Messages in this thread | | | Subject | Re: [PATCH] memory ordering in __kfifo primitives | From | Stelian Pop <> | Date | Thu, 10 Aug 2006 18:23:04 +0200 |
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Le jeudi 10 août 2006 à 09:11 -0700, Paul E. McKenney a écrit : > On Thu, Aug 10, 2006 at 05:47:22PM +0200, Stelian Pop wrote: > > Le jeudi 10 août 2006 à 08:39 -0700, Paul E. McKenney a écrit : > > > 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, > > > > ... and the internal lock can be supplied by the user at kfifo_alloc() > > time. > > Would that really work for them? Looks to me like it would result > in self-deadlock if they passed in session->lock.
Yeah, it will deadlock if the lock is already taken before calling __kfifo_get and __kfifo_put.
> Or did you have something else in mind for them?
What I had in mind is to replace all occurences of: kfifo_alloc(..., NULL); ... spin_lock(&session->lock) __kfifo_get() spin_unlock()
with the simpler: kfifo_alloc(..., &session->lock) ... kfifo_get()
As for the occurences of: ... spin_lock(&session->lock) do_something(); __kifo_get();
well, there is not much we can do about them...
Let's take this problem differently: is a memory barrier cheaper than a spinlock ?
If the answer is yes as I suspect, why should the kfifo API force the user to take a spinlock ?
Stelian. -- Stelian Pop <stelian@popies.net>
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