Messages in this thread | | | Date | Sat, 29 Jun 2013 16:23:10 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless update of refcount |
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On 06/29/2013 01:45 PM, Linus Torvalds wrote: > Sorry for not commenting earlier, I was traveling and keeping email to > a minimum.. > > On Wed, Jun 26, 2013 at 10:43 AM, Waiman Long<Waiman.Long@hp.com> wrote: >> This patch introduces a new spinlock_refcount.h header file to be >> included by kernel code that want to do a lockless update of reference >> count protected by a spinlock. > So I really like the concept, but the implementation is a mess, and > tries to do too much, while actually achieving too little. > > I do not believe you should care about debug spinlocks at all, and > just leave them be. Have a simple fallback code that defaults to > regular counts and spinlocks, and have any debug cases just use that.
I was concern that people might want to have the same behavior even when spinlock debugging was on. Apparently, this is not really needed. Now I can just disable the optimization and fall back to the old path when spinlock debugging is on.
> But more importantly, I think this needs to be architecture-specific, > and using<linux/spinlock_refcount.h> to try to do some generic 64-bit > cmpxchg() version is a bad bad idea.
Yes, I can put the current implementation into asm-generic/spinlock_refcount.h. Now I need to put an asm/spinlock_refcount.h into every arch's include/asm directory. Right? I don't think there is a mechanism in the build script to create a symlink from asm to generic-asm when a header file is missing. Is it the general rule that we should have a linux/spinlock_refcount.h that include asm/spinlock_refcount.h instead of including asm/spinlock_refcount.h directly?
> We have several architectures coming up that have memory transaction > support, and the "spinlock with refcount" is a perfect candidate for a > transactional memory implementation. So when introducing a new atomic > like this that is very performance-critical and used for some very > core code, I really think architectures would want to make their own > optimized versions. > > These things should also not be inlined, I think. > > So I think the concept is good, but I think the implementation needs > more thought. > > Linus
Thank for the comment. I will try to come up with a version that is acceptable to all stakeholders.
Regards, Longman
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