| Message ID | 20190821091506.21196-5-ming.lei@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | block: don't acquire .sysfs_lock before removing mq & iosched kobjects | expand |
On 8/21/19 2:15 AM, Ming Lei wrote: > blk_mq_realloc_hw_ctxs() is called from blk_mq_init_allocated_queue() > and blk_mq_update_nr_hw_queues(). For the former caller, the kobject > isn't exposed to userspace yet. For the latter caller, sysfs/debugfs > is un-registered before updating nr_hw_queues. > > On the other hand, commit 2f8f1336a48b ("blk-mq: always free hctx after > request queue is freed") moves freeing hctx into queue's release > handler, so there won't be race with queue release path too. > > So don't hold q->sysfs_lock in blk_mq_realloc_hw_ctxs(). How about mentioning that the locking at the start of blk_mq_update_nr_hw_queues() serializes all blk_mq_realloc_hw_ctxs() calls that happen after a queue has been registered in sysfs? Anyway: Reviewed-by: Bart Van Assche <bvanassche@acm.org>
On Wed, Aug 21, 2019 at 08:56:36AM -0700, Bart Van Assche wrote: > On 8/21/19 2:15 AM, Ming Lei wrote: > > blk_mq_realloc_hw_ctxs() is called from blk_mq_init_allocated_queue() > > and blk_mq_update_nr_hw_queues(). For the former caller, the kobject > > isn't exposed to userspace yet. For the latter caller, sysfs/debugfs > > is un-registered before updating nr_hw_queues. > > > > On the other hand, commit 2f8f1336a48b ("blk-mq: always free hctx after > > request queue is freed") moves freeing hctx into queue's release > > handler, so there won't be race with queue release path too. > > > > So don't hold q->sysfs_lock in blk_mq_realloc_hw_ctxs(). > > How about mentioning that the locking at the start of > blk_mq_update_nr_hw_queues() serializes all blk_mq_realloc_hw_ctxs() calls > that happen after a queue has been registered in sysfs? This patch is actually wrong because elevator switch still may happen during updating nr_hw_queues, since only hctx sysfs entries are un-registered, and "queue/scheduler" is still visible to userspace. So I will drop this patch in V3. Thanks, Ming
diff --git a/block/blk-mq.c b/block/blk-mq.c index b0ee0cac737f..d4c8692aca1f 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -2768,8 +2768,6 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, int i, j, end; struct blk_mq_hw_ctx **hctxs = q->queue_hw_ctx; - /* protect against switching io scheduler */ - mutex_lock(&q->sysfs_lock); for (i = 0; i < set->nr_hw_queues; i++) { int node; struct blk_mq_hw_ctx *hctx; @@ -2820,7 +2818,6 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set, hctxs[j] = NULL; } } - mutex_unlock(&q->sysfs_lock); } /*
blk_mq_realloc_hw_ctxs() is called from blk_mq_init_allocated_queue() and blk_mq_update_nr_hw_queues(). For the former caller, the kobject isn't exposed to userspace yet. For the latter caller, sysfs/debugfs is un-registered before updating nr_hw_queues. On the other hand, commit 2f8f1336a48b ("blk-mq: always free hctx after request queue is freed") moves freeing hctx into queue's release handler, so there won't be race with queue release path too. So don't hold q->sysfs_lock in blk_mq_realloc_hw_ctxs(). Cc: Christoph Hellwig <hch@infradead.org> Cc: Hannes Reinecke <hare@suse.com> Cc: Greg KH <gregkh@linuxfoundation.org> Cc: Mike Snitzer <snitzer@redhat.com> Cc: Bart Van Assche <bvanassche@acm.org> Signed-off-by: Ming Lei <ming.lei@redhat.com> --- block/blk-mq.c | 3 --- 1 file changed, 3 deletions(-)