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    Subject[PATCH 5.13 112/800] powerpc/stacktrace: Fix spurious "stale" traces in raise_backtrace_ipi()
    Date
    From: Michael Ellerman <mpe@ellerman.id.au>

    commit 7c6986ade69e3c81bac831645bc72109cd798a80 upstream.

    In raise_backtrace_ipi() we iterate through the cpumask of CPUs, sending
    each an IPI asking them to do a backtrace, but we don't wait for the
    backtrace to happen.

    We then iterate through the CPU mask again, and if any CPU hasn't done
    the backtrace and cleared itself from the mask, we print a trace on its
    behalf, noting that the trace may be "stale".

    This works well enough when a CPU is not responding, because in that
    case it doesn't receive the IPI and the sending CPU is left to print the
    trace. But when all CPUs are responding we are left with a race between
    the sending and receiving CPUs, if the sending CPU wins the race then it
    will erroneously print a trace.

    This leads to spurious "stale" traces from the sending CPU, which can
    then be interleaved messily with the receiving CPU, note the CPU
    numbers, eg:

    [ 1658.929157][ C7] rcu: Stack dump where RCU GP kthread last ran:
    [ 1658.929223][ C7] Sending NMI from CPU 7 to CPUs 1:
    [ 1658.929303][ C1] NMI backtrace for cpu 1
    [ 1658.929303][ C7] CPU 1 didn't respond to backtrace IPI, inspecting paca.
    [ 1658.929362][ C1] CPU: 1 PID: 325 Comm: kworker/1:1H Tainted: G W E 5.13.0-rc2+ #46
    [ 1658.929405][ C7] irq_soft_mask: 0x01 in_mce: 0 in_nmi: 0 current: 325 (kworker/1:1H)
    [ 1658.929465][ C1] Workqueue: events_highpri test_work_fn [test_lockup]
    [ 1658.929549][ C7] Back trace of paca->saved_r1 (0xc0000000057fb400) (possibly stale):
    [ 1658.929592][ C1] NIP: c00000000002cf50 LR: c008000000820178 CTR: c00000000002cfa0

    To fix it, change the logic so that the sending CPU waits 5s for the
    receiving CPU to print its trace. If the receiving CPU prints its trace
    successfully then the sending CPU just continues, avoiding any spurious
    "stale" trace.

    This has the added benefit of allowing all CPUs to print their traces in
    order and avoids any interleaving of their output.

    Fixes: 5cc05910f26e ("powerpc/64s: Wire up arch_trigger_cpumask_backtrace()")
    Cc: stable@vger.kernel.org # v4.18+
    Reported-by: Nathan Lynch <nathanl@linux.ibm.com>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Link: https://lore.kernel.org/r/20210625140408.3351173-1-mpe@ellerman.id.au
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/powerpc/kernel/stacktrace.c | 26 ++++++++++++++++++++------
    1 file changed, 20 insertions(+), 6 deletions(-)

    --- a/arch/powerpc/kernel/stacktrace.c
    +++ b/arch/powerpc/kernel/stacktrace.c
    @@ -172,17 +172,31 @@ static void handle_backtrace_ipi(struct

    static void raise_backtrace_ipi(cpumask_t *mask)
    {
    + struct paca_struct *p;
    unsigned int cpu;
    + u64 delay_us;

    for_each_cpu(cpu, mask) {
    - if (cpu == smp_processor_id())
    + if (cpu == smp_processor_id()) {
    handle_backtrace_ipi(NULL);
    - else
    - smp_send_safe_nmi_ipi(cpu, handle_backtrace_ipi, 5 * USEC_PER_SEC);
    - }
    + continue;
    + }

    - for_each_cpu(cpu, mask) {
    - struct paca_struct *p = paca_ptrs[cpu];
    + delay_us = 5 * USEC_PER_SEC;
    +
    + if (smp_send_safe_nmi_ipi(cpu, handle_backtrace_ipi, delay_us)) {
    + // Now wait up to 5s for the other CPU to do its backtrace
    + while (cpumask_test_cpu(cpu, mask) && delay_us) {
    + udelay(1);
    + delay_us--;
    + }
    +
    + // Other CPU cleared itself from the mask
    + if (delay_us)
    + continue;
    + }
    +
    + p = paca_ptrs[cpu];

    cpumask_clear_cpu(cpu, mask);


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