Message-ID: <51BBE14D.2050605@hp.com>
Date: Fri, 14 Jun 2013 23:36:45 -0400
From: Waiman Long <waiman.long@hp.com>
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
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        Ingo Molnar <mingo@elte.hu>, ????????? <laijs@cn.fujitsu.com>,
        Dipankar Sarma <dipankar@in.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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Subject: Re: [PATCH RFC ticketlock] Auto-queued ticketlock
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On 06/14/2013 09:26 PM, Benjamin Herrenschmidt wrote:
> On Fri, 2013-06-14 at 14:17 -0400, Waiman Long wrote:
>> With some minor changes, the current patch can be modified to support
>> debugging lock for 32-bit system. For 64-bit system, we can apply a
>> similar concept for debugging lock with cmpxchg_double. However, for
>> architecture that does not have cmpxchg_double support, it will be out
>> of luck and we probably couldn't support the same feature in debugging
>> mode. It will have to fall back to taking the lock.
> That means only x86_64 and s390 would benefit from it ... I'm sure we can do better :-)
>
> Cheers,
> Ben.

On second thought, using cmpxchg_double may not be such a good idea 
after all as it requires a 16-byte alignment, at least for x86-64. 
Another possible alternative is to integrate the reference count 
directly into the spinlock_t data structure immediately after 
arch_spinlock_t for this special case. If CONFIG_GENERIC_LOCKBREAK is 
not defined, there will be a 4-byte hole that can be used. Otherwise, 
the spinlock_t structure will have an 8 byte size increase. I suppose 
that others won't be too upset for an 8-byte increase in size when 
spinlock debugging is turned on.

Regards,
Longman
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