nrf_fstorage_write() triggers HARD FAULT only when called from usage fault handler

Question

We are implementing a system where diagnostics info is saved to FLASH when one of the error handlers is called. Our code collects diagnostic registers and stack info, and saves them to a predetermined block in FLASH, 
 We use our own library which is a wrapper on nrf_storage_write(): 
 
 volatile bool flash_waiting = false ; 
 void flash_raw_wait_until_done ( void ); 
 void flash_raw_callback ( nrf_fstorage_evt_t * p_evt ); 
 
 NRF_FSTORAGE_DEF ( nrf_fstorage_t flash_blocks ) = 
 { 
 .evt_handler = flash_raw_callback , 
 .start_addr = FLASH_START , 
 .end_addr = FLASH_END 
 }; 
 
 void flash_raw_init ( void ) 
 { 
 PROJECT_ERROR_CHECK ( nrf_fstorage_init ( & flash_blocks , & nrf_fstorage_sd , NULL )); 
 }; 
 
 void flash_raw_write_block ( const uint32_t * block_start , const uint32_t * src , const uint8_t n_words ) 
 { 
 PROJECT_CHECK_NOT_NULL ( block_start ); 
 PROJECT_CHECK_NOT_NULL ( src ); 
 PROJECT_LOG_DEBUG ( "WR %d wds to 0x %08x fr 0x %04x " , n_words , block_start , src ); 
 flash_waiting = true ; 
 ret_code_t rc = nrf_fstorage_write (& flash_blocks , ( uint32_t ) block_start , src , n_words * ( sizeof ( uint32_t )), NULL ); 
 //ret_code_t rc = 0; 
 PROJECT_LOG_DEBUG ( "nrf_fstorage_write returned 0x %04x " , rc ); 
 
 flash_raw_wait_until_done (); 
 } 
 
 void flash_raw_wait_until_done ( void ) 
 { 
 uint32_t t ; 
 for ( t = 0 ; t < FLASH_BLK_WRITE_TIMEOUT_MS && flash_waiting ; t ++) nrf_delay_ms ( 1 ); 
 if ( flash_waiting ) 
 { 
 PROJECT_LOG_ERROR ( "FLASH operation timed out." ); 
 } 
 else 
 { 
 PROJECT_LOG_DEBUG ( "FLASH OK %d ms" , t ); 
 } 
 
 } 
 
 void flash_raw_callback ( nrf_fstorage_evt_t * p_evt ) 
 { 
 if ( p_evt -> result != NRF_SUCCESS) 
 { 
 PROJECT_LOG_ERROR ( "FLASH failure 0x %04x " , p_evt -> result ); 
 } 
 flash_waiting = false ; 
 } 
 
