linux-kernel - Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY

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Message-ID: <CAMmz+Y=eq=S+gZaRefVUrfB7LDRVfD5UdpkfQXS0zvnHdNt0XA@mail.gmail.com>
Date:   Wed, 8 Nov 2017 08:59:22 -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

On Wed, Nov 8, 2017 at 7:57 AM, Jiri Olsa <jolsa@...hat.com> wrote:
> On Wed, Nov 08, 2017 at 07:51:10AM -0800, Milind Chabbi wrote:
>> On Wed, Nov 8, 2017 at 7:12 AM, Jiri Olsa <jolsa@...hat.com> wrote:
>>
>> > > I am not able to fully understand your concern.
>> > > Can you point to a code file and line related to your observation?
>> > > The patch is modeled after the existing modify_user_hw_breakpoint() function
>> > > present in events/hw_breakpoint.c; don't you see this problem in that code?
>> >
>> > the reserve_bp_slot/release_bp_slot functions manage
>> > counts for current breakpoints based on its type
>> >
>> > those counts are cumulated in here:
>> >   static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
>> >
>> > you allow to change the breakpoint type, so I'd expect
>> > to see some code that release slot count for old type
>> > and take new one (if it's available)
>> >
>> > jirka
>>
>>
>> Why is this not a concern for modify_user_hw_breakpoint() function?
>
> I don't know ;-)
>
> jirka

Jirka,

I carefully looked at bp_cpuinfo[] and nr_slots[] data structures.
nr_slots[] is an array of length two (one slot of TYPE_INST and
another for TYPE_DATA).
The accounting "thinks" that there is one limit on the number of
instruction breakpoints and another limit on the number of data
breakpoints.
The assumption is clearly broken; for example, on x86 there exists a
limit on the *total* number of all breakpoints disregarding their kind
and the code has failed to capture this aspect.

As such, modify_user_hw_breakpoint() makes no attempt to keep the
counts correct. Instead, it simply tries to change and install a new
breakpoint and fails if the hardware disallows.
This can lead to a situation where, say on x86, someone creates 4
TYPE_DATA breakpoints, then changes one of them to TYPE_INS via
modify_user_hw_breakpoint() and then releases the TYPE_INS breakpoint.
Since the accounting still thinks that there are four TYPE_DATA
breakpoints, it will disallow creating a new TYPE_DATA breakpoint,
although there is place for one TYPE_DATA breakpoint.

This convinces me that the problem and the solution are outside of
this current patch.
Do you agree?

-Milind