kdb: Gid rid of implicit setting of the current task / regs

Some (but not all?) of the kdb backtrace paths would cause the
kdb_current_task and kdb_current_regs to remain changed.  As discussed
in a review of a previous patch [1], this doesn't seem intuitive, so
let's fix that.

...but, it turns out that there's actually no longer any reason to set
the current task / current regs while backtracing anymore anyway.  As
of commit 2277b49258 ("kdb: Fix stack crawling on 'running' CPUs
that aren't the master") if we're backtracing on a task running on a
CPU we ask that CPU to do the backtrace itself.  Linux can do that
without anything fancy.  If we're doing backtrace on a sleeping task
we can also do that fine without updating globals.  So this patch
mostly just turns into deleting a bunch of code.

[1] https://lore.kernel.org/r/20191010150735.dhrj3pbjgmjrdpwr@holly.lan

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20191109111624.4.Ibc3d982bbeb9e46872d43973ba808cd4c79537c7@changeid
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

kernel/debug/kdb/kdb_bt.c

@@ -119,7 +119,6 @@ kdb_bt_cpu(unsigned long cpu)
 		return;
 	}
 
-	kdb_set_current_task(kdb_tsk);
 	kdb_bt1(kdb_tsk, ~0UL, false);
 }
 
@@ -166,10 +165,8 @@ kdb_bt(int argc, const char **argv)
 		if (diag)
 			return diag;
 		p = find_task_by_pid_ns(pid, &init_pid_ns);
-		if (p) {
-			kdb_set_current_task(p);
+		if (p)
 			return kdb_bt1(p, ~0UL, false);
-		}
 		kdb_printf("No process with pid == %ld found\n", pid);
 		return 0;
 	} else if (strcmp(argv[0], "btt") == 0) {
@@ -178,11 +175,9 @@ kdb_bt(int argc, const char **argv)
 		diag = kdbgetularg((char *)argv[1], &addr);
 		if (diag)
 			return diag;
-		kdb_set_current_task((struct task_struct *)addr);
 		return kdb_bt1((struct task_struct *)addr, ~0UL, false);
 	} else if (strcmp(argv[0], "btc") == 0) {
 		unsigned long cpu = ~0;
-		struct task_struct *save_current_task = kdb_current_task;
 		if (argc > 1)
 			return KDB_ARGCOUNT;
 		if (argc == 1) {
@@ -204,7 +199,6 @@ kdb_bt(int argc, const char **argv)
 				kdb_bt_cpu(cpu);
 				touch_nmi_watchdog();
 			}
-			kdb_set_current_task(save_current_task);
 		}
 		return 0;
 	} else {

kernel/debug/kdb/kdb_main.c

@@ -1138,7 +1138,7 @@ static void kdb_dumpregs(struct pt_regs *regs)
 	console_loglevel = old_lvl;
 }
 
-void kdb_set_current_task(struct task_struct *p)
+static void kdb_set_current_task(struct task_struct *p)
 {
 	kdb_current_task = p;
 

kernel/debug/kdb/kdb_private.h

@@ -240,7 +240,6 @@ extern void *debug_kmalloc(size_t size, gfp_t flags);
 extern void debug_kfree(void *);
 extern void debug_kusage(void);
 
-extern void kdb_set_current_task(struct task_struct *);
 extern struct task_struct *kdb_current_task;
 extern struct pt_regs *kdb_current_regs;