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On Thu, 2012-03-08 at 16:44 -0500, Steven Rostedt wrote: > On Thu, 2012-03-08 at 22:37 +0100, Peter Zijlstra wrote: > > > > Now when the original task releases the lock again, the other task can > > > take it just like it does on mainline. > > > > Now interleave it with a third task of even higher priority that puts > > the spinner to sleep. > > So? It will eventually have to allow the task to run. Adding a "third > higher priority" task can cause problems in any other part of the -rt > kernel. > > We don't need to worry about priority inversion. If the higher task > blocks on the original task, it will boost its priority (even if it does > the adaptive spin) which will again boost the task that it preempted. > > Now we may need to add a sched_yield() in the adaptive spin to let the > other task run. That's not what I mean,.. task-A (cpu0) task-B (cpu1) task-C (cpu1) lock ->d_lock lock ->i_lock lock ->d_lock <-------------- preempts B trylock ->i_lock While is is perfectly normal, the result is that A stops spinning and goes to sleep. Now B continues and loops ad infinitum because it keeps getting ->d_lock before A because its cache hot on cpu1 and waking A takes a while etc.. No progress guarantee -> fail. Test-and-set spinlocks have unbounded latency and we've hit pure starvation cases in mainline. In fact it was so bad mainline had to grow ticket locks to cope -- we don't want to rely on anything like this in RT. -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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