McKenney: Stupid RCU Tricks: rcutorture Catches an RCU Bug

On his blog, Paul McKenney investigates a bug in read-copy update (RCU) in preparation for the 3.19 merge window. "Of course, we all have specific patches that we are suspicious of. So my next step was to revert suspect patches and to otherwise attempt to outguess the bug. Unfortunately, I quickly learned that the bug is difficult to reproduce, requiring something like 100 hours of focused rcutorture testing. Bisection based on 100-hour tests would have consumed the remainder of 2014 and a significant fraction of 2015, so something better was required. In fact, something way better was required because there was only a very small number of failures, which meant that the expected test time to reproduce the bug might well have been 200 hours or even 300 hours instead of my best guess of 100 hours."

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