Re: furwocks: Fast Userspace Read/Write Locks

["Peter Wächtler" <pwaechtler@loewe-komp.de>]

Rusty Russell wrote:

> In message <20020307153228.3A6773FE06@smtp.linux.ibm.com> you write:
> 
>>On Thursday 07 March 2002 07:50 am, Arjan van de Ven wrote:
>>
>>>Rusty Russell wrote:
>>>
>>>>This is a userspace implementation of rwlocks on top of futexes.
>>>>
>>>question: if rwlocks aren't actually slower in the fast path than
>>>futexes,
>>>would it make sense to only do the rw variant and in some userspace
>>>layer
>>>map "traditional" semaphores to write locks ?
>>>Saves half the implementation and testing....
>>>
>>I m not in favor of that. The dominant lock will be mutexes.
>>
> 
> To clarify: I'd love this, but rwlocks in the kernel aren't even
> vaguely fair.  With a steady stream of overlapping readers, a writer
> will never get the lock.
> 
> Hope that clarifies,


But you talk about the current implementation, don't you?
Is there something to prevent an implementation of rwlocks in the
kernel, where a wrlock will lock (postponed) further rdlock requests?

I mean: the wrlocker prevents newly rdlocks to succeed and waits for the
current rdlockers to release the lock an then gets the lock..