[PATCH] Add the pthread_rwlock_unlock() different point explanation between Linux and POSIX

[Zhang Xiliang <zhangxiliang-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>]

I tested the unlock order of rwlock in Linux with SCHED_FIFO and SCHED_RR, and found 
that when readers blocked has higher priority than writers, writers always shall 
take precedence over readers.

A write process                         <--unlock the rwlock
  ->A higher priority read process      <--block
  ->A lower priority write process      <--acquired rwlock

In pthread_rwlock_rdlock() manual, it says: If the Thread Execution Scheduling option is
supported, when threads executing with the scheduling policies SCHED_FIFO, SCHED_RR, or 
SCHED_SPORADIC are waiting on the lock, they shall acquire the lock in priority order
when the lock becomes available.

Signed-off-by: Zhang Xiliang <zhangxiliang-BthXqXjhjHXQFUHtdCDX3A@public.gmane.org>
---
 man3p/pthread_rwlock_unlock.3p |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

diff --git a/man3p/pthread_rwlock_unlock.3p b/man3p/pthread_rwlock_unlock.3p
index 3e726d0..4508536 100644
--- a/man3p/pthread_rwlock_unlock.3p
+++ b/man3p/pthread_rwlock_unlock.3p
@@ -49,6 +49,11 @@ locks.
 .LP
 Results are undefined if any of these functions are called with an
 uninitialized read-write lock.
+.SH DIFFERENCE BETWEEN LINUX and POSIX
+.LP
+In Linux, when threads executing with the scheduling policies SCHED_FIFO, 
+SCHED_RR are waiting on the block, write locks always shall take precedence 
+over read lockswhich have higher priority.
 .SH RETURN VALUE
 .LP
 If successful, the \fIpthread_rwlock_unlock\fP() function shall return

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