Date: Fri, 13 Jan 2006 14:22:14 -0500 From: John Baldwin <jhb@freebsd.org> To: freebsd-hackers@freebsd.org Cc: prime <guomingyan@gmail.com>, "Kamal R. Prasad" <kamalp@acm.org> Subject: Re: How priority propagation works on read/write lock? Message-ID: <200601131422.15208.jhb@freebsd.org> In-Reply-To: <1fa17f810601130220h521590banff7d775a8bd4eaa6@mail.gmail.com> References: <1fa17f810601122232l25551bc5n4e4a01ff6b7921e@mail.gmail.com> <ac7deb50601130146y4abd573dn2ac47622374c1551@mail.gmail.com> <1fa17f810601130220h521590banff7d775a8bd4eaa6@mail.gmail.com>
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On Friday 13 January 2006 05:20 am, prime wrote: > On 1/13/06, Kamal R. Prasad <kamalp@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@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 > > > _______________________________________________ > > > freebsd-hackers@freebsd.org mailing list > > > http://lists.freebsd.org/mailman/listinfo/freebsd-hackers > > > To unsubscribe, send any mail to > > > "freebsd-hackers-unsubscribe@freebsd.org " > > 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@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
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