| Message ID | 20210608071903.431195-2-ming.lei@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | block: fix race between adding wbt and normal IO | expand |
On 6/8/21 12:19 AM, Ming Lei wrote: > static inline void rq_qos_add(struct request_queue *q, struct rq_qos *rqos) > { > + /* > + * No IO can be in-flight when adding rqos, so freeze queue, which > + * is fine since we only support rq_qos for blk-mq queue > + */ > + blk_mq_freeze_queue(q); > rqos->next = q->rq_qos; > q->rq_qos = rqos; > + blk_mq_unfreeze_queue(q); > > if (rqos->ops->debugfs_attrs) > blk_mq_debugfs_register_rqos(rqos); > @@ -110,12 +117,18 @@ static inline void rq_qos_del(struct request_queue *q, struct rq_qos *rqos) > { > struct rq_qos **cur; > > + /* > + * No IO can be in-flight when removing rqos, so freeze queue, > + * which is fine since we only support rq_qos for blk-mq queue > + */ > + blk_mq_freeze_queue(q); > for (cur = &q->rq_qos; *cur; cur = &(*cur)->next) { > if (*cur == rqos) { > *cur = rqos->next; > break; > } > } > + blk_mq_unfreeze_queue(q); > > blk_mq_debugfs_unregister_rqos(rqos); > } Although this patch looks like an improvement to me, I think we also need protection against concurrent rq_qos_add() and rq_qos_del() calls, e.g. via a mutex. Thanks, Bart.
On Tue, Jun 08, 2021 at 08:04:00AM -0700, Bart Van Assche wrote: > On 6/8/21 12:19 AM, Ming Lei wrote: > > static inline void rq_qos_add(struct request_queue *q, struct rq_qos *rqos) > > { > > + /* > > + * No IO can be in-flight when adding rqos, so freeze queue, which > > + * is fine since we only support rq_qos for blk-mq queue > > + */ > > + blk_mq_freeze_queue(q); > > rqos->next = q->rq_qos; > > q->rq_qos = rqos; > > + blk_mq_unfreeze_queue(q); > > > > if (rqos->ops->debugfs_attrs) > > blk_mq_debugfs_register_rqos(rqos); > > @@ -110,12 +117,18 @@ static inline void rq_qos_del(struct request_queue *q, struct rq_qos *rqos) > > { > > struct rq_qos **cur; > > > > + /* > > + * No IO can be in-flight when removing rqos, so freeze queue, > > + * which is fine since we only support rq_qos for blk-mq queue > > + */ > > + blk_mq_freeze_queue(q); > > for (cur = &q->rq_qos; *cur; cur = &(*cur)->next) { > > if (*cur == rqos) { > > *cur = rqos->next; > > break; > > } > > } > > + blk_mq_unfreeze_queue(q); > > > > blk_mq_debugfs_unregister_rqos(rqos); > > } > > Although this patch looks like an improvement to me, I think we also > need protection against concurrent rq_qos_add() and rq_qos_del() calls, > e.g. via a mutex. Fine, one spinlock should be enough, will do it in V3. Thanks, Ming
diff --git a/block/blk-rq-qos.h b/block/blk-rq-qos.h index 2bc43e94f4c4..c9dccb344312 100644 --- a/block/blk-rq-qos.h +++ b/block/blk-rq-qos.h @@ -7,6 +7,7 @@ #include <linux/blk_types.h> #include <linux/atomic.h> #include <linux/wait.h> +#include <linux/blk-mq.h> #include "blk-mq-debugfs.h" @@ -99,8 +100,14 @@ static inline void rq_wait_init(struct rq_wait *rq_wait) static inline void rq_qos_add(struct request_queue *q, struct rq_qos *rqos) { + /* + * No IO can be in-flight when adding rqos, so freeze queue, which + * is fine since we only support rq_qos for blk-mq queue + */ + blk_mq_freeze_queue(q); rqos->next = q->rq_qos; q->rq_qos = rqos; + blk_mq_unfreeze_queue(q); if (rqos->ops->debugfs_attrs) blk_mq_debugfs_register_rqos(rqos); @@ -110,12 +117,18 @@ static inline void rq_qos_del(struct request_queue *q, struct rq_qos *rqos) { struct rq_qos **cur; + /* + * No IO can be in-flight when removing rqos, so freeze queue, + * which is fine since we only support rq_qos for blk-mq queue + */ + blk_mq_freeze_queue(q); for (cur = &q->rq_qos; *cur; cur = &(*cur)->next) { if (*cur == rqos) { *cur = rqos->next; break; } } + blk_mq_unfreeze_queue(q); blk_mq_debugfs_unregister_rqos(rqos); }
Yi reported several kernel panics on: [16687.001777] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000008 ... [16687.163549] pc : __rq_qos_track+0x38/0x60 or [ 997.690455] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000020 ... [ 997.850347] pc : __rq_qos_done+0x2c/0x50 Turns out it is caused by race between adding rq qos(wbt) and normal IO because rq_qos_add can be run when IO is being submitted, fix this issue by freezing queue before adding/deleting rq qos to queue. rq_qos_exit() needn't to freeze queue because it is called after queue has been frozen. iolatency calls rq_qos_add() during allocating queue, so freezing won't add delay because queue usage refcount works at atomic mode at that time. iocost calls rq_qos_add() when writing cgroup attribute file, that is fine to freeze queue at that time since we usually freeze queue when storing to queue sysfs attribute, meantime iocost only exists on the root cgroup. wbt_init calls it in blk_register_queue() and queue sysfs attribute store(queue_wb_lat_store() when write it 1st time in case of !BLK_WBT_MQ), the following patch will speedup the queue freezing in wbt_init. Reported-by: Yi Zhang <yi.zhang@redhat.com> Cc: Bart Van Assche <bvanassche@acm.org> Signed-off-by: Ming Lei <ming.lei@redhat.com> --- block/blk-rq-qos.h | 13 +++++++++++++ 1 file changed, 13 insertions(+)