Re: [PATCH 2/3] blk-cgroup: store blkcg in bio instead of blkg

[Nilay Shroff]

On 8/23/26 7:00 PM, Yu Kuai wrote:
> @@ -257,20 +262,21 @@ static inline bool bio_issue_as_root_blkg(struct bio *bio)
>    * @blkcg: blkcg of interest
>    * @q: request_queue of interest
>    *
>    * Lookup a blkg for the @blkcg - @q pair, whether it is online or dying.
>    *
> - * Must be called in a RCU critical section.
> + * Must be called with either RCU read lock or queue_lock held.
>    *
>    * This does not acquire a reference.  The caller must already hold one, or
>    * have the blkg pinned by I/O.
>    */
>   static inline struct blkcg_gq *blkg_lookup_any(struct blkcg *blkcg,
>   					       struct request_queue *q)
>   {
> -	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
> -			 "blkg_lookup_any() requires an RCU read lock");
> +	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&
> +			 !lockdep_is_held(&q->queue_lock),
> +			 "blkg_lookup_any() requires an RCU read lock or queue_lock");
>   
>   	return rhashtable_lookup(&q->blkg_hash, &blkcg->css.id,
>   				 blkg_hash_params);
>   }

The changes look good to me. However, rhashtable_lookup() is annotated
with __must_hold_shared(RCU). Since we now officially support Clang
context annotations in the block layer, compiling with the latest
Clang/LLVM generates a number of context warnings, for example:

block/blk-cgroup.h:277:9: warning: calling function 'rhashtable_lookup'
requires holding __ctx_lock_RCU 'RCU' [-Wthread-safety-analysis]
  277 |         return rhashtable_lookup(&q->blkg_hash, &blkcg->css.id,
      |                ^

The same warning is reported from several other block layer source
files that include blk-cgroup.h.

This indicates that rhashtable_lookup() requires the RCU read lock
from Clang's context-analysis perspective. Looking at the call paths of
blkg_lookup_any(), it appears that not all callers hold
rcu_read_lock() when invoking it.

I think we should fix those call paths to acquire rcu reader lock
before calling blkg_lookup_any(), rather than weakening or bypassing
the context annotation in blkg_lookup_any().

Thanks,
--Nilay