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Date: Thu, 19 Jul 2007 00:05:49 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Dave Airlie <airlied@linux.ie>
Cc: linux-kernel@vger.kernel.org
Subject: cmpxchg is not available to generic code
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arm:

drivers/char/drm/drm_lock.c: In function `drm_lock_take':
drivers/char/drm/drm_lock.c:221: error: implicit declaration of function `cmpxchg'

You might be able to use atomic_cmpxchg, which _is_ present
on all architectures.  Or use a spinlock.

What's that code doing anyway?  driver-private locking primitives?