42 lines
1.1 KiB
Plaintext
42 lines
1.1 KiB
Plaintext
KERNEL THREADS
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Freezer
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Upon entering a suspended state the system will freeze all
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tasks. This is done by delivering pseudosignals. This affects
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kernel threads, too. To successfully freeze a kernel thread
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the thread has to check for the pseudosignal and enter the
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refrigerator. Code to do this looks like this:
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do {
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hub_events();
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wait_event_interruptible(khubd_wait, !list_empty(&hub_event_list));
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if (current->flags & PF_FREEZE)
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refrigerator(PF_FREEZE);
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} while (!signal_pending(current));
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from drivers/usb/core/hub.c::hub_thread()
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The Unfreezable
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Some kernel threads however, must not be frozen. The kernel must
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be able to finish pending IO operations and later on be able to
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write the memory image to disk. Kernel threads needed to do IO
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must stay awake. Such threads must mark themselves unfreezable
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like this:
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/*
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* This thread doesn't need any user-level access,
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* so get rid of all our resources.
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*/
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daemonize("usb-storage");
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current->flags |= PF_NOFREEZE;
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from drivers/usb/storage/usb.c::usb_stor_control_thread()
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Such drivers are themselves responsible for staying quiet during
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the actual snapshotting.
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