Re: [LKP] [writeback] 8bc4ad9498: INFO: suspicious RCU usage. ]

[Jens Axboe]

On 09/07/2016 08:04 PM, Ye Xiaolong wrote:
> On 09/08, Fengguang Wu wrote:
> > Hi Jens and Tejun,
> >
> > > The command line was in the original email:
> > >
> > > $ qemu-system-x86_64 -enable-kvm -cpu Westmere -m 512M
> > >
> > > And agree, in general it'd be nice if there was a link to the image as
> > > well, so that folks can reproduce.
> >
> > Yes we have a reproduce script internally, however need to polish it
> > up and make it general usable by kernel developers.
> >
> > CC Philip. It'd be helpful to give more priority to the "lkp qemu"
> > reproduce command.
> >
> > For the warning here, it does not depend on any initrd, I just
> > confirmed this simple script can reproduce the problem.
>
> Yes, just confirmed that v4.8-rc5 doesn't have this warning, while kernel
> for the updated commit (fa0e2cb1af765691fabd329f03cad563a0eebf18) on branch
> wb-buf-throttle could reproduce it.

Anyone willing to try the below patch?

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ef61bda76317..a7bb2e6bcaef 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -3764,9 +3764,11 @@ static void check_blkcg_changed(struct cfq_io_cq 
*cic, struct bio *bio)
 	struct cfq_data *cfqd = cic_to_cfqd(cic);
 	struct cfq_queue *cfqq;
 	uint64_t serial_nr;
+	bool nonroot_cg;

 	rcu_read_lock();
 	serial_nr = bio_blkcg(bio)->css.serial_nr;
+	nonroot_cg = bio_blkcg(bio) != &blkcg_root;
 	rcu_read_unlock();

 	/*
@@ -3781,7 +3783,7 @@ static void check_blkcg_changed(struct cfq_io_cq 
*cic, struct bio *bio)
 	 * do proper throttling of writes. Turn off wbt for that
 	 * case.
 	 */
-	if (bio_blkcg(bio) != &blkcg_root) {
+	if (nonroot_cg) {
 		struct request_queue *q = cfqd->queue;

 		if (q->rq_wb)

--
Jens Axboe