Re: [PATCH v5 15/36] srcu: Support Clang's context analysis

On 12/19/25 7:40 AM, Marco Elver wrote:
> +/*
> + * No-op helper to denote that ssp must be held. Because SRCU-protected pointers
> + * should still be marked with __rcu_guarded, and we do not want to mark them
> + * with __guarded_by(ssp) as it would complicate annotations for writers, we
> + * choose the following strategy: srcu_dereference_check() calls this helper
> + * that checks that the passed ssp is held, and then fake-acquires 'RCU'.
> + */
> +static inline void __srcu_read_lock_must_hold(const struct srcu_struct *ssp) __must_hold_shared(ssp) { }
>   
>   /**
>    * srcu_dereference_check - fetch SRCU-protected pointer for later dereferencing
> @@ -223,9 +233,15 @@ static inline int srcu_read_lock_held(const struct srcu_struct *ssp)
>    * to 1.  The @c argument will normally be a logical expression containing
>    * lockdep_is_held() calls.
>    */
> -#define srcu_dereference_check(p, ssp, c) \
> -	__rcu_dereference_check((p), __UNIQUE_ID(rcu), \
> -				(c) || srcu_read_lock_held(ssp), __rcu)
> +#define srcu_dereference_check(p, ssp, c)					\
> +({										\
> +	__srcu_read_lock_must_hold(ssp);					\
> +	__acquire_shared_ctx_lock(RCU);					\
> +	__auto_type __v = __rcu_dereference_check((p), __UNIQUE_ID(rcu),	\
> +				(c) || srcu_read_lock_held(ssp), __rcu);	\
> +	__release_shared_ctx_lock(RCU);					\
> +	__v;									\
> +})

Hi Marco,

The above change is something I'm not happy about. The original
implementation of the srcu_dereference_check() macro shows that it is
sufficient to either hold an SRCU reader lock or the updater lock ('c').
The addition of "__srcu_read_lock_must_hold()" will cause compilation to
fail if the caller doesn't hold an SRCU reader lock. I'm concerned that
this will either lead to adding __no_context_analysis to SRCU updater
code that uses srcu_dereference_check() or to adding misleading
__assume_ctx_lock(ssp) annotations in SRCU updater code.

Thanks,

Bart.