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

[Jiri Olsa <jolsa@redhat.com>]

On Wed, Nov 08, 2017 at 08:59:22AM -0800, Milind Chabbi wrote:
> 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.

there's the CONFIG_HAVE_MIXED_BREAKPOINTS_REGS that puts DATA and INST
under one count on x86.. but that seems to be the enabled only for:

	arch/sh/Kconfig:        select HAVE_MIXED_BREAKPOINTS_REGS
	arch/x86/Kconfig:       select HAVE_MIXED_BREAKPOINTS_REGS

> 
> 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?

I'll leave this decision to maintainer ;-) but seems better to fix
the interface before we add any new dependent function calls

jirka