On 6/2/22 13:46, Tejun Heo wrote:
On Thu, Jun 02, 2022 at 01:26:10PM -0400, Waiman Long wrote:
On 6/2/22 12:58, Tejun Heo wrote:
Hello,
On Thu, Jun 02, 2022 at 09:35:43AM -0400, Waiman Long wrote:
@@ -2011,9 +2076,16 @@ void blk_cgroup_bio_start(struct bio *bio)
}
bis->cur.ios[rwd]++;
+ if (!READ_ONCE(bis->lnode.next)) {
+ struct llist_head *lhead = per_cpu_ptr(blkcg->lhead, cpu);
+
+ llist_add(&bis->lnode, lhead);
+ percpu_ref_get(&bis->blkg->refcnt);
Hmm... what guarantees that more than one threads race here? llist assumes
that there's a single writer for a given llist_node and the ref count would
be off too, right?
The llist_add() function is atomic. It calls into llist_add_batch() in
lib/llist.c which uses cmpxchg() to make the change. There is a non-atomic
version __llist_add() which may be problematic in this case. Note that irq
is disabled in the u64_stats_update* critical section, there shouldn't be a
racing thread running in the same cpu. Other cpus will modify their own
version of lhead. Perhaps the non-atomic version can be used here as well.
Ah, right, this is per-cpu, so there can be no second writer trying to add
the same node at the same time. Can you add a comment explaining the overall
design / behavior? Other than that, please feel free to add
Acked-by: Tejun Heo <tj@xxxxxxxxxx>
Thanks.
OK, I will send another patch to document the design in
block/blk-cgroup.c. I don't want to touch this patch unless I need to
correct some code here.
Thanks,
Longman
