diff options
Diffstat (limited to 'Documentation/power/freezing-of-tasks.txt')
| -rw-r--r-- | Documentation/power/freezing-of-tasks.txt | 31 |
1 files changed, 19 insertions, 12 deletions
diff --git a/Documentation/power/freezing-of-tasks.txt b/Documentation/power/freezing-of-tasks.txt index d5c65e8d6a3..38b57248fd6 100644 --- a/Documentation/power/freezing-of-tasks.txt +++ b/Documentation/power/freezing-of-tasks.txt @@ -19,12 +19,13 @@ we only consider hibernation, but the description also applies to suspend). Namely, as the first step of the hibernation procedure the function freeze_processes() (defined in kernel/power/process.c) is called. It executes try_to_freeze_tasks() that sets TIF_FREEZE for all of the freezable tasks and -sends a fake signal to each of them. A task that receives such a signal and has -TIF_FREEZE set, should react to it by calling the refrigerator() function -(defined in kernel/power/process.c), which sets the task's PF_FROZEN flag, -changes its state to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is -cleared for it. Then, we say that the task is 'frozen' and therefore the set of -functions handling this mechanism is called 'the freezer' (these functions are +either wakes them up, if they are kernel threads, or sends fake signals to them, +if they are user space processes. A task that has TIF_FREEZE set, should react +to it by calling the function called refrigerator() (defined in +kernel/power/process.c), which sets the task's PF_FROZEN flag, changes its state +to TASK_UNINTERRUPTIBLE and makes it loop until PF_FROZEN is cleared for it. +Then, we say that the task is 'frozen' and therefore the set of functions +handling this mechanism is referred to as 'the freezer' (these functions are defined in kernel/power/process.c and include/linux/freezer.h). User space processes are generally frozen before kernel threads. @@ -35,21 +36,27 @@ task enter refrigerator() if the flag is set. For user space processes try_to_freeze() is called automatically from the signal-handling code, but the freezable kernel threads need to call it -explicitly in suitable places. The code to do this may look like the following: +explicitly in suitable places or use the wait_event_freezable() or +wait_event_freezable_timeout() macros (defined in include/linux/freezer.h) +that combine interruptible sleep with checking if TIF_FREEZE is set and calling +try_to_freeze(). The main loop of a freezable kernel thread may look like the +following one: + set_freezable(); do { hub_events(); - wait_event_interruptible(khubd_wait, - !list_empty(&hub_event_list)); - try_to_freeze(); - } while (!signal_pending(current)); + wait_event_freezable(khubd_wait, + !list_empty(&hub_event_list) || + kthread_should_stop()); + } while (!kthread_should_stop() || !list_empty(&hub_event_list)); (from drivers/usb/core/hub.c::hub_thread()). If a freezable kernel thread fails to call try_to_freeze() after the freezer has set TIF_FREEZE for it, the freezing of tasks will fail and the entire hibernation operation will be cancelled. For this reason, freezable kernel -threads must call try_to_freeze() somewhere. +threads must call try_to_freeze() somewhere or use one of the +wait_event_freezable() and wait_event_freezable_timeout() macros. After the system memory state has been restored from a hibernation image and devices have been reinitialized, the function thaw_processes() is called in |