In aosp/1979327 we attempted to prevent tasks with pending signals and
PF_FREEZER_SKIP from being immediately rescheduled, because such tasks
would crash the kernel if run while no capable CPUs were online. This was
implemented by declining to immediately reschedule them unless various
conditions were met. However, this ended up causing signals to fail to
be delivered if the signal was received while a task is processing a
syscall, such as futex(2), that will block with PF_FREEZER_SKIP set,
as the kernel relies on a check for TIF_SIGPENDING after setting the
task state to TASK_INTERRUPTIBLE in order to deliver such a signal.
This patch is an alternative solution to the original problem that
avoids introducing the signal delivery bug. It works by changing
how freezer_should_skip() is implemented. Instead of just checking
PF_FREEZER_SKIP, we also use the on_rq field to check whether the task
is not on a runqueue. In this way we ensure that a task that will be
immediately rescheduled will not return true from freezer_should_skip(),
and the task will block the freezer unless it is actually taken off
the runqueue.
Signed-off-by: Peter Collingbourne <pcc@google.com>
Bug: 202918514
Bug: 251700836
Change-Id: I3f9b705ce9ad2ca1d2df959f43cf05bef78560f8
5723e2465c