Messages in this thread | | | Date | Thu, 10 Aug 2006 08:39:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] memory ordering in __kfifo primitives |
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On Thu, Aug 10, 2006 at 04:26:53PM +0200, Stelian Pop wrote: > Le jeudi 10 août 2006 à 06:41 -0700, Paul E. McKenney a écrit : > > > I am happy to go either way -- the patch with the memory barriers > > (which does have the side-effect of slowing down kfifo_get() and > > kfifo_put(), by the way), or a patch removing the comments saying > > that it is OK to invoke __kfifo_get() and __kfifo_put() without > > locking. > > > > Any other thoughts on which is better? (1) the memory barriers or > > (2) requiring the caller hold appropriate locks across calls to > > __kfifo_get() and __kfifo_put()? > > If someone wants to use explicit locking, he/she can go with kfifo_get() > instead of the __ version.
However, the kfifo_get()/kfifo_put() interfaces use the internal lock, which cannot be used by the caller to protect other code surrounding the call to kfifo_get()/kfifo_put(). See for example the ISCSI use, where they have a session->lock that, among other things, protects their __kfifo_get()/__kfifo_put() calls.
> I'd rather keep the __kfifo_get() and __kfifo_put() functions lockless, > so I say go for (1) even if there is a tiny price to pay for corectness.
If we require the caller to supply the locks for __kfifo_get() and __kfifo_put(), then we have -both- correctness -and- better performance. And the only current user of __kfifo_get()/__kfifo_put() stated that they could easily expand their session->lock to cover all such calls, and that doing so would not hurt their performance.
So, are you sure? And if so, why?
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