[bug report] PM / reboot: Eliminate race between reboot and suspend

[Dan Carpenter <dan.carpenter@oracle.com>]

Hello Pingfan Liu,

The patch 55f2503c3b69: "PM / reboot: Eliminate race between reboot
and suspend" from Jul 31, 2018, leads to the following static checker
warning:

	kernel/reboot.c:395 __do_sys_reboot()
	error: double unlocked 'system_transition_mutex' (orig line 387)

kernel/reboot.c
   310  SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
   311                  void __user *, arg)
   312  {
   313          struct pid_namespace *pid_ns = task_active_pid_ns(current);
   314          char buffer[256];
   315          int ret = 0;
   316  
   317          /* We only trust the superuser with rebooting the system. */
   318          if (!ns_capable(pid_ns->user_ns, CAP_SYS_BOOT))
   319                  return -EPERM;
   320  
   321          /* For safety, we require "magic" arguments. */
   322          if (magic1 != LINUX_REBOOT_MAGIC1 ||
   323                          (magic2 != LINUX_REBOOT_MAGIC2 &&
   324                          magic2 != LINUX_REBOOT_MAGIC2A &&
   325                          magic2 != LINUX_REBOOT_MAGIC2B &&
   326                          magic2 != LINUX_REBOOT_MAGIC2C))
   327                  return -EINVAL;
   328  
   329          /*
   330           * If pid namespaces are enabled and the current task is in a child
   331           * pid_namespace, the command is handled by reboot_pid_ns() which will
   332           * call do_exit().
   333           */
   334          ret = reboot_pid_ns(pid_ns, cmd);
   335          if (ret)
   336                  return ret;
   337  
   338          /* Instead of trying to make the power_off code look like
   339           * halt when pm_power_off is not set do it the easy way.
   340           */
   341          if ((cmd == LINUX_REBOOT_CMD_POWER_OFF) && !pm_power_off)
   342                  cmd = LINUX_REBOOT_CMD_HALT;
   343  
   344          mutex_lock(&system_transition_mutex);
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This used to be reboot_mutex.

   345          switch (cmd) {
   346          case LINUX_REBOOT_CMD_RESTART:
   347                  kernel_restart(NULL);
   348                  break;
   349  
   350          case LINUX_REBOOT_CMD_CAD_ON:
   351                  C_A_D = 1;
   352                  break;
   353  
   354          case LINUX_REBOOT_CMD_CAD_OFF:
   355                  C_A_D = 0;
   356                  break;
   357  
   358          case LINUX_REBOOT_CMD_HALT:
   359                  kernel_halt();
   360                  do_exit(0);
   361                  panic("cannot halt");
   362  
   363          case LINUX_REBOOT_CMD_POWER_OFF:
   364                  kernel_power_off();
   365                  do_exit(0);
   366                  break;
   367  
   368          case LINUX_REBOOT_CMD_RESTART2:
   369                  ret = strncpy_from_user(&buffer[0], arg, sizeof(buffer) - 1);
   370                  if (ret < 0) {
   371                          ret = -EFAULT;
   372                          break;
   373                  }
   374                  buffer[sizeof(buffer) - 1] = '\0';
   375  
   376                  kernel_restart(buffer);
   377                  break;
   378  
   379  #ifdef CONFIG_KEXEC_CORE
   380          case LINUX_REBOOT_CMD_KEXEC:
   381                  ret = kernel_kexec();
   382                  break;
   383  #endif
   384  
   385  #ifdef CONFIG_HIBERNATION
   386          case LINUX_REBOOT_CMD_SW_SUSPEND:
   387                  ret = hibernate();
                        ^^^^^^^^^^^^^^^^^
The first thing that hibernate() does is call lock_system_sleep() which
use to take the pm_mutex, but now it takes system_transition_mutex so
it will just hang forever.

   388                  break;
   389  #endif
   390  
   391          default:
   392                  ret = -EINVAL;
   393                  break;
   394          }
   395          mutex_unlock(&system_transition_mutex);
   396          return ret;
   397  }

regards,
dan carpenter