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 14:48:40 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     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 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.

Oleg.