Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

[Peter Zijlstra]

On Tue, Oct 01, 2013 at 09:28:15AM +0200, Ingo Molnar wrote:
> That I mostly agree with, except that without a serious usecase do we have 
> a guarantee that bugs in fancies queueing in rwsems gets ironed out?

Methinks mmap_sem is still a big enough lock to work out a locking
primitive :-)

In fact, try something like this from userspace:

n-threads:

  pthread_mutex_lock(&mutex);
  foo = mmap();
  pthread_mutex_lock(&mutex);

  /* work */

  pthread_mutex_unlock(&mutex);
  munma(foo);
  pthread_mutex_unlock(&mutex);

vs

n-threads:

  foo = mmap();
  /* work */
  munmap(foo);


I've had reports that the former was significantly faster than the
latter.
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