Date: Fri, 14 Mar 2003 14:31:12 -0500 (EST) 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> Subject: RE: cvs commit: src/lib/libpthread/thread thr_rwlock.c Message-ID: <XFMail.20030314143112.jhb@FreeBSD.org> In-Reply-To: <XFMail.20030314142317.jhb@FreeBSD.org>
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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/
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