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[Tim Gardner <tim.gardner@xxxxxxxxxxxxx>]

** Also affects: linux (Ubuntu Trusty)
   Importance: Undecided
       Status: New

** Also affects: linux (Ubuntu Utopic)
   Importance: Medium
       Status: Triaged

** Changed in: linux (Ubuntu Trusty)
       Status: New => Fix Committed

** Changed in: linux (Ubuntu Trusty)
     Assignee: (unassigned) => Craig Magina (craig.magina)

** Changed in: linux (Ubuntu Utopic)
       Status: Triaged => In Progress

** Changed in: linux (Ubuntu Utopic)
    Milestone: None => ubuntu-14.10

** Changed in: linux (Ubuntu Utopic)
     Assignee: (unassigned) => Craig Magina (craig.magina)

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https://bugs.launchpad.net/bugs/1359514

Title:
  pci-xgene-msi: fixed deadlock in irq_set_affinity

Status in “linux” package in Ubuntu:
  In Progress
Status in “linux” source package in Trusty:
  Fix Committed
Status in “linux” source package in Utopic:
  In Progress

Bug description:
  [IMPACT]
  Issue:  CPU affinity is changed while irqbalance is running.

      + Problem explanation:

        - Old code

          + Call xgene_msi_cascade function (CPU x)

          + raw_spin_lock(&desc->lock);  (CPU x)

          + Goto generic_handle_irq (CPU x)

          + The CPU x doesn't have a chance to exit the
  xgene_msi_cascade function to unlock desc->lock before Linux scheduce
  and call xgene_msi_set_affinity (irqbalance is caller) in the same CPU
  x

          + In irq_set_affinity, call raw_spin_lock_irqsave(&desc->lock,
  flags) which cause deadlock to CPU x because it disables preempt

        - New code

          + Use chained_irq_enter and exit as the standard way to
  cascade interrupt functions.

  [TEST CASE]
  Turn off irqbalance
  Run a single tcp stream
  Randomly change the affinity of the receiving ring:

  @ ethtool -S $INTF | grep rx[0-9].*_pac @ - to detect the rx ring
  @ grep $INTF /proc/interrupts@ - to find it's interrupt
  @ printf "%x" $(( 2 ** $((RANDOM % 8)) )) > /proc/irq/$IRQ/smp_affinity @ - to change the affinity

  [Regression Potential]
  Fix specific to the xgene pci msi code.

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