[PATCH RFC v2 04/15] hazptr: add scoped_guard() support

[Kunwu Chan <kunwu.chan@gmail.com>]

Add a scoped_guard() class for hazptr so that callers can pair
hazptr_acquire() with hazptr_release() on scope exit.

The guard holds a pointer to a caller-provided hazptr_ctx rather
than embedding it.  The ctx must remain at a stable address while
the hazard pointer is held because the slow path can chain its
backup slot into the overflow list by address.

Suggested-by: Boqun Feng <boqun@kernel.org>
Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
 include/linux/hazptr.h | 51 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/include/linux/hazptr.h b/include/linux/hazptr.h
index d1670121947a..275cbe54b075 100644
--- a/include/linux/hazptr.h
+++ b/include/linux/hazptr.h
@@ -320,6 +320,57 @@ void hazptr_release(struct hazptr_ctx *ctx, void *addr)
 		hazptr_unchain_backup_slot(ctx);
 }
 
+/**
+ * struct hazptr_guard - cleanup.h guard for a single hazard pointer
+ *
+ * @ctx: Pointer to the caller-provided hazard-pointer context.
+ * @addr: The address returned by the matching hazptr_acquire().
+ */
+struct hazptr_guard {
+	struct hazptr_ctx *ctx;
+	void *addr;
+};
+
+/**
+ * hazptr_guard_acquire - Acquire a hazard pointer for scoped_guard() use
+ *
+ * @ctx: The caller-provided hazard-pointer context.
+ * @addr_p: Pointer to the pointer that is to be hazard-pointer protected.
+ *
+ * Acquire a hazard pointer using @ctx and return a guard that
+ * cleanup.h releases via hazptr_release() on scope exit.  The guard
+ * only carries the ctx pointer and the address returned by
+ * hazptr_acquire().
+ */
+static inline struct hazptr_guard
+hazptr_guard_acquire(struct hazptr_ctx *ctx, void * const *addr_p)
+{
+	struct hazptr_guard g;
+
+	g.ctx = ctx;
+	g.addr = hazptr_acquire(ctx, addr_p);
+	return g;
+}
+
+DEFINE_CLASS(hazptr, struct hazptr_guard,
+	     hazptr_release(_T.ctx, _T.addr),
+	     hazptr_guard_acquire(ctx, addr_p),
+	     struct hazptr_ctx *ctx, void * const *addr_p)
+DEFINE_CLASS_IS_UNCONDITIONAL(hazptr)
+
+/*
+ * The ctx must remain at a stable address while the hazard pointer
+ * is held because the slow path can chain its backup slot
+ * into the overflow list by address.
+ *
+ *	struct hazptr_ctx ctx;
+ *	void *obj = ptr;
+ *
+ *	scoped_guard(hazptr, &ctx, &obj) {
+ *		...
+ *	}
+ */
+
 void hazptr_init(void);
 
 #endif /* _LINUX_HAZPTR_H */
-- 
2.43.0