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Date:   Thu, 9 Nov 2017 10:59:49 -0800
From:   Milind Chabbi <chabbi.milind@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Milind Chabbi <chabbi.milind@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel@...r.kernel.org,
        Michael Kerrisk-manpages <mtk.manpages@...il.com>,
        linux-man@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
        Andi Kleen <ak@...ux.intel.com>,
        Kan Liang <kan.liang@...el.com>,
        Hari Bathini <hbathini@...ux.vnet.ibm.com>,
        Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>,
        Jin Yao <yao.jin@...ux.intel.com>
Subject: Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT

SNIP

On Thu, Nov 9, 2017 at 5:12 AM, Jiri Olsa <jolsa@...hat.com> wrote:
>
>
> how about something like below (untested)
>
> looks like there's no irq caller for modify_user_hw_breakpoint,
> so we should be fine with locking nr_bp_mutex
>
> jirka
>
>
> ---
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 3f8cb1e14588..f062b68399ea 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -448,6 +448,8 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
>         else
>                 perf_event_disable(bp);
>
> +       release_bp_slot(bp);
> +
>         bp->attr.bp_addr = attr->bp_addr;
>         bp->attr.bp_type = attr->bp_type;
>         bp->attr.bp_len = attr->bp_len;
> @@ -455,9 +457,9 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
>         if (attr->disabled)
>                 goto end;
>
> -       err = validate_hw_breakpoint(bp);
> +       err = reserve_bp_slot(bp);
>         if (!err)
> -               perf_event_enable(bp);
> +               err = validate_hw_breakpoint(bp);
>
>         if (err) {
>                 bp->attr.bp_addr = old_addr;
> @@ -469,6 +471,7 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
>                 return err;
>         }
>
> +       perf_event_enable(bp);
>  end:
>         bp->attr.disabled = attr->disabled;
>

We can do this accounting only if bp->attr.bp_type != attr->bp_type.

-Milind

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