MIPS: ptrace: Avoid smp_processor_id() in preemptible code

ptrace_{get,set}_watch_regs access current_cpu_data to get the watch
register count/masks, which calls smp_processor_id(). However they are
run in preemptible context and therefore trigger warnings like so:

[ 6340.092000] BUG: using smp_processor_id() in preemptible [00000000] code: gdb/367
[ 6340.092000] caller is ptrace_get_watch_regs+0x44/0x220

Since the watch register count/masks should be the same across all
CPUs, use boot_cpu_data instead. Note that this may need to change in
future should a heterogenous system be supported where the count/masks
are not the same across all CPUs (the current code is also incorrect
for this scenario - current_cpu_data here would not necessarily be
correct for the CPU that the target task will execute on).

Signed-off-by: Alex Smith <[e-mail]>
Reviewed-by: Paul Burton <[e-mail]>
Cc: [e-mail]
Patchwork: https://patchwork.linux-mips.org/patch/6879/
Signed-off-by: Ralf Baechle <[e-mail]>

arch/mips/kernel/ptrace.c

···
 	enum pt_watch_style style;
 	int i;
 
-	if (!cpu_has_watch || current_cpu_data.watch_reg_use_cnt == 0)
+	if (!cpu_has_watch || boot_cpu_data.watch_reg_use_cnt == 0)
 		return -EIO;
 	if (!access_ok(VERIFY_WRITE, addr, sizeof(struct pt_watch_regs)))
 		return -EIO;
···
 #endif
 
 	__put_user(style, &addr->style);
-	__put_user(current_cpu_data.watch_reg_use_cnt,
+	__put_user(boot_cpu_data.watch_reg_use_cnt,
 		   &addr->WATCH_STYLE.num_valid);
-	for (i = 0; i < current_cpu_data.watch_reg_use_cnt; i++) {
+	for (i = 0; i < boot_cpu_data.watch_reg_use_cnt; i++) {
 		__put_user(child->thread.watch.mips3264.watchlo[i],
 			   &addr->WATCH_STYLE.watchlo[i]);
 		__put_user(child->thread.watch.mips3264.watchhi[i] & 0xfff,
 			   &addr->WATCH_STYLE.watchhi[i]);
-		__put_user(current_cpu_data.watch_reg_masks[i],
+		__put_user(boot_cpu_data.watch_reg_masks[i],
 			   &addr->WATCH_STYLE.watch_masks[i]);
 	}
 	for (; i < 8; i++) {
···
 	unsigned long lt[NUM_WATCH_REGS];
 	u16 ht[NUM_WATCH_REGS];
 
-	if (!cpu_has_watch || current_cpu_data.watch_reg_use_cnt == 0)
+	if (!cpu_has_watch || boot_cpu_data.watch_reg_use_cnt == 0)
 		return -EIO;
 	if (!access_ok(VERIFY_READ, addr, sizeof(struct pt_watch_regs)))
 		return -EIO;
 	/* Check the values. */
-	for (i = 0; i < current_cpu_data.watch_reg_use_cnt; i++) {
+	for (i = 0; i < boot_cpu_data.watch_reg_use_cnt; i++) {
 		__get_user(lt[i], &addr->WATCH_STYLE.watchlo[i]);
 #ifdef CONFIG_32BIT
 		if (lt[i] & __UA_LIMIT)
···
 			return -EINVAL;
 	}
 	/* Install them. */
-	for (i = 0; i < current_cpu_data.watch_reg_use_cnt; i++) {
+	for (i = 0; i < boot_cpu_data.watch_reg_use_cnt; i++) {
 		if (lt[i] & 7)
 			watch_active = 1;
 		child->thread.watch.mips3264.watchlo[i] = lt[i];