Date: Mon, 17 Mar 2003 03:08:40 +1100 From: Darren Reed <darrenr@reed.wattle.id.au> To: David Xu <davidxu@FreeBSD.org> Cc: John Baldwin <jhb@FreeBSD.org>, cvs-all@FreeBSD.org, cvs-src@FreeBSD.org, src-committers@FreeBSD.org Subject: Re: cvs commit: src/lib/libpthread/thread thr_rwlock.c Message-ID: <200303161608.DAA02164@avalon.reed.wattle.id.au> In-Reply-To: <200303150347.h2F3lLjK069888@freefall.freebsd.org>
next in thread | previous in thread | raw e-mail | index | archive | help
So what you're saying is this design prevents bad code being written.
And the problem is ?
In some email I received from David Xu, sie wrote:
> This design prevents a thread to get a reader lock recursively when
> there is a writter blocked on a rwlock.
>
>
> ----- Original Message -----
> From: "John Baldwin" <jhb@FreeBSD.org>
> To: "John Baldwin" <jhb@FreeBSD.org>
> Cc: <src-committers@FreeBSD.org>; <cvs-src@FreeBSD.org>; <cvs-all@FreeBSD.org>; "David Xu" <davidxu@FreeBSD.org>
> Sent: Saturday, March 15, 2003 3:31 AM
> Subject: RE: cvs commit: src/lib/libpthread/thread thr_rwlock.c
>
>
> >
> > On 14-Mar-2003 John Baldwin wrote:
> > >
> > > On 14-Mar-2003 David Xu wrote:
> > >> davidxu 2003/03/13 17:02:47 PST
> > >>
> > >> FreeBSD src repository
> > >>
> > >> Modified files:
> > >> lib/libpthread/thread thr_rwlock.c
> > >> Log:
> > >> Fix a bug in rwlock. When a rwlock was locked by reader threads, a
> > >> writter thread can block reader threads to get read lock.
> > >
> > > That's not a bug. That is a very common way of implementing reader
> > > writer locks. The idea is that if a writer is waiting for the lock
> > > you make later read requests wait for the lock so that they don't
> > > starve the writer. This is how Solaris rw locks work for example.
> > > The in-kernel sx locks don't currently work that way, but that
> > > may change at some point in the future. For more discussion on why
> > > Solaris chose this route, go find a copy of Solaris Internals.
> > >
> > > You probably should revert this and find out if this was an
> > > intentional design decision rather than a "bug".
> >
> > Looking at the diff a bit more:
> >
> > @@ -157,7 +157,7 @@
> > return(ret);
> >
> > /* give writers priority over readers */
> > - while (prwlock->blocked_writers || prwlock->state < 0) {
> > + while (prwlock->state < 0) {
> > ret = pthread_cond_wait(&prwlock->read_signal, &prwlock->lock);
> >
> > if (ret != 0) {
> >
> > The comment above the while loop seems to indicate that this was
> > indeed a design choice. As a result of this change the comment no
> > longer applies. Please revert.
> >
> > --
> >
> > John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/
> > "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
>
>
> .
>
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe cvs-all" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200303161608.DAA02164>