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@xxxxxx> 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
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
