__compute_return_epc() uses CFC1 instruction which might result in a

coprocessor unusable exception since the process can lose its fpu
context by preemption.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/branch.c

@@ -12,6 +12,7 @@
 #include <asm/branch.h>
 #include <asm/cpu.h>
 #include <asm/cpu-features.h>
+#include <asm/fpu.h>
 #include <asm/inst.h>
 #include <asm/ptrace.h>
 #include <asm/uaccess.h>
@@ -161,10 +162,13 @@ int __compute_return_epc(struct pt_regs *regs)
 	 * And now the FPA/cp1 branch instructions.
 	 */
 	case cop1_op:
-		if (!cpu_has_fpu)
-			fcr31 = current->thread.fpu.soft.fcr31;
-		else
+		preempt_disable();
+		if (is_fpu_owner())
 			asm volatile("cfc1\t%0,$31" : "=r" (fcr31));
+		else
+			fcr31 = current->thread.fpu.hard.fcr31;
+		preempt_enable();
+
 		bit = (insn.i_format.rt >> 2);
 		bit += (bit != 0);
 		bit += 23;

include/asm-mips/fpu.h

@@ -80,9 +80,14 @@ do {									\
 
 #define clear_fpu_owner()	clear_thread_flag(TIF_USEDFPU)
 
+static inline int __is_fpu_owner(void)
+{
+	return test_thread_flag(TIF_USEDFPU);
+}
+
 static inline int is_fpu_owner(void)
 {
-	return cpu_has_fpu && test_thread_flag(TIF_USEDFPU);
+	return cpu_has_fpu && __is_fpu_owner();
 }
 
 static inline void own_fpu(void)
@@ -127,7 +132,7 @@ static inline void restore_fp(struct task_struct *tsk)
 static inline fpureg_t *get_fpu_regs(struct task_struct *tsk)
 {
 	if (cpu_has_fpu) {
-		if ((tsk == current) && is_fpu_owner())
+		if ((tsk == current) && __is_fpu_owner())
 			_save_fp(current);
 		return tsk->thread.fpu.hard.fpr;
 	}