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    SubjectRe: [PATCH] perf: Fix intel shared extra msr allocation
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    On Fri, Jun 1, 2012 at 5:35 PM, Stephane Eranian <eranian@google.com> wrote:
    > On Fri, Jun 1, 2012 at 5:20 AM, Yan, Zheng <zheng.z.yan@intel.com> wrote:
    >> From: "Yan, Zheng" <zheng.z.yan@intel.com>
    >>
    >> intel_shared_reg_get/put_constraints() can be indirectly called
    >> by validate_group(). In that case, they should avoid modifying
    >> the perf_event date structure because the event can be already
    >> in active state. Otherwise the shared extra msr's reference
    >> count will be left in inconsistent state.
    >>
    > I understand the problem but I am wondering if you actually saw
    > it in real life. The reason I am asking is because  of the way
    > validate_group() collects the events and how they are added
    > to sibling_list. The new event is added at the tail. Thus it will
    > come last, and will get to __intel_shared_reg_get_constraints()
    > last, thus I am wondering if it can really modify the programming
    > on the existing events.

    The real problem is from __intel_shared_reg_put_constraints(). it set
    reg->alloc to 0 and decreases fake_cpuc->shared_regs->regs[reg->idx]'s
    reference count. Later when deleting the event, put_constraints() will find
    reg->alloc is 0 and it won't decrease the shared msr's reference count.

    Run 'perf stat --group -a -C 0 -e LLC-loads -e LLC-stores sleep 1" on
    Nehalem can trigger the bug.

    >
    > See more comments inline.
    >
    >> Signed-off-by: Zheng Yan <zheng.z.yan@intel.com>
    >> ---
    >>  arch/x86/kernel/cpu/perf_event_intel.c |   31 +++++++++++++++++++++++--------
    >>  1 files changed, 23 insertions(+), 8 deletions(-)
    >>
    >> diff --git a/arch/x86/kernel/cpu/perf_event_intel.c b/arch/x86/kernel/cpu/perf_event_intel.c
    >> index 166546e..10840d0 100644
    >> --- a/arch/x86/kernel/cpu/perf_event_intel.c
    >> +++ b/arch/x86/kernel/cpu/perf_event_intel.c
    >> @@ -1119,11 +1119,21 @@ intel_bts_constraints(struct perf_event *event)
    >>        return NULL;
    >>  }
    >>
    >> -static bool intel_try_alt_er(struct perf_event *event, int orig_idx)
    >> +static bool intel_try_alt_er(struct perf_event *event, int *idx,
    >> +                            int orig_idx, bool fake_cpuc)
    >>  {
    >> -       if (!(x86_pmu.er_flags & ERF_HAS_RSP_1))
    >> +       if (!(x86_pmu.er_flags & ERF_HAS_RSP_1) || *idx != orig_idx)
    >>                return false;
    >>
    >> +       /* don't modify the event structure if the cpuc is faked */
    >> +       if (fake_cpuc) {
    >> +               if (*idx == EXTRA_REG_RSP_0)
    >> +                       *idx = EXTRA_REG_RSP_1;
    >> +               else if (*idx == EXTRA_REG_RSP_1)
    >> +                       *idx = EXTRA_REG_RSP_0;
    >> +               return (*idx != orig_idx);
    >> +       }
    >> +
    > I understand that.
    >
    >>        if (event->hw.extra_reg.idx == EXTRA_REG_RSP_0) {
    >>                event->hw.config &= ~INTEL_ARCH_EVENT_MASK;
    >>                event->hw.config |= 0x01bb;
    >> @@ -1139,6 +1149,7 @@ static bool intel_try_alt_er(struct perf_event *event, int orig_idx)
    >>        if (event->hw.extra_reg.idx == orig_idx)
    >>                return false;
    >>
    >> +       *idx = event->hw.extra_reg.idx;
    >>        return true;
    >>  }
    >>
    >> @@ -1155,16 +1166,18 @@ __intel_shared_reg_get_constraints(struct cpu_hw_events *cpuc,
    >>                                   struct hw_perf_event_extra *reg)
    >>  {
    >>        struct event_constraint *c = &emptyconstraint;
    >> +       struct intel_shared_regs *shared_regs = cpuc->shared_regs;
    >>        struct er_account *era;
    >>        unsigned long flags;
    >>        int orig_idx = reg->idx;
    >> +       int idx = orig_idx;
    >>
    >> -       /* already allocated shared msr */
    >> -       if (reg->alloc)
    >> +       /* shared msr is already allocated and cpuc is not faked */
    >> +       if (reg->alloc && shared_regs->core_id != -1)
    >>                return NULL; /* call x86_get_event_constraint() */
    >>
    > I don't understand what you need this stuff. Shared_regs is faked as well.

    The event can be in active state, we should avoid clearing reg->alloc.

    Regards
    Yan, Zheng
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