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Peter Zijlstra (Intel) authored
A split-lock occurs when an atomic instruction operates on data that spans two cache lines. In order to maintain atomicity the core takes a global bus lock. This is typically >1000 cycles slower than an atomic operation within a cache line. It also disrupts performance on other cores (which must wait for the bus lock to be released before their memory operations can complete). For real-time systems this may mean missing deadlines. For other systems it may just be very annoying. Some CPUs have the capability to raise an #AC trap when a split lock is attempted. Provide a command line option to give the user choices on how to handle this: split_lock_detect= off - not enabled (no traps for split locks) warn - warn once when an application does a split lock, but allow it to continue running. fatal - Send SIGBUS to applications that cause split lock On systems that support split lock detection the default is "warn". Note that if the kernel hits a split lock in any mode other than "off" it will OOPs. One implementation wrinkle is that the MSR to control the split lock detection is per-core, not per thread. This might result in some short lived races on HT systems in "warn" mode if Linux tries to enable on one thread while disabling on the other. Race analysis by Sean Christopherson: - Toggling of split-lock is only done in "warn" mode. Worst case scenario of a race is that a misbehaving task will generate multiple #AC exceptions on the same instruction. And this race will only occur if both siblings are running tasks that generate split-lock #ACs, e.g. a race where sibling threads are writing different values will only occur if CPUx is disabling split-lock after an #AC and CPUy is re-enabling split-lock after *its* previous task generated an #AC. - Transitioning between off/warn/fatal modes at runtime isn't supported and disabling is tracked per task, so hardware will always reach a steady state that matches the configured mode. I.e. split-lock is guaranteed to be enabled in hardware once all _TIF_SLD threads have been scheduled out. Signed-off-by:Peter Zijlstra (Intel) <peterz@infradead.org> Co-developed-by:
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