Re: [PATCH v3 3/3] arm64, compiler-context-analysis: Permit alias analysis through __READ_ONCE() with CONFIG_LTO=y

[David Laight <david.laight.linux@gmail.com>]

On Fri, 30 Jan 2026 14:28:26 +0100
Marco Elver <elver@google.com> wrote:

> 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/20260121110704.221498346@infradead.org [1]
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202601221040.TeM0ihff-lkp@intel.com/
> Cc: Peter Zijlstra <peterz@infradead.org>
> Tested-by: Boqun Feng <boqun@kernel.org>
> Signed-off-by: Marco Elver <elver@google.com>

LGTM (for an obscure definition of G).

Reviewed-by: David Laight <david.laight.linux@gmail.com>

> ---
> v3:
> * Use 'typeof(*__ret)'.
> * Commit message.
> 
> v2:
> * Rebase.
> ---
>  arch/arm64/include/asm/rwonce.h | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/rwonce.h b/arch/arm64/include/asm/rwonce.h
> index 42c9e8429274..b7de74d4bf07 100644
> --- a/arch/arm64/include/asm/rwonce.h
> +++ b/arch/arm64/include/asm/rwonce.h
> @@ -45,8 +45,12 @@
>   */
>  #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)			\
> @@ -71,7 +75,7 @@
>  	default:							\
>  		__u.__val = *(volatile typeof(*__x) *)__x;		\
>  	}								\
> -	__u.__val;							\
> +	*__ret;								\
>  })
>  
>  #endif	/* !BUILD_VDSO */