Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT

From: Milind Chabbi <chabbi.milind@gmail.com>
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>
Subject: Re: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT
Date: Wed, 8 Nov 2017 08:59:22 -0800	[thread overview]
Message-ID: <CAMmz+Y=eq=S+gZaRefVUrfB7LDRVfD5UdpkfQXS0zvnHdNt0XA@mail.gmail.com> (raw)
In-Reply-To: <20171108155741.GA12627@krava>

On Wed, Nov 8, 2017 at 7:57 AM, Jiri Olsa <jolsa@redhat.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@redhat.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