 Everything works fine when we do this from an application error (i.e. we call the error handler ourselves). 
 However when we call this code from the Usage Fault Handler (after first saving the diag info), the call to nrf_storage_wr() seems to immediately cause another fault which is escalted to a Hard Fault. (We are testing this by deliberately triggering a divide by zero). Log output: 
 INF main.c 112: kamikaze by DIVZERO! ERR arm_fault_handling.c 82: CFSR: 02000000 HFSR: 00000000 ERR arm_fault_handling.c 83: DFSR: 00000000 AFSR: 00000000 ERR arm_fault_handling.c 84: LR: fffffff9 MMFAR: e000ed34 ERR arm_fault_handling.c 85: BFAR: e000ed38 PC: 0002eeca ERR arm_fault_handling.c 86: SP: 2000ff98 TOS: 20010000 ERR arm_fault_handling.c 87: Stack depth: 26 ERR arm_fault_handling.c 90: stack[00] 2000ff98 ERR arm_fault_handling.c 90: stack[01] 00000001 ERR arm_fault_handling.c 90: stack[02] 00030e97 ERR arm_fault_handling.c 90: stack[03] 00000000 ERR arm_fault_handling.c 90: stack[04] 2000ffd4 ERR arm_fault_handling.c 90: stack[05] fffffff9 ERR arm_fault_handling.c 90: stack[06] 00000000 ERR arm_fault_handling.c 90: stack[07] 00030ff0 ERR arm_fault_handling.c 90: stack[08] 00000001 ERR arm_fault_handling.c 90: stack[09] 00000000 ERR arm_fault_handling.c 90: stack[10] 0000000a ERR arm_fault_handling.c 90: stack[11] 000264c3 ERR arm_fault_handling.c 90: stack[12] 00026856 ERR arm_fault_handling.c 90: stack[13] 01000200 ERR arm_fault_handling.c 90: stack[14] 000264af ERR arm_fault_handling.c 90: stack[15] 2000ffe0 ERR arm_fault_handling.c 90: stack[16] 00030e24 ERR arm_fault_handling.c 90: stack[17] 01000000 ERR arm_fault_handling.c 90: stack[18] 2000ffe8 ERR arm_fault_handling.c 90: stack[19] 01046709 ERR arm_fault_handling.c 90: stack[20] 2000fff8 ERR arm_fault_handling.c 90: stack[21] 000267a9 ERR arm_fault_handling.c 90: stack[22] 00030fe0 ERR arm_fault_handling.c 90: stack[23] 00000000 ERR arm_fault_handling.c 90: stack[24] 00000000 ERR arm_fault_handling.c 90: stack[25] 00026297 DBG project_err_debug.c 108: Priv mode? 1 DBG _flash_storage.c 76: writing forensics to 0x00076000 DBG _flash_storage_raw.c 36: WR 62 wds to 0x00076000 fr 0x20004 ERR arm_fault_handling.c 82: CFSR: 02000000 HFSR: 40000000 ERR arm_fault_handling.c 83: DFSR: 00000000 AFSR: 00000000 ERR arm_fault_handling.c 84: LR: fffffff1 MMFAR: e000ed34 ERR arm_fault_handling.c 85: BFAR: e000ed38 PC: 0002ed22 ERR arm_fault_handling.c 86: SP: 2000fe38 TOS: 20010000 ERR arm_fault_handling.c 87: Stack depth: 32 ERR arm_fault_handling.c 90: stack[00] 2000fe38 ERR arm_fault_handling.c 90: stack[01] 00000001 ERR arm_fault_handling.c 90: stack[02] 2000fe3c ERR arm_fault_handling.c 90: stack[03] 0002a195 ERR arm_fault_handling.c 90: stack[04] 2000fe70 ERR arm_fault_handling.c 90: stack[05] fffffff1 ERR arm_fault_handling.c 90: stack[06] 00076000 ERR arm_fault_handling.c 90: stack[07] 20004000 ERR arm_fault_handling.c 90: stack[08] 0000003e ERR arm_fault_handling.c 90: stack[09] 20004000 ERR arm_fault_handling.c 90: stack[10] 0000000a ERR arm_fault_handling.c 90: stack[11] 00029ef9 ERR arm_fault_handling.c 90: stack[12] 00029e10 ERR arm_fault_handling.c 90: stack[13] 21000006 ERR arm_fault_handling.c 90: stack[14] 20003aa0 ERR arm_fault_handling.c 90: stack[15] 200039ec ERR arm_fault_handling.c 90: stack[16] 00000000 ERR arm_fault_handling.c 90: stack[17] 20004000 ERR arm_fault_handling.c 90: stack[18] 00076000 ERR arm_fault_handling.c 90: stack[19] 0000003e ERR arm_fault_handling.c 90: stack[20] 2000fe90 ERR arm_fault_handling.c 90: stack[21] 00029fbb ERR arm_fault_handling.c 90: stack[22] 2000feb8 ERR arm_fault_handling.c 90: stack[23] 20003a8c ERR arm_fault_handling.c 90: stack[24] 2000fea0 ERR arm_fault_handling.c 90: stack[25] 0002a053 ERR arm_fault_handling.c 90: stack[26] 2000fea8 ERR arm_fault_handling.c 90: stack[27] 0002a1fd ERR arm_fault_handling.c 90: stack[28] 000000f8 ERR arm_fault_handling.c 90: stack[29] 20004000 ERR arm_fault_handling.c 90: stack[30] 00076000 ERR arm_fault_handling.c 90: stack[31] 20002b10 DBG project_err_debug.c 108: Priv mode? 1 DBG _flash_storage.c 76: writing forensics to 0x00076000 DBG _flash_storage_raw.c 36: WR 62 wds to 0x00076000 fr 0x20004 DBG _flash_storage_raw.c 40: nrf_fstorage_write returned 0x0000 ERR _flash_storage_raw.c 60: FLASH operation timed out. 
 Why would this be? Do the ARM (Hard Fault, Usage Fault etc) handlers set some flag or place the system in some mode that causes a FLASH write to fail?

danmcb · Accepted Answer

I seem to have resolved the issue. I needed to clear the PENDING flag in the handler. Now the handler works correctly. 
 
 void UsageFault_Handler ( void ) 
 { 
 SAVE_DIAGNOSTIC_REGISTERS ; 
 SCB -> SHCSR &= !( SCB_SHCSR_USGFAULTPENDED_Msk ); 
 project_fatal_error ( "USAGE FAULT" ); 
 }