Support for "split lock" detection:

  - Atomic operations (lock prefixed instructions) which span two cache
    lines have to acquire the global bus lock. This is at least 1k cycles
    slower than an atomic operation within a cache line and disrupts
    performance on other cores. Aside of performance disruption this is
    a unpriviledged form of DoS.

    Some newer CPUs have the capability to raise an #AC trap when such an
    operation is attempted. The detection is by default enabled in warning
    mode which will warn once when a user space application is caught. A
    command line option allows to disable the detection or to select fatal
    mode which will terminate offending applications with SIGBUS.
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