mon, 6 Aug 2018 18:34:36 +0200	[thread overview]
Message-ID: <20180806163436.GH7840@redhat.com> (raw)
In-Reply-To: <20180806150836.GD16446@krava>

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@kernel.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?

It seems I am totally confused and need to sleep ;)

Oleg.