[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: Paolo Galtieri <[e-mail]>
Cc: Kumar Gala <[e-mail]>
Cc: Matt Porter <[e-mail]>
Cc: Tom Rini <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/ppc/kernel/process.c

···
 
 void exit_thread(void)
 {
+	preempt_disable();
 	if (last_task_used_math == current)
 		last_task_used_math = NULL;
 	if (last_task_used_altivec == current)
···
 	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)
···
 	if (last_task_used_spe == current)
 		last_task_used_spe = NULL;
 #endif
+	preempt_enable();
 }
 
 void
···
 	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)
···
 	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