Lets say time-spent-outside-spinlock == time-spent-in-spinlock and
number-of-cpus == 2.
1 < 2 , so it should livelock according to you...
There is off-by-one bug in the condition. It should be:
(time_spent_in_spinlock + time_spent_outside_spinlock) /
time_spent_in_spinlock < number_of_cpus
... or if you divide it by time_spent_in_spinlock:
time_spent_outside_spinlock / time_spent_in_spinlock + 1 < number_of_cpus
...but afaict this should work okay. Even if spinlocks are very
unfair, as long as time-outside and time-inside comes in big chunks,
it should work.
If you are unlucky, one cpu may stall for a while, but... I see no
livelock.
If some rogue threads (and it may not even be intetional) call the same
syscall stressing the one spinlock all the time, other syscalls needing
the same spinlock may stall.
Fortunately, they'll unstall with probability of 1... so no, I do not
think this is real problem.
If someone takes semaphore in syscall (we do), same problem may
happen, right...? Without need for 2048 cpus. Maybe semaphores/mutexes
are fair (or mostly fair) these days, but rwlocks may not be or
something.
Pavel-
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html