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@xxxxxxxxxx> 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@xxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
