[PATCH] ppc: fix floating point register corruption

I recently discovered a bug on PPC which causes the floating point
registers to get corrupted when CONFIG_PREEMPT=y.

The problem occurred while running a multi threaded Java application that
does floating point.  The problem could be reproduced in anywhere from 2 to
6 hours.  With the patch I have included below it ran for over a week
without failure.
Signed-off-by: NPaolo Galtieri <pgaltieri@mvista.com>
Cc: Kumar Gala <galak@gate.crashing.org>
Cc: Matt Porter <mporter@kernel.crashing.org>
Cc: Tom Rini <trini@kernel.crashing.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

arch/ppc/kernel/process.c

@@ -417,6 +417,7 @@ void show_regs(struct pt_regs * regs)
 
 void exit_thread(void)
 {
+	preempt_disable();
 	if (last_task_used_math == current)
 		last_task_used_math = NULL;
 	if (last_task_used_altivec == current)
@@ -425,10 +426,12 @@ void exit_thread(void)
 	if (last_task_used_spe == current)
 		last_task_used_spe = NULL;
 #endif
+	preempt_enable();
 }
 
 void flush_thread(void)
 {
+	preempt_disable();
 	if (last_task_used_math == current)
 		last_task_used_math = NULL;
 	if (last_task_used_altivec == current)
@@ -437,6 +440,7 @@ void flush_thread(void)
 	if (last_task_used_spe == current)
 		last_task_used_spe = NULL;
 #endif
+	preempt_enable();
 }
 
 void
@@ -535,6 +539,7 @@ void start_thread(struct pt_regs *regs, unsigned long nip, unsigned long sp)
 	regs->nip = nip;
 	regs->gpr[1] = sp;
 	regs->msr = MSR_USER;
+	preempt_disable();
 	if (last_task_used_math == current)
 		last_task_used_math = NULL;
 	if (last_task_used_altivec == current)
@@ -543,6 +548,7 @@ void start_thread(struct pt_regs *regs, unsigned long nip, unsigned long sp)
 	if (last_task_used_spe == current)
 		last_task_used_spe = NULL;
 #endif
+	preempt_enable();
 	memset(current->thread.fpr, 0, sizeof(current->thread.fpr));
 	current->thread.fpscr.val = 0;
 #ifdef CONFIG_ALTIVEC