How priority propagation works on read/write lock?

[prime]

On 1/13/06, Kamal R. Prasad <kamalp at acm.org> wrote:
>
> Priority need not be propagated to readers as they will not block other
> readers.
> Most likely, you only need to propagate to the writer to avoid priority
> inversron.
>
> regards
> -kamal
>
>
> On 1/13/06, prime <guomingyan at gmail.com> wrote:
>
> > 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.
> > --
> > Three passions, simple but overwhelmingly strong, have governed my life:
> >
> > the longing for love, the search for knowledge, and unbearable pity for
> > the suffering of mankind.
> >                         ---------Bertrand Russell
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> > "
> >
>
>
  Thanks your reply.
But readers may block writers, aren't they?
  For example, there are three threads,A,B and C, and a read/write lock
rwlock1 ,and a mutex mtx1.
1.A lock mtx1,
2.B get the read lock of rwlock1 and then want
  to get mtx1,but mtx1 is locked by A,so B has to
  wait on mtx1.
3.C want to get the write lock of rwlock1 and it
   has to wait,because rwlock1 is read locked by B.
Now if C's priority < A's priority(in numerical), then we get priority
inversion.

How can avoid this priority inversion?
Thanks.

--
Three passions, simple but overwhelmingly strong, have governed my life:
the longing for love, the search for knowledge, and unbearable pity for
the suffering of mankind.
                         ---------Bertrand Russell