Re: spinlock requests (was RE: [Xen-devel] [Patch] don't spin with irq disabled)

[Juergen Gross <juergen.gross@xxxxxxxxxxxxxxxxxxx>]

Dan Magenheimer wrote:
> Thanks Juergen.  Do you know of any GPLv2 code that implements
> this improved rwlock solution?  (I don't think Linux does,
> does it?)

Good question.
I just looked into the Linux code and decided not to analyse it. :-)
I have implemented a solution for our BS2000 system on Xen. It is just
a rather simple state machine using the cmpxchg instruction for the
update of the (structured) lock word.
If there is common interest for this solution I could prepare a patch.


Juergen

>> -----Original Message-----
>> From: Juergen Gross [mailto:juergen.gross@xxxxxxxxxxxxxxxxxxx]
>> Sent: Monday, March 30, 2009 12:12 AM
>> To: Dan Magenheimer
>> Cc: Keir Fraser; Jan Beulich; xen-devel@xxxxxxxxxxxxxxxxxxx
>> Subject: Re: spinlock requests (was RE: [Xen-devel] [Patch] don't spin
>> with irq disabled)
>>
>> if you are planning to use rw_locks you should be aware that 
>> the current
>> implementation in Xen is sub-optimal on systems with high 
>> processor counts.
>> Read locks always succeed when other readers are already 
>> holding the lock,
>> even if a writer is waiting for the lock. If there are many 
>> potential readers
>> they might (in theory) lock out a writer for rather long times.
>> A better solution would be to stop further readers to acquire 
>> the lock if a
>> writer is waiting for it.



-- 
Juergen Gross                             Principal Developer
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