selftests/bpf: Add task work BUSY window test

Add a regression test for reusing a bpf_task_work context while its
scheduling irq_work is still BUSY. A test that schedules from a thread
pinned to on

selftests/bpf: Add task work BUSY window test

Add a regression test for reusing a bpf_task_work context while its
scheduling irq_work is still BUSY. A test that schedules from a thread
pinned to one CPU never observes that window: irq_work_queue() self-IPIs
the local CPU and the invocation completes before the thread's next
syscall.

Use an fexit program on task_work_add() to delay the scheduling
handler's return with a time- and iteration-bounded busy-wait, after
the first-round callback was published and before the handler's tail
state transition. The target task attempts to reuse the same context
from a later syscall, strictly after its own callback wrapper returned,
so the context is STANDBY at that point. A valid window requires the
fexit program to observe the attempt's acknowledgment before returning,
which proves the attempt ran while the old handler, and thus the BUSY
irq_work, was still in flight. With the map and program references
held and no competing scheduler, the only possible source of -EBUSY for
that attempt is the BUSY check in the acquire path. Check that the
attempt is rejected with -EBUSY, that scheduling succeeds on a bounded
retry once BUSY clears, and that the second callback runs exactly once.

Each attempt uses dedicated workers and a fresh map value. Reset the
shared slots before enabling the workers. Stop and join them at the
end of the attempt, then wait for their TIDs to disappear from PID
lookup before checking the final callback count or starting another
attempt. join alone can return before exit_task_work(); a signal-zero
tgkill probe still finds the worker until after its exit task_work has
run. Bound that wait and fail on unexpected probe errors. Callback
deliveries are also matched against the window's generation.

Missed windows are counted and retried within a bounded attempt budget;
hard failures fail the test. Record the busy-wait elapsed time after
worker cleanup as a diagnostic, without asserting a wall-clock bound,
since host vCPU preemption can stretch wall-clock time without affecting
the gate behavior under test.

Skip when fewer than two CPUs are available or when the loaded kconfig
extern identifies CONFIG_PREEMPT_RT. Check that extern before attaching
programs or starting workers. Load and attach errors fail the test
instead of being inferred from tracefs file paths and reported as
unsupported.

Signed-off-by: Yun Lu <luyun@kylinos.cn>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261012061915.231254-5-luyun_611@163.com

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