Re: Are PRRWLock read locks reentrant?

On 11/18/2008 04:49 PM, Wan-Teh Chang wrote:
> On Tue, Nov 18, 2008 at 1:39 PM, Nathan Kinder<[email protected]>  wrote:
>> I know that a PRLock is not reentrant, but is getting a read lock on a
>> PRRWLock using PR_RWLock_Rlock() reentrant?
>
> I checked the implementation.  The answer is Yes, it is reentrant.
> This is reasonable because a reader-writer lock allows multiple
> readers, including a reader acquiring it multiple times.  See also
> the pthread_rwlock_rdlock man page at
> http://opengroup.org/onlinepubs/007908799/xsh/pthread_rwlock_rdlock.html
> which says:
>
>    A thread may hold multiple concurrent read locks on rwlock (that is,
>    successfully call the pthread_rwlock_rdlock() function n times). If so,
>    the thread must perform matching unlocks (that is, it must call the
>    pthread_rwlock_unlock() function n times).
>
> Wan-Teh

Sorry to bring up such an old message thread, but an issue has come up 
related to re-entrant use of PR_RWLock_Rlock().

The way PR_RWLock works is that a waiting writer will block any threads
attempting to get a new read lock.  If you use read locks in a 
re-entrant manor, a request for a write lock between the two read lock 
calls will cause ad eadlock (the writer is waiting for the reader to 
exit, and the reader can'tget the re-entrant lock since the writer is 
waiting).  I am running into this deadlock in my application.

I'd like to propose that we modify PR_RWLock to behave differently when 
a re-entrant read lock is made.  If a thread already holds a read lock 
and tries to get another read lock, this should be allowed, even if a 
writer is waiting on the write lock.  Any other threads attempting to 
get a read lock will have to wait on the writer since it is given 
priority.  This approach would prevent the writer from being starved due 
to many active readers, yet it would also allow for safe re-entrant use 
of read locks without chance of a deadlock.

Does the above proposal sound feasible?

Thanks,
-NGK