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From: Milind Chabbi <chabbi.milind@gmail.com>
Date: Mon, 13 Nov 2017 00:02:56 -0800
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Subject: Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT
To: Jiri Olsa <jolsa@redhat.com>
Cc: Milind Chabbi <chabbi.milind@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>, linux-kernel@vger.kernel.org,
        Michael Kerrisk-manpages <mtk.manpages@gmail.com>,
        linux-man@vger.kernel.org, Michael Ellerman <mpe@ellerman.id.au>,
        Andi Kleen <ak@linux.intel.com>, Kan Liang <kan.liang@intel.com>,
        Hari Bathini <hbathini@linux.vnet.ibm.com>,
        Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>,
        Jin Yao <yao.jin@linux.intel.com>
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SNIP

On Sun, Nov 12, 2017 at 11:46 PM, Jiri Olsa <jolsa@redhat.com> wrote:

> but you closed fd4 before openning fd5..?

Yes, that is correct. I closed fd4. The reason is by closing fd4, we
are having a total of 3 hardware breakpoints active, but we are making
the software counting in the kernel think that four TYPE_DATA
breakpoints active. The counting should have disallowed us from
creating fd5 as per the following logic in the kernel:

static int __reserve_bp_slot(struct perf_event *bp)

{
 ....

        /* Flexible counters need to keep at least one slot */
        if (slots.pinned + (!!slots.flexible) > nr_slots[type])
                return -ENOSPC;
....
}