MIPS: Fix race condition with FPU thread task flag during context switch.

[ralf@linux-mips.org: Cosmetic changes; also fixed up r2300_switch.S and
octeon_switch.S which needed similar modifications.]

Signed-off-by: Leonid Yegoshin <yegoshin@mips.com>
Signed-off-by: Steven J. Hill <sjhill@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3784/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/switch_to.h

@@ -22,7 +22,7 @@ struct task_struct;
  * switch_to(n) should switch tasks to task nr n, first
  * checking that n isn't the current task, in which case it does nothing.
  */
-extern asmlinkage void *resume(void *last, void *next, void *next_ti);
+extern asmlinkage void *resume(void *last, void *next, void *next_ti, u32 __usedfpu);
 
 extern unsigned int ll_bit;
 extern struct task_struct *ll_task;
@@ -66,11 +66,13 @@ do {									\
 
 #define switch_to(prev, next, last)					\
 do {									\
+	u32 __usedfpu;							\
 	__mips_mt_fpaff_switch_to(prev);				\
 	if (cpu_has_dsp)						\
 		__save_dsp(prev);					\
 	__clear_software_ll_bit();					\
-	(last) = resume(prev, next, task_thread_info(next));		\
+	__usedfpu = test_and_clear_tsk_thread_flag(prev, TIF_USEDFPU);	\
+	(last) = resume(prev, next, task_thread_info(next), __usedfpu);	\
 } while (0)
 
 #define finish_arch_switch(prev)					\

arch/mips/kernel/octeon_switch.S

@@ -31,7 +31,7 @@
 
 /*
  * task_struct *resume(task_struct *prev, task_struct *next,
- *                     struct thread_info *next_ti)
+ *                     struct thread_info *next_ti, int usedfpu)
  */
 	.align	7
 	LEAF(resume)

arch/mips/kernel/r2300_switch.S

@@ -43,7 +43,7 @@
 
 /*
  * task_struct *resume(task_struct *prev, task_struct *next,
- *                     struct thread_info *next_ti) )
+ *                     struct thread_info *next_ti, int usedfpu)
  */
 LEAF(resume)
 	mfc0	t1, CP0_STATUS
@@ -51,18 +51,9 @@ LEAF(resume)
 	cpu_save_nonscratch a0
 	sw	ra, THREAD_REG31(a0)
 
-	/*
-	 * check if we need to save FPU registers
-	 */
-	lw	t3, TASK_THREAD_INFO(a0)
-	lw	t0, TI_FLAGS(t3)
-	li	t1, _TIF_USEDFPU
-	and	t2, t0, t1
-	beqz	t2, 1f
-	nor	t1, zero, t1
+	beqz	a3, 1f
 
-	and	t0, t0, t1
-	sw	t0, TI_FLAGS(t3)
+	PTR_L	t3, TASK_THREAD_INFO(a0)
 
 	/*
 	 * clear saved user stack CU1 bit

arch/mips/kernel/r4k_switch.S

@@ -41,7 +41,7 @@
 
 /*
  * task_struct *resume(task_struct *prev, task_struct *next,
- *                     struct thread_info *next_ti)
+ *                     struct thread_info *next_ti, int usedfpu)
  */
 	.align	5
 	LEAF(resume)
@@ -53,16 +53,10 @@
 	/*
 	 * check if we need to save FPU registers
 	 */
+
+	beqz    a3, 1f
+
 	PTR_L	t3, TASK_THREAD_INFO(a0)
-	LONG_L	t0, TI_FLAGS(t3)
-	li	t1, _TIF_USEDFPU
-	and	t2, t0, t1
-	beqz	t2, 1f
-	nor	t1, zero, t1
-
-	and	t0, t0, t1
-	LONG_S	t0, TI_FLAGS(t3)
-
 	/*
 	 * clear saved user stack CU1 bit
 	 */