Avoid crashes during fault handler

If the fault handler was hit before the stdio console was used and
initialised, the initialisation code caused a "mutex in ISR" trap,
stopping the register dump from happening.

Temporarily set the `error_in_progress` flag at the top of the fault
handler, and restore it before calling `mbed_error`. Take the
opportunity to suppress fault dumps on recursive crashes, much as is
done inside `mbed_error`.

platform/mbed_interface.h

@@ -168,7 +168,7 @@ void mbed_error_vprintf(const char *format, va_list arg) MBED_PRINTF(1, 0);
  *       FileHandle::write of the stderr device is. The default
  *       serial console is safe, either buffered or not. If the
  *       console has not previously been initialized, an attempt
- *       to use this from interrupt may during console initialization.
+ *       to use this from interrupt may crash during console initialization.
  *       Special handling of `mbed_error` relaxes various system traps
  *       to increase the chance of initialization working.
  *

platform/source/TARGET_CORTEX_M/mbed_fault_handler.c

@@ -21,6 +21,7 @@
 #include <string.h>
 
 #include "device.h"
+#include "mbed_atomic.h"
 #include "mbed_error.h"
 #include "mbed_interface.h"
 #include "mbed_crash_data_offsets.h"
@@ -39,41 +40,49 @@ mbed_fault_context_t fault_context;
 mbed_fault_context_t *const mbed_fault_context = &fault_context;
 #endif
 
+extern bool mbed_error_in_progress;
+
 //This is a handler function called from Fault handler to print the error information out.
 //This runs in fault context and uses special functions(defined in mbed_rtx_fault_handler.c) to print the information without using C-lib support.
 void mbed_fault_handler(uint32_t fault_type, const mbed_fault_context_t *mbed_fault_context_in)
 {
     mbed_error_status_t faultStatus = MBED_SUCCESS;
 
-    mbed_error_printf("\n++ MbedOS Fault Handler ++\n\nFaultType: ");
+    /* Need to set the flag to ensure prints do not trigger a "mutex in ISR" trap
+     * if they're first prints since boot and we have to init the I/O system.
+     */
+    if (!core_util_atomic_exchange_bool(&mbed_error_in_progress, true)) {
+        mbed_error_printf("\n++ MbedOS Fault Handler ++\n\nFaultType: ");
 
-    switch (fault_type) {
+        switch (fault_type) {
-        case MEMMANAGE_FAULT_EXCEPTION:
+            case MEMMANAGE_FAULT_EXCEPTION:
-            mbed_error_printf("MemManageFault");
+                mbed_error_printf("MemManageFault");
-            faultStatus = MBED_ERROR_MEMMANAGE_EXCEPTION;
+                faultStatus = MBED_ERROR_MEMMANAGE_EXCEPTION;
-            break;
+                break;
 
-        case BUS_FAULT_EXCEPTION:
+            case BUS_FAULT_EXCEPTION:
-            mbed_error_printf("BusFault");
+                mbed_error_printf("BusFault");
-            faultStatus = MBED_ERROR_BUSFAULT_EXCEPTION;
+                faultStatus = MBED_ERROR_BUSFAULT_EXCEPTION;
-            break;
+                break;
 
-        case USAGE_FAULT_EXCEPTION:
+            case USAGE_FAULT_EXCEPTION:
-            mbed_error_printf("UsageFault");
+                mbed_error_printf("UsageFault");
-            faultStatus = MBED_ERROR_USAGEFAULT_EXCEPTION;
+                faultStatus = MBED_ERROR_USAGEFAULT_EXCEPTION;
-            break;
+                break;
 
-        //There is no way we can hit this code without getting an exception, so we have the default treated like hardfault
+            //There is no way we can hit this code without getting an exception, so we have the default treated like hardfault
-        case HARD_FAULT_EXCEPTION:
+            case HARD_FAULT_EXCEPTION:
-        default:
+            default:
-            mbed_error_printf("HardFault");
+                mbed_error_printf("HardFault");
-            faultStatus = MBED_ERROR_HARDFAULT_EXCEPTION;
+                faultStatus = MBED_ERROR_HARDFAULT_EXCEPTION;
-            break;
+                break;
+        }
+        mbed_error_printf("\n\nContext:");
+        print_context_info();
+
+        mbed_error_printf("\n\n-- MbedOS Fault Handler --\n\n");
+        core_util_atomic_store_bool(&mbed_error_in_progress, false);
     }
-    mbed_error_printf("\n\nContext:");
-    print_context_info();
-
-    mbed_error_printf("\n\n-- MbedOS Fault Handler --\n\n");
 
     //Now call mbed_error, to log the error and halt the system
     mbed_error(faultStatus, "Fault exception", (unsigned int)mbed_fault_context_in, NULL, 0);

platform/source/mbed_error.c

@@ -51,7 +51,7 @@ static void print_error_report(const mbed_error_ctx *ctx, const char *, const ch
 #define ERROR_REPORT(ctx, error_msg, error_filename, error_line) ((void) 0)
 #endif
 
-static bool error_in_progress;
+bool mbed_error_in_progress;
 static core_util_atomic_flag halt_in_progress = CORE_UTIL_ATOMIC_FLAG_INIT;
 static int error_count = 0;
 static mbed_error_ctx first_error_ctx = {0};
@@ -121,7 +121,7 @@ static MBED_NORETURN void mbed_halt_system(void)
 WEAK MBED_NORETURN void error(const char *format, ...)
 {
     // Prevent recursion if error is called again during store+print attempt
-    if (!core_util_atomic_exchange_bool(&error_in_progress, true)) {
+    if (!core_util_atomic_exchange_bool(&mbed_error_in_progress, true)) {
         handle_error(MBED_ERROR_UNKNOWN, 0, NULL, 0, MBED_CALLER_ADDR());
         ERROR_REPORT(&last_error_ctx, "Fatal Run-time error", NULL, 0);
 
@@ -298,7 +298,7 @@ int mbed_get_error_count(void)
 //Reads the fatal error occurred" flag
 bool mbed_get_error_in_progress(void)
 {
-    return core_util_atomic_load_bool(&error_in_progress);
+    return core_util_atomic_load_bool(&mbed_error_in_progress);
 }
 
 //Sets a non-fatal error
@@ -311,7 +311,7 @@ mbed_error_status_t mbed_warning(mbed_error_status_t error_status, const char *e
 WEAK MBED_NORETURN mbed_error_status_t mbed_error(mbed_error_status_t error_status, const char *error_msg, unsigned int error_value, const char *filename, int line_number)
 {
     // Prevent recursion if error is called again during store+print attempt
-    if (!core_util_atomic_exchange_bool(&error_in_progress, true)) {
+    if (!core_util_atomic_exchange_bool(&mbed_error_in_progress, true)) {
         //set the error reported
         (void) handle_error(error_status, error_value, filename, line_number, MBED_CALLER_ADDR());