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

On 9/6/26 5:09 PM, yu kuai wrote:

Hi,

在 2026/9/6 18:56, Nilay Shroff 写道:

quoted

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 agree that this is not a runtime correctness issue when the lookup is already
protected by q->queue_lock.

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.

Since rhashtable_lookup() is annotated with __must_hold_shared(RCU), Clang's
context analysis still reports warnings for these call paths. While this
doesn't cause a build failure (and only generates warning) when CONFIG_WERROR
is not enabled, it could do so for configurations that enable CONFIG_WERROR=y.

So IMO, moving the rcu_read_lock() protection into this patch makes sense.
This would avoid introducing the Clang warnings/errors in this patch, and then
you may accordingly adjust your subsequent series where the q->queue_lock protection
is removed and the blkg lookup paths are protected by RCU.

I also noticed that some paths already take q->queue_lock followed by rcu_read_lock(),
so adding the RCU protection here should be consistent with those existing paths.

Thanks,
--Nilay