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    SubjectRe: [PATCH] perf/core: fast breakpoint modification via _IOC_MODIFY_BREAKPOINT
    On Sun, Nov 12, 2017 at 11:09:23AM -0800, Milind Chabbi wrote:
    > ,
    >
    > On Thu, Nov 9, 2017 at 10:59 AM, Milind Chabbi <chabbi.milind@gmail.com> wrote:
    > > SNIP
    > >
    > > On Thu, Nov 9, 2017 at 5:12 AM, Jiri Olsa <jolsa@redhat.com> wrote:
    > >>
    > >>
    > >> how about something like below (untested)
    > >>
    > >> looks like there's no irq caller for modify_user_hw_breakpoint,
    > >> so we should be fine with locking nr_bp_mutex
    > >>
    > >> jirka
    > >>
    > >>
    > >> ---
    > >> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
    > >> index 3f8cb1e14588..f062b68399ea 100644
    > >> --- a/kernel/events/hw_breakpoint.c
    > >> +++ b/kernel/events/hw_breakpoint.c
    > >> @@ -448,6 +448,8 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
    > >> else
    > >> perf_event_disable(bp);
    > >>
    > >> + release_bp_slot(bp);
    > >> +
    > >> bp->attr.bp_addr = attr->bp_addr;
    > >> bp->attr.bp_type = attr->bp_type;
    > >> bp->attr.bp_len = attr->bp_len;
    > >> @@ -455,9 +457,9 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
    > >> if (attr->disabled)
    > >> goto end;
    > >>
    > >> - err = validate_hw_breakpoint(bp);
    > >> + err = reserve_bp_slot(bp);
    > >> if (!err)
    > >> - perf_event_enable(bp);
    > >> + err = validate_hw_breakpoint(bp);
    > >>
    > >> if (err) {
    > >> bp->attr.bp_addr = old_addr;
    > >> @@ -469,6 +471,7 @@ int modify_user_hw_breakpoint(struct perf_event *bp, struct perf_event_attr *att
    > >> return err;
    > >> }
    > >>
    > >> + perf_event_enable(bp);
    > >> end:
    > >> bp->attr.disabled = attr->disabled;
    > >>
    > >
    > > We can do this accounting only if bp->attr.bp_type != attr->bp_type.
    > >
    > > -Milind
    >
    >
    > Jirka,
    >
    > Neither of us seems to fully understand the convoluted logic used in
    > breakpoint counting.

    yea, I was hoping some of the guys would take over ;-)

    the problem I have with the patch above is that we could
    fail to reserve the slot at the end, which is not what
    the caller might expect

    >
    > I tested the following sequence on an x86 machine, which has four
    > debug registers (without your suggested patch for counting
    > correction).
    >
    > fd1 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR1
    > fd2 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR2
    > fd3 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR3
    > fd4 = perf_event_open(...); //BP_TYPE= HW_BREAKPOINT_RW @ ADDR4
    > ioctl(fd4, MODIFY, ...); // change fd4 to BP_TYPE= HW_BREAKPOINT_X @ ADDR5
    > close(fd4);
    > fd5 = perf_event_open(); //BP_TYPE=RW @ ADDR6
    >
    > We expected fd5 to fail because four BP_TYPE=TYPE_DATA are in use as
    > per the accounting, but in reality, fd5 was successfully opened.

    but you closed fd4 before openning fd5..?

    >
    > Is the accounting accidentally working on x86?
    > Is there another architecture where TYPE_DATA and TYPE_INS are counted
    > differently?

    [jolsa@krava linux-perf]$ grep -r HAVE_MIXED_BREAKPOINTS_REGS arch/*
    arch/Kconfig:config HAVE_MIXED_BREAKPOINTS_REGS
    arch/sh/Kconfig: select HAVE_MIXED_BREAKPOINTS_REGS
    arch/x86/Kconfig: select HAVE_MIXED_BREAKPOINTS_REGS

    I'll try to check on it this week

    jirka

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