debug_core: disable hw_breakpoints on all cores in kgdb_cpu_enter()

The slave cpus do not have the hw breakpoints disabled upon entry to
the debug_core and as a result could cause unrecoverable recursive
faults on badly placed breakpoints, or get out of sync with the arch
specific hw breakpoint operations.

This patch addresses the problem by invoking kgdb_disable_hw_debug()
earlier in kgdb_enter_cpu for each cpu that enters the debug core.

The hw breakpoint dis/enable flow should be:

master_debug_cpu   slave_debug_cpu
         \              /
          kgdb_cpu_enter
                |
        kgdb_disable_hw_debug --> uninstall pre-enabled hw_breakpoint
                |
 do add/rm dis/enable operates to hw_breakpoints on master_debug_cpu..
                |
        correct_hw_break --> correct/install the enabled hw_breakpoint
                |
           leave_kgdb

Signed-off-by: Dongdong Deng <[email protected]>
Signed-off-by: Jason Wessel <[email protected]>

kernel/debug/debug_core.c

@@ -485,6 +485,9 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs)
 	int error;
 	int i, cpu;
 	int trace_on = 0;
+
+	kgdb_disable_hw_debug(ks->linux_regs);
+
 acquirelock:
 	/*
 	 * Interrupts will be restored by the 'trap return' code, except when
@@ -569,8 +572,6 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs)
 	if (dbg_io_ops->pre_exception)
 		dbg_io_ops->pre_exception();
 
-	kgdb_disable_hw_debug(ks->linux_regs);
-
 	/*
 	 * Get the passive CPU lock which will hold all the non-primary
 	 * CPU in a spin state while the debugger is active
@@ -661,6 +662,8 @@ static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs)
 		else
 			kgdb_sstep_pid = 0;
 	}
+	if (arch_kgdb_ops.correct_hw_break)
+		arch_kgdb_ops.correct_hw_break();
 	if (trace_on)
 		tracing_on();
 	/* Free kgdb_active */