Currently, livepatch gradually migrate the system from an unpatched to a patched state (or vice versa). Each task drops its TIF_PATCH_PENDING itself when crossing the kernel/user space boundary or it is cleared using the stack checking approach. If there is a task which sleeps on a patched function, the whole transition can get stuck indefinitely.
Livepatch has means which can be used in these cases. The transition can be cancelled and/or immediate flag may be used for the live patch. On the other hand it might be useful to poke the system a little bit and help the transition to finish by doing so. That is what the fake signal can be used for. A task sleeping/waiting in the kernel gets TIF_SIGPENDING set, it handles it and during that its TIF_PATCH_PENDING is cleared. Kthreads are only woken up, they do not handle signals suitably. Still, there are cases which neither fake signal can solve. A task can sleep uninterruptibly without reacting to signals at all. Even then, it may be safe to clear the task's TIF_PATCH_PENDING. As a last resort, admin may force such clearing for all tasks in the system with this patch set. We use the fake signal in SLES for a long time. Moreover, we don't have a stack checking there, so we rely on the fake signal a lot. We send it automatically and periodically. The first patch is only preparatory. It introduces the sysfs attribute through which both actions are performed. The second patch adds the fake signal and the third one forced clearing of the flag. Changes from v1: - better wording, typos, comments, documentation - Libor, Josh - symbolic names in sysfs instead of numbers - Libor - exit_to_usermode_loop(), call klp_update_patch_state() before do_signal() - Oleg - better names - Josh - mutex and WARN_ON_ONCE not added to klp_force_transitions() - Petr, Josh - handle idle tasks in klp_force_transitions() too - Josh TODO: Now there is a sysfs attribute called "force", which provides two functionalities, "signal" and "force" (previously "unmark"). I haven't managed to come up with better names. Proposals are welcome. On the other hand I do not mind it much. Miroslav Benes (3): livepatch: Add force sysfs attribute livepatch: send a fake signal to all blocking tasks livepatch: force transition process to finish Documentation/ABI/testing/sysfs-kernel-livepatch | 18 +++++++ Documentation/livepatch/livepatch.txt | 14 +++++ arch/powerpc/kernel/signal.c | 6 +-- arch/x86/entry/common.c | 6 +-- kernel/livepatch/core.c | 50 ++++++++++++++++++ kernel/livepatch/transition.c | 65 ++++++++++++++++++++++++ kernel/livepatch/transition.h | 2 + kernel/signal.c | 4 +- 8 files changed, 158 insertions(+), 7 deletions(-) -- 2.13.3