rust: add SRCU abstraction

The immediate motivation is the Tyr reset infrastructure [1] which needs
to serialize reset sensitive hardware access against reset and teardown
paths. That reset series started to require many independent dependencies
so this SRCU support is split out as a standalone Rust API to keep the
reset series focused on the reset logic and easier to review, rebase and
land.


Changes since v9:

- SRCU initialization restructuring is now a separate patch (the first
  one in this series).
- Comment fix on C initialization functions.


Changes since v8:

- Refactor srcu init handling to provide better init API (motivated by
  the "mutex.h" initializers.
- Use warn_on! instead of pr_warn! on leaked guard detection.


Changes since v7:

- Moved synchronize_srcu() call inside "if srcu_readers_active()"
  condition in "impl PinnedDrop for Srcu".
- Improved comments on the synchronize_srcu() call in
  "impl PinnedDrop for Srcu".


Changes since v6:

- Removed "CONFIG_DEBUG_LOCK_ALLOC" condition from "rust/helpers/srcu.c"
  and created a simple wrapper inside "include/linux/srcu.h" for it.


Changes since v5:

- Created separate srcu_readers_active() variants for "srcutiny.h" and
  "srcutree.h".


Changes since v4:

- Exposed srcu_readers_active from C side and wired it to the Rust
  helpers.
- Used srcu_readers_active() in SRCU drop and logged with pr_warn if
  there are leaked guards during the drop.


Changes since v3 (which are for Sashiko notes [2]):

- Added rust helpers for srcu_barrier() and synchronize_srcu_expedited()
  so the abstraction builds with CONFIG_TINY_SRCU, where these are
  static inline functions.
- Added missing INVARIANT comment in Srcu::new() about why the type
  invariants hold after successful initialization.


Changes since v2:

- Removed closure-based API.
- Added #[doc(hidden)] on new_srcu macro.
- Added #[must_use..] on srcu::Guard.
- Improved the clean-up path (PinnedDrop implementation) which
  eventually made read_lock safe with leaked guards.


Changes since v1:

- Made the owned SRCU read-side guard API unsafe and added a safe closure
  based helper for callers that do not need to keep the guard. This is to
  avoid UB on the C side cleanup_srcu_struct where the SRCU struct is freed
  while there are still active guards, which can happen if the caller leaks
  the guard e.g., with mem::forget().
- Improved doc comments.


v1: https://lore.kernel.org/all/20260428103437.156236-1-work@...
v2: https://lore.kernel.org/all/20260502162833.34334-1-work@o...
v3: https://lore.kernel.org/all/20260522054228.114814-1-work@...
v4: https://lore.kernel.org/all/20260525175706.124910-1-work@...
v5: https://lore.kernel.org/all/20260527174120.510447-1-work@...
v6: https://lore.kernel.org/all/20260527203615.163688-1-work@...
v7: https://lore.kernel.org/all/20260528062810.256212-1-work@...
v8: https://lore.kernel.org/all/20260529114449.112066-1-work@...
v9: https://lore.kernel.org/all/20260529134004.396743-1-work@...

[1]: https://lore.kernel.org/all/20260416171728.205141-1-work@...
[2]: https://sashiko.dev/#/patchset/20260522054228.114814-1-wo...

Onur Özkan (5):
  srcu: make init_srcu_struct() consistently wrap __init_srcu_struct()
  rust: helpers: add SRCU helpers
  srcu: expose srcu_readers_active()
  rust: sync: add SRCU abstraction
  MAINTAINERS: add Rust SRCU files to SRCU entry

 MAINTAINERS              |   3 +
 include/linux/srcu.h     |  29 ++++---
 include/linux/srcutiny.h |  13 +++
 include/linux/srcutree.h |  24 ++++++
 kernel/rcu/srcutiny.c    |  14 ++--
 kernel/rcu/srcutree.c    |  36 ++-------
 rust/helpers/helpers.c   |   1 +
 rust/helpers/srcu.c      |  35 ++++++++
 rust/kernel/sync.rs      |   2 +
 rust/kernel/sync/srcu.rs | 171 +++++++++++++++++++++++++++++++++++++++
 10 files changed, 282 insertions(+), 46 deletions(-)
 create mode 100644 rust/helpers/srcu.c
 create mode 100644 rust/kernel/sync/srcu.rs

-- 
2.51.2