That's an interesting concept. How could this actually be used
to protect a particular resource? Do all users of that
resource have to claim both the lock and the semaphore before
they may access it?
There are a number of locks (such as pagecache_lock) which in the
great majority of cases are held for a short period, but are
occasionally held for a long period. So these locks are not
a performance problem, they are not a scalability problem but
they *are* a worst-case-latency problem.
> I'm stating the obvious here, and re-iterating what you said, and that is that
> we need to carefully pick the correct primitive for the job. Unless there's
> something very unusual in the Linux implementation that I've missed, a
> spinlock is a "cheaper" method of protecting a short critical section, and
> should be chosen.
> I know the BKL is a semantically a little unusual (the automatic release on
> sleep stuff), but even so, isn't
> actually equivalent to
> If so, it's no great surprise that performance dropped given that we replaced
> a spinlock (albeit one guarding somewhat more than the critical section) with
> a semaphore.
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