How priority propagation works on read/write lock?

[prime]

Hi hackers,
   I have a question about how priority propagation works on
read/write lock.On locks that have only one owner at a determinate
moment,we can simply propagate the priority to the owner of lock,but
read/write lock may have many owners at some time,so how can we know
who are the owners?
   I browse the OpenSolaris' read/write lock implementation,and find
that, it simply treats the owner of the lock as NULL when readers own
the read/write lock.In this way,we can not propagate our priority to
all threads that block us.

Thanks very much.
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