[PATCH] misra: address Rule 11.3 in spin_unlock_common()

[Dmytro Prokopchuk1 <dmytro_prokopchuk1@xxxxxxxx>]

The generic 'add_sized()' macro performs type-based pointer casts, which
violate MISRA C Rule 11.3 (cast between pointer to object type and pointer
to a different object type).
Replace the use of 'add_sized(&t->head, 1)' with the type-specific
'add_u16_sized(&t->head, 1)' in the 'spin_unlock_common()' function.

A BUILD_BUG_ON(sizeof(t->head) != sizeof(uint16_t)); check is added to
ensure the field size matches expectations at build time.

On RISC-V, the atomic addition function for 16-bit values was missing,
causing build failures when called 'add_u16_sized()'. Generate a static
inline implementation of 'add_u16_sized()' for uint16_t.

Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@xxxxxxxx>
---
Test CI pipeline:
https://gitlab.com/xen-project/people/dimaprkp4k/xen/-/pipelines/2136333330
---
 xen/arch/riscv/include/asm/atomic.h | 10 ++++++++++
 xen/common/spinlock.c               |  3 ++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/xen/arch/riscv/include/asm/atomic.h 
b/xen/arch/riscv/include/asm/atomic.h
index 8e0425cea0..2676061725 100644
--- a/xen/arch/riscv/include/asm/atomic.h
+++ b/xen/arch/riscv/include/asm/atomic.h
@@ -111,6 +111,16 @@ static always_inline void _add_sized(volatile void *p,
     }
 }
 
+#define build_add_sized(name, type)                                     \
+static inline void name(volatile type *addr, unsigned long val)         \
+{                                                                       \
+    volatile type *ptr = addr;                                          \
+    write_atomic(ptr, read_atomic(ptr) + val);                          \
+}
+
+build_add_sized(add_u16_sized, uint16_t)
+#undef build_add_sized
+
 #define add_sized(p, x)                                 \
 ({                                                      \
     typeof(*(p)) x_ = (x);                              \
diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
index 0389293b09..d9dc9998e6 100644
--- a/xen/common/spinlock.c
+++ b/xen/common/spinlock.c
@@ -367,7 +367,8 @@ static void always_inline 
spin_unlock_common(spinlock_tickets_t *t,
     LOCK_PROFILE_REL;
     rel_lock(debug);
     arch_lock_release_barrier();
-    add_sized(&t->head, 1);
+    BUILD_BUG_ON(sizeof(t->head) != sizeof(uint16_t));
+    add_u16_sized(&t->head, 1);
     arch_lock_signal();
     preempt_enable();
 }
-- 
2.43.0