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To: "Vineet Gupta" <vineetg76@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: Usage semantics of atomic_set ( )
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From: Roland Dreier <rdreier@cisco.com>
Date: Fri, 11 Jan 2008 15:16:40 -0800
In-Reply-To: <9f4f8abe0801111457t7535069ar1f98cc37d5cf8d40@mail.gmail.com> (Vineet Gupta's message of "Fri, 11 Jan 2008 14:57:47 -0800")
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 > I'm trying to implement atomic ops for a CPU which has no inherent
 > support for Read-Modify-Write Ops. Instead of using a global spin lock
 > which protects all the atomic APIs, I want to use a spin lock per
 > instance of atomic_t. This works well when atomic_t is unitary and
 > statically initialized using ATOMIC_INIT (where I can reset the
 > spinlock_t as well). However if atomic_t var is embedded within a
 > bigger struct which is allocated dynamically how to I init the
 > embedded spin lock. atomic_set ( ) is the closest choice, however I
 > don't think it's current usage in kernel code qualifies it to be
 > "initializer only".

A simple way to handle this might be to use a separate array of
spinlocks and hash each atomic_t to one entry in the array.  You could
look in asm-parisc and arch/parisc to see an implementation of this
that is already in the kernel.

 - R.