s390/unwind: make reuse_sp default when unwinding pt_regs

Currently unwinder yields 2 entries when pt_regs are met:
sp="address of pt_regs itself" ip=pt_regs->psw
sp=pt_regs->gprs[15] ip="r14 from stack frame pointed by pt_regs->gprs[15]"

And neither of those 2 states (combination of sp and ip) ever happened.

reuse_sp has been introduced by commit a1d863ac ("s390/unwind: fix
mixing regs and sp"). reuse_sp=true makes unwinder keen to produce the
following result, when pt_regs are given (as an arg to unwind_start):
sp=pt_regs->gprs[15] ip=pt_regs->psw
sp=pt_regs->gprs[15] ip="r14 from stack frame pointed by pt_regs->gprs[15]"

The first state is an actual state in which a task was when pt_regs were
collected. The second state is marked unreliable and is for debugging
purposes to cover the case when a task has been interrupted in between
stack frame allocation and writing back_chain - in this case r14 might
show an actual caller.

Make unwinder behaviour enabled via reuse_sp=true default and drop the
special case handling.
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/include/asm/unwind.h

@@ -35,7 +35,6 @@ struct unwind_state {
 	struct task_struct *task;
 	struct pt_regs *regs;
 	unsigned long sp, ip;
-	bool reuse_sp;
 	int graph_idx;
 	bool reliable;
 	bool error;

arch/s390/kernel/unwind_bc.c

@@ -46,16 +46,7 @@ bool unwind_next_frame(struct unwind_state *state)
 
 	regs = state->regs;
 	if (unlikely(regs)) {
-		if (state->reuse_sp) {
-			sp = state->sp;
-			state->reuse_sp = false;
-		} else {
-			sp = READ_ONCE_NOCHECK(regs->gprs[15]);
-			if (unlikely(outside_of_stack(state, sp))) {
-				if (!update_stack_info(state, sp))
-					goto out_err;
-			}
-		}
+		sp = state->sp;
 		sf = (struct stack_frame *) sp;
 		ip = READ_ONCE_NOCHECK(sf->gprs[8]);
 		reliable = false;
@@ -81,6 +72,11 @@ bool unwind_next_frame(struct unwind_state *state)
 			if (READ_ONCE_NOCHECK(regs->psw.mask) & PSW_MASK_PSTATE)
 				goto out_stop;
 			ip = READ_ONCE_NOCHECK(regs->psw.addr);
+			sp = READ_ONCE_NOCHECK(regs->gprs[15]);
+			if (unlikely(outside_of_stack(state, sp))) {
+				if (!update_stack_info(state, sp))
+					goto out_err;
+			}
 			reliable = true;
 		}
 	}
@@ -107,7 +103,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 {
 	struct stack_info *info = &state->stack_info;
 	unsigned long *mask = &state->stack_mask;
-	bool reliable, reuse_sp;
+	bool reliable;
 	struct stack_frame *sf;
 	unsigned long ip;
 
@@ -134,12 +130,10 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	if (regs) {
 		ip = READ_ONCE_NOCHECK(regs->psw.addr);
 		reliable = true;
-		reuse_sp = true;
 	} else {
 		sf = (struct stack_frame *) sp;
 		ip = READ_ONCE_NOCHECK(sf->gprs[8]);
 		reliable = false;
-		reuse_sp = false;
 	}
 
 	ip = ftrace_graph_ret_addr(state->task, &state->graph_idx, ip, NULL);
@@ -148,6 +142,5 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	state->sp = sp;
 	state->ip = ip;
 	state->reliable = reliable;
-	state->reuse_sp = reuse_sp;
 }
 EXPORT_SYMBOL_GPL(__unwind_start);