[PATCH] powerpc: Avoid potential FP corruption with preempt and UP

Heikki Lindholm pointed out that there was a potential race with the
lazy CPU state (FP, VR, EVR) stuff if preempt is enabled.  The race
is that in the process of restoring FP state on sigreturn, the task
gets preempted by a user task that wants to use the FPU.  It will take
an FP unavailable exception, which will write the current FPU state
to the thread_struct, overwriting the values which sigreturn has
stored.  Note that this can only happen on UP since we don't implement
lazy CPU state on SMP.

The fix is to flush the lazy CPU state before updating the
thread_struct.  To do this we re-use the flush_lazy_cpu_state()
function from process.c.

Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/process.c

@@ -201,13 +201,13 @@ int dump_spe(struct pt_regs *regs, elf_vrregset_t *evrregs)
 }
 #endif /* CONFIG_SPE */
 
+#ifndef CONFIG_SMP
 /*
  * If we are doing lazy switching of CPU state (FP, altivec or SPE),
  * and the current task has some state, discard it.
  */
-static inline void discard_lazy_cpu_state(void)
+void discard_lazy_cpu_state(void)
 {
-#ifndef CONFIG_SMP
 	preempt_disable();
 	if (last_task_used_math == current)
 		last_task_used_math = NULL;
@@ -220,8 +220,8 @@ static inline void discard_lazy_cpu_state(void)
 		last_task_used_spe = NULL;
 #endif
 	preempt_enable();
-#endif /* CONFIG_SMP */
 }
+#endif /* CONFIG_SMP */
 
 int set_dabr(unsigned long dabr)
 {