reboot: refactor and comment the cpu selection code

Small improvements to the code, without changing the way it works:

 - use a local variable, to avoid a small time lapse where reboot_cpu
   can have an invalid value

 - comment the code which is not easy to understand at a glance

 - merge two identical code blocks into one

 - replace pointer arithmetics with equivalent array syntax

Link: https://lkml.kernel.org/r/20201103214025.116799-4-mcroce@linux.microsoft.com
Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Pavel Tatashin <pasha.tatashin@soleen.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Robin Holt <robinmholt@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/reboot.c b/kernel/reboot.c
index 2a18b76..aa3bfd6 100644 (file)
--- a/kernel/reboot.c
+++ b/kernel/reboot.c
@@ -553,20 +553,24 @@ static int __init reboot_setup(char *str)
                        break;
 
                case 's':
-                       if (isdigit(*(str+1)))
-                               reboot_cpu = simple_strtoul(str+1, NULL, 0);
-                       else if (str[1] == 'm' && str[2] == 'p' &&
-                                                       isdigit(*(str+3)))
-                               reboot_cpu = simple_strtoul(str+3, NULL, 0);
-                       else
+                       /*
+                        * reboot_cpu is s[mp]#### with #### being the processor
+                        * to be used for rebooting. Skip 's' or 'smp' prefix.
+                        */
+                       str += str[1] == 'm' && str[2] == 'p' ? 3 : 1;
+
+                       if (isdigit(str[0])) {
+                               int cpu = simple_strtoul(str, NULL, 0);
+
+                               if (cpu >= num_possible_cpus()) {
+                                       pr_err("Ignoring the CPU number in reboot= option. "
+                                       "CPU %d exceeds possible cpu number %d\n",
+                                       cpu, num_possible_cpus());
+                                       break;
+                               }
+                               reboot_cpu = cpu;
+                       } else
                                *mode = REBOOT_SOFT;
-                       if (reboot_cpu >= num_possible_cpus()) {
-                               pr_err("Ignoring the CPU number in reboot= option. "
-                                      "CPU %d exceeds possible cpu number %d\n",
-                                      reboot_cpu, num_possible_cpus());
-                               reboot_cpu = 0;
-                               break;
-                       }
                        break;
 
                case 'g':