Merge branch 'bpf-introduce-deferred-task-context-execution'

Mykyta Yatsenko says:

====================
bpf: Introduce deferred task context execution

From: Mykyta Yatsenko <yatsenko@meta.com>

This patch introduces a new mechanism for BPF programs to schedule
deferred execution in the context of a specific task using the kernel’s
task_work infrastructure.

The new bpf_task_work interface enables BPF use cases that
require sleepable subprogram execution within task context, for example,
scheduling sleepable function from the context that does not
allow sleepable, such as NMI.

Introduced kfuncs bpf_task_work_schedule_signal() and
bpf_task_work_schedule_resume() for scheduling BPF callbacks correspond
to different modes used by task_work (TWA_SIGNAL or TWA_RESUME).

The implementation manages scheduling state via metadata objects (struct
bpf_task_work_context). Pointers to bpf_task_work_context are stored
in BPF map values. State transitions are handled via an atomic
state machine (bpf_task_work_state) to ensure correctness under
concurrent usage and deletion, lifetime is guarded by refcounting and
RCU Tasks Trace.
Kfuncs call task_work_add() indirectly via irq_work to avoid locking in
potentially NMI context.

Changelog:
---
v7 -> v8
v7: https://lore.kernel.org/bpf/20250922232611.614512-1-mykyta.yatsenko5@gmail.com/
 * Fix unused variable warning in patch 1
 * Decrease stress test time from 2 to 1 second
 * Went through CI warnings, other than unused variable, there are just
 2 new in kernel/bpf/helpers.c related to newly introduced kfuncs, these
 look expected.

v6 -> v7
v6: https://lore.kernel.org/bpf/20250918132615.193388-1-mykyta.yatsenko5@gmail.com/
 * Added stress test
 * Extending refactoring in patch 1
 * Changing comment and removing one check for map->usercnt in patch 7

v5 -> v6
v5: https://lore.kernel.org/bpf/20250916233651.258458-1-mykyta.yatsenko5@gmail.com/
 * Fixing readability in verifier.c:check_map_field_pointer()
 * Removing BUG_ON from helpers.c

v4 -> v5
v4:
https://lore.kernel.org/all/20250915201820.248977-1-mykyta.yatsenko5@gmail.com/
 * Fix invalid/null pointer dereference bug, reported by syzbot
 * Nits in selftests

v3 -> v4
v3: https://lore.kernel.org/all/20250905164508.1489482-1-mykyta.yatsenko5@gmail.com/
 * Modify async callback return value processing in verifier, to allow
non-zero return values.
 * Change return type of the callback from void to int, as verifier
expects scalar value.
 * Switched to void* for bpf_map API kfunc arguments to avoid casts.
 * Addressing numerous nits and small improvements.

v2 -> v3
v2: https://lore.kernel.org/all/20250815192156.272445-1-mykyta.yatsenko5@gmail.com/
 * Introduce ref counting
 * Add patches with minor verifier and btf.c refactorings to avoid code
duplication
 * Rework initiation of the task work scheduling to handle race with map
usercnt dropping to zero
====================

Link: https://patch.msgid.link/20250923112404.668720-1-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>