kdb,debug_core: adjust master cpu switch logic against new debug_core locking

The kdb shell needs to enforce switching back to the original CPU that
took the exception before restoring normal kernel execution.  Resuming
from a different CPU than what took the original exception will cause
problems with spin locks that are freed from the a different processor
than had taken the lock.

The special logic in dbg_cpu_switch() can go away entirely with
because the state of what cpus want to be masters or slaves will
remain unchanged between entry and exit of the debug_core exception
context.
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

kernel/debug/debug_core.c

@@ -460,19 +460,6 @@ static int kgdb_reenter_check(struct kgdb_state *ks)
 	return 1;
 }
 
-static void dbg_cpu_switch(int cpu, int next_cpu)
-{
-	/* Mark the cpu we are switching away from as a slave when it
-	 * holds the kgdb_active token.  This must be done so that the
-	 * that all the cpus wait in for the debug core will not enter
-	 * again as the master. */
-	if (cpu == atomic_read(&kgdb_active)) {
-		kgdb_info[cpu].exception_state |= DCPU_IS_SLAVE;
-		kgdb_info[cpu].exception_state &= ~DCPU_WANT_MASTER;
-	}
-	kgdb_info[next_cpu].exception_state |= DCPU_NEXT_MASTER;
-}
-
 static void dbg_touch_watchdogs(void)
 {
 	touch_softlockup_watchdog_sync();
@@ -638,7 +625,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
 		if (error == DBG_PASS_EVENT) {
 			dbg_kdb_mode = !dbg_kdb_mode;
 		} else if (error == DBG_SWITCH_CPU_EVENT) {
-			dbg_cpu_switch(cpu, dbg_switch_cpu);
+			kgdb_info[dbg_switch_cpu].exception_state |=
+				DCPU_NEXT_MASTER;
 			goto cpu_loop;
 		} else {
 			kgdb_info[cpu].ret_state = error;

kernel/debug/kdb/kdb_debugger.c

@@ -86,7 +86,7 @@ int kdb_stub(struct kgdb_state *ks)
 	}
 	/* Set initial kdb state variables */
 	KDB_STATE_CLEAR(KGDB_TRANS);
-	kdb_initial_cpu = ks->cpu;
+	kdb_initial_cpu = atomic_read(&kgdb_active);
 	kdb_current_task = kgdb_info[ks->cpu].task;
 	kdb_current_regs = kgdb_info[ks->cpu].debuggerinfo;
 	/* Remove any breakpoints as needed by kdb and clear single step */
@@ -105,7 +105,6 @@ int kdb_stub(struct kgdb_state *ks)
 		ks->pass_exception = 1;
 		KDB_FLAG_SET(CATASTROPHIC);
 	}
-	kdb_initial_cpu = ks->cpu;
 	if (KDB_STATE(SSBPT) && reason == KDB_REASON_SSTEP) {
 		KDB_STATE_CLEAR(SSBPT);
 		KDB_STATE_CLEAR(DOING_SS);

kernel/debug/kdb/kdb_main.c

@@ -1749,13 +1749,13 @@ static int kdb_go(int argc, const char **argv)
 	int nextarg;
 	long offset;
 
+	if (raw_smp_processor_id() != kdb_initial_cpu) {
+		kdb_printf("go must execute on the entry cpu, "
+			   "please use \"cpu %d\" and then execute go\n",
+			   kdb_initial_cpu);
+		return KDB_BADCPUNUM;
+	}
 	if (argc == 1) {
-		if (raw_smp_processor_id() != kdb_initial_cpu) {
-			kdb_printf("go <address> must be issued from the "
-				   "initial cpu, do cpu %d first\n",
-				   kdb_initial_cpu);
-			return KDB_ARGCOUNT;
-		}
 		nextarg = 1;
 		diag = kdbgetaddrarg(argc, argv, &nextarg,
 				     &addr, &offset, NULL);