An idea of remove MUTEX_WAKE_ALL

[prime]

Hi hackers,
   I have an idea about remove the kernel option MUTEX_WAKE_ALL.
   When we unlock the mutex(in _mtx_unlock_sleep),we can directly
give the lock to the first thread waiting on the turnstile.And a
thread gets the mutex after he returned from turnstile_wait so he
can simply jump out the _obtain_lock loop in _mtx_lock_sleep.
This makes a mutex always be owned by a thread when there are threads
waiting on the turnstile,so priority inheritance can work now.
   This idea need only a few changes in kern/kern_mutex.c .But when
NO_ADAPTIVE_MUTEXS not set,it makes threads that spinning on other CPU
to get the mutex have to spin for a long time,and this makes the short
term mutex more expensive(maybe should use spin mutex instead).

What do think about the idea? Thanks.

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