How priority propagation works on read/write lock?

[John Baldwin]

On Friday 13 January 2006 05:20 am, prime wrote:
> 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.

I think you just kind of punt and do a best effort.  Trying to manage a list 
of current read lock holders would be a bit PITA.

-- 
John Baldwin <jhb at FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org