[PATCH] Infrastructure for atomic user accesses

Well the optimum solution there would be to create and use
`inc_preempt_count_non_preempt()'.  I don't see any
way of embedding this in kmap_atomic() or copy_to_user_atomic()
without loss of flexibility or incurring a double-inc somewhere.

arch/i386/mm/fault.c

@@ -181,10 +181,10 @@ asmlinkage void do_page_fault(struct pt_regs *regs, unsigned long error_code)
 	info.si_code = SEGV_MAPERR;
 
 	/*
-	 * If we're in an interrupt or have no user
-	 * context, we must not take the fault..
+	 * If we're in an interrupt, have no user context or are running in an
+	 * atomic region then we must not take the fault..
 	 */
-	if (in_interrupt() || !mm)
+	if (preempt_count() || !mm)
 		goto no_context;
 
 	down_read(&mm->mmap_sem);

include/linux/preempt.h

@@ -5,19 +5,29 @@
 
 #define preempt_count() (current_thread_info()->preempt_count)
 
+#define inc_preempt_count() \
+do { \
+	preempt_count()++; \
+} while (0)
+
+#define dec_preempt_count() \
+do { \
+	preempt_count()--; \
+} while (0)
+
 #ifdef CONFIG_PREEMPT
 
 extern void preempt_schedule(void);
 
 #define preempt_disable() \
 do { \
-	preempt_count()++; \
+	inc_preempt_count(); \
 	barrier(); \
 } while (0)
 
 #define preempt_enable_no_resched() \
 do { \
-	preempt_count()--; \
+	dec_preempt_count(); \
 	barrier(); \
 } while (0)
 
@@ -34,6 +44,9 @@ do { \
 		preempt_schedule(); \
 } while (0)
 
+#define inc_preempt_count_non_preempt()	do { } while (0)
+#define dec_preempt_count_non_preempt()	do { } while (0)
+
 #else
 
 #define preempt_disable()		do { } while (0)
@@ -41,6 +54,13 @@ do { \
 #define preempt_enable()		do { } while (0)
 #define preempt_check_resched()		do { } while (0)
 
+/*
+ * Sometimes we want to increment the preempt count, but we know that it's
+ * already incremented if the kernel is compiled for preemptibility.
+ */
+#define inc_preempt_count_non_preempt()	inc_preempt_count()
+#define dec_preempt_count_non_preempt()	dec_preempt_count()
+
 #endif
 
 #endif /* __LINUX_PREEMPT_H */