On Tue, Dec 22, 2020 at 11:22:19AM +0000, John Garry wrote: > Resend without ppvk@xxxxxxxxxxxxxx, which bounces for me > > On 22/12/2020 02:13, Bart Van Assche wrote: > > On 12/21/20 10:47 AM, John Garry wrote: > >> Yes, I agree, and I'm not sure what I wrote to give that impression. > >> > >> About "root partition", above, I'm just saying that / is mounted on a > >> sda partition: > >> > >> root@ubuntu:/home/john# mount | grep sda > >> /dev/sda2 on / type ext4 (rw,relatime,errors=remount-ro,stripe=32) > >> /dev/sda1 on /boot/efi type vfat > >> (rw,relatime,fmask=0077,dmask=0077,codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro) > > Hi John, > > > > Hi Bart, Ming, > > > Thanks for the clarification. I