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

Hi,

在 2026/9/6 18:56, Nilay Shroff 写道:

On 8/23/26 7:00 PM, Yu Kuai wrote:

quoted

@@ -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 was not aware of the above clang warning, this should not be a real problem
because queue_lock should have the same protection as rcu, and I was trying
not to make this patch too complex.

I add the rcu_read_lock() in the following patch to covert queue_lock to blkcg_mutex:

[RFC PATCH v3 3/6] blk-cgroup: protect blkgs with blkcg_mutex - Yu Kuai <https://lore.kernel.org/all/20260823152926.1043863-4-yukuai@kernel.org/ (local)>

Since this is related to compile warning, I'll move the rcu protection to this
patch.

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

-- 
Thanks,
Kuai