git: kernel - Fix IPI signaling issue, add a few assertions

Matthew Dillon dillon at
Fri Nov 11 22:38:04 PST 2011

commit c17a6852f80ce59ea0641ae01d0b3c52f0584101
Author: Matthew Dillon <dillon at>
Date:   Fri Nov 11 22:31:27 2011 -0800

    kernel - Fix IPI signaling issue, add a few assertions
    * Change the low-level IPI code to physically disable interrupts when
      waiting for the ICR status bit to clear and issuing a new IPI.  It
      appears that on Intel cpus it is possible for (with circumstantial
      evidence) a LAPIC EOI to busy the command sequencer.
      Thus if interrupts are enabled inside a critical section, even if all
      they do is EOI the LAPIC and IRET, this can prevent an IPI from being
      sent if the interrupt occurs at just the right moment during an IPI
    * Because IPIs are already limited to one per target cpu at any given
      moment via gd->gd_npoll we can also do away with the ipiq polling
      code that was inside the ICR wait.
    * Add a few assertions to try to catch other possible MP problems.
      Assert that TDF_TSLEEPQ is not set when freeing a thread.
      Assert that the cpusync ipiq does not overflow.
      Assert that the vm_object hold count does not go negative.
    Reported-by: ftigeot

Summary of changes:
 sys/kern/lwkt_ipiq.c           |   10 ++++++
 sys/kern/lwkt_thread.c         |    3 +-
 sys/platform/pc64/apic/lapic.c |   64 ++++++++++++++++++++-------------------
 sys/vm/vm_object.c             |    1 +
 4 files changed, 46 insertions(+), 32 deletions(-)

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