[PATCH] ppc32: fix destroy_context() race condition

[Guillaume Autran <gautran@mrv.com>]

Fix for a race condition when a task gets preempted by another task
while executing the destroy_context(...) in a FEW_CONTEXTS environment.
mm->context == NO_CONTEXT but the context_map may indicate all contexts
are in use.
The solution to this problem is to disable kernel preemption while
destroying a MMU context.

Signed-off-by: Guillaume Autran <gautran@mrv.com>

---

diff -Nru a/include/asm-ppc/mmu_context.h b/include/asm-ppc/mmu_context.h
--- a/include/asm-ppc/mmu_context.h	2005-06-17 15:48:29.000000000 -0400
+++ b/include/asm-ppc/mmu_context.h	2005-07-05 08:58:46.000000000 -0400
@@ -149,6 +149,7 @@
 */
static inline void destroy_context(struct mm_struct *mm)
{
+	preempt_disable();
	if (mm->context != NO_CONTEXT) {
		clear_bit(mm->context, context_map);
		mm->context = NO_CONTEXT;
@@ -156,6 +157,7 @@
		atomic_inc(&nr_free_contexts);
#endif
	}
+	preempt_enable();
}

static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,