arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y

When enabling Clang's Context Analysis (aka. Thread Safety Analysis) on
kernel/futex/core.o (see Peter's changes at [1]), in arm64 LTO builds we
could see:

| kernel/futex/core.c:982:1: warning: spinlock 'atomic ? __u.__val : q->lock_ptr' is still held at the end of function [-Wthread-safety-analysis]
|      982 | }
|          | ^
|    kernel/futex/core.c:976:2: note: spinlock acquired here
|      976 |         spin_lock(lock_ptr);
|          |         ^
| kernel/futex/core.c:982:1: warning: expecting spinlock 'q->lock_ptr' to be held at the end of function [-Wthread-safety-analysis]
|      982 | }
|          | ^
|    kernel/futex/core.c:966:6: note: spinlock acquired here
|      966 | void futex_q_lockptr_lock(struct futex_q *q)
|          |      ^
|    2 warnings generated.

Where we have:

	extern void futex_q_lockptr_lock(struct futex_q *q) __acquires(q->lock_ptr);
	..
	void futex_q_lockptr_lock(struct futex_q *q)
	{
		spinlock_t *lock_ptr;

		/*
		 * See futex_unqueue() why lock_ptr can change.
		 */
		guard(rcu)();
	retry:
>>		lock_ptr = READ_ONCE(q->lock_ptr);
		spin_lock(lock_ptr);
	...
	}

At the time of the above report (prior to removal of the 'atomic' flag),
Clang Thread Safety Analysis's alias analysis resolved 'lock_ptr' to
'atomic ?  __u.__val : q->lock_ptr' (now just '__u.__val'), and used
this as the identity of the context lock given it cannot "see through"
the inline assembly; however, we want 'q->lock_ptr' as the canonical
context lock.

While for code generation the compiler simplified to '__u.__val' for
pointers (8 byte case -> 'atomic' was set), TSA's analysis (a) happens
much earlier on the AST, and (b) would be the wrong deduction.

Now that we've gotten rid of the 'atomic' ternary comparison, we can
return '__u.__val' through a pointer that we initialize with '&x', but
then update via a pointer-to-pointer. When READ_ONCE()'ing a context
lock pointer, TSA's alias analysis does not invalidate the initial alias
when updated through the pointer-to-pointer, and we make it effectively
"see through" the __READ_ONCE().

Code generation is unchanged.

Link: https://lkml.kernel.org/r/[email protected] [1]
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Cc: Peter Zijlstra <[email protected]>
Tested-by: Boqun Feng <[email protected]>
Reviewed-by: David Laight <[email protected]>
Signed-off-by: Marco Elver <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

Author: melver

arch/arm64/include/asm/rwonce.h

@@ -42,8 +42,12 @@
  */
 #define __READ_ONCE(x)							\
 ({									\
-	typeof(&(x)) __x = &(x);					\
-	union { __rwonce_typeof_unqual(*__x) __val; char __c[1]; } __u;	\
+	auto __x = &(x);						\
+	auto __ret = (__rwonce_typeof_unqual(*__x) *)__x;		\
+	/* Hides alias reassignment from Clang's -Wthread-safety. */	\
+	auto __retp = &__ret;						\
+	union { typeof(*__ret) __val; char __c[1]; } __u;		\
+	*__retp = &__u.__val;						\
 	switch (sizeof(x)) {						\
 	case 1:								\
 		asm volatile(__LOAD_RCPC(b, %w0, %1)			\
@@ -68,7 +72,7 @@
 	default:							\
 		__u.__val = *(volatile typeof(*__x) *)__x;		\
 	}								\
-	__u.__val;							\
+	*__ret;								\
 })
 
 #endif	/* !BUILD_VDSO */