fs/resctrl: Change last_cmd_status custom during input parsing

A pattern of usage of last_cmd_status was introduced during its enabling in
commit

  c377dcfbee80 ("x86/intel_rdt: Add diagnostics when writing the schemata file")

and since copied throughout resctrl to result in the following custom:

  ..._write()
  {
  	/* Early parsing of input, exit on failure. */

  	/* Obtain rdtgroup_mutex */
  	rdt_last_cmd_clear(); /* Clear last_cmd_status buffer */

  	/*
  	 * Act on user command, failures result in detail
  	 * error message in last_cmd_status buffer via
  	 * rdt_last_cmd_puts()/rdt_last_cmd_printf().
  	 */

  	/* Release rdtgroup_mutex */
  }

If resctrl exits with failure during early parsing of input there are two
possible scenarios:

 - The last_cmd_status buffer is empty and a user's read of
   info/last_cmd_status returns "ok".

 - The last_cmd_status buffer contains details from an earlier ...write()
   failure and a user's read of info/last_cmd_status returns this outdated
   error description.

Writing to a resctrl file is considered a "resctrl command" and the resctrl
documentation states the following about the last_cmd_status file:

  "If the command failed, it will provide more information that can be
   conveyed in the error returns from file operations."

Neither of the current scenarios is correct behavior.

Move early input parsing to be done with rdtgroup_mutex held after the
last_cmd_status buffer is cleared. Let info/last_cmd_status be accurate
when an error is encountered during parsing of user command.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Ben Horgan <ben.horgan@arm.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Tested-by: Babu Moger <babu.moger@amd.com>
Link: https://patch.msgid.link/c9eba0ef3b9a72b845b4ae02ecd7c098ed8bf06f.1782857711.git.reinette.chatre@intel.com

0 files changed, 0 insertions, 0 deletions