linux-kernel - Re: [PATCH 4/5] perf/hw_breakpoint: Set breakpoint as disabled in modify_user_hw

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Date:   Thu, 9 Aug 2018 18:30:15 +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>,
        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>,
        Milind Chabbi <chabbi.milind@...il.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH 4/5] perf/hw_breakpoint: Set breakpoint as disabled in
 modify_user_hw_breakpoint error path

On Thu, Aug 09, 2018 at 04:17:13PM +0200, Oleg Nesterov wrote:
> On 08/09, Jiri Olsa wrote:
> >
> > @@ -523,8 +523,10 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
> >  		perf_event_disable(bp);
> >
> >  	err = modify_user_hw_breakpoint_check(bp, attr, false);
> > -	if (err)
> > +	if (err) {
> > +		bp->attr.disabled = 1;
> >  		return err;
> 
> Yes, but on the second thought... Can't we simply do
> 
> 	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.
> 		 * So call the function directly after making sure we are targeting the
> 		 * current task.
> 		 */
> 		if (irqs_disabled() && bp->ctx && bp->ctx->task == current)
> 			perf_event_disable_local(bp);
> 		else
> 			perf_event_disable(bp);
> 
> 		err = modify_user_hw_breakpoint_check(bp, attr, false);
> 
> 		if (!bp.attr->disabled)
> 			perf_event_enable(bp);
> 
> 		return err;
> 	}
> 
> instead of this and the next patch?
> 
> We can do this because (as you pointed out) validate_hw_breakpoint() has already
> gone in -tip tree, and (afaics) modify_user_hw_breakpoint_check() never changes
> perf_event's state on failure, so we can safely "restart" this bp if it was enabled
> before.
> 
> 1. This is what we had before the recent f67b15037a7a50c57f72e69a6d59941ad90a0f0f
>    "perf/hwbp: Simplify the perf-hwbp code, fix documentation".
> 
>    Note that this commit was actually the bug fix which introduced another problem
>    fixed by your 2/5.
> 
>    But see above, perf_event_enable() is no longer unsafe after
>    modify_user_hw_breakpoint_check(), we can restore the previous semantics.
> 
> 2. This matches perf_event_modify_breakpoint(). Which btw can be simplified a bit,
>    it too can simply do
> 
> 		if (!bp->attr.disabled)
> 			_perf_event_enable(bp);
> 
> 		return err;

yep, seems good.. will send new version

thanks,
jirka