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    Subject[PATCH 5.10 085/103] powerpc/perf: Invoke per-CPU variable access with disabled interrupts
    Date
    From: Athira Rajeev <atrajeev@linux.vnet.ibm.com>

    commit f66de7ac4849eb42a7b18e26b8ee49e08130fd27 upstream.

    The power_pmu_event_init() callback access per-cpu variable
    (cpu_hw_events) to check for event constraints and Branch Stack
    (BHRB). Current usage is to disable preemption when accessing the
    per-cpu variable, but this does not prevent timer callback from
    interrupting event_init. Fix this by using 'local_irq_save/restore'
    to make sure the code path is invoked with disabled interrupts.

    This change is tested in mambo simulator to ensure that, if a timer
    interrupt comes in during the per-cpu access in event_init, it will be
    soft masked and replayed later. For testing purpose, introduced a
    udelay() in power_pmu_event_init() to make sure a timer interrupt arrives
    while in per-cpu variable access code between local_irq_save/resore.
    As expected the timer interrupt was replayed later during local_irq_restore
    called from power_pmu_event_init. This was confirmed by adding
    breakpoint in mambo and checking the backtrace when timer_interrupt
    was hit.

    Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
    Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/1606814880-1720-1-git-send-email-atrajeev@linux.vnet.ibm.com
    Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/powerpc/perf/core-book3s.c | 10 ++++++----
    1 file changed, 6 insertions(+), 4 deletions(-)

    --- a/arch/powerpc/perf/core-book3s.c
    +++ b/arch/powerpc/perf/core-book3s.c
    @@ -1884,7 +1884,7 @@ static bool is_event_blacklisted(u64 ev)
    static int power_pmu_event_init(struct perf_event *event)
    {
    u64 ev;
    - unsigned long flags;
    + unsigned long flags, irq_flags;
    struct perf_event *ctrs[MAX_HWEVENTS];
    u64 events[MAX_HWEVENTS];
    unsigned int cflags[MAX_HWEVENTS];
    @@ -1992,7 +1992,9 @@ static int power_pmu_event_init(struct p
    if (check_excludes(ctrs, cflags, n, 1))
    return -EINVAL;

    - cpuhw = &get_cpu_var(cpu_hw_events);
    + local_irq_save(irq_flags);
    + cpuhw = this_cpu_ptr(&cpu_hw_events);
    +
    err = power_check_constraints(cpuhw, events, cflags, n + 1);

    if (has_branch_stack(event)) {
    @@ -2003,13 +2005,13 @@ static int power_pmu_event_init(struct p
    event->attr.branch_sample_type);

    if (bhrb_filter == -1) {
    - put_cpu_var(cpu_hw_events);
    + local_irq_restore(irq_flags);
    return -EOPNOTSUPP;
    }
    cpuhw->bhrb_filter = bhrb_filter;
    }

    - put_cpu_var(cpu_hw_events);
    + local_irq_restore(irq_flags);
    if (err)
    return -EINVAL;


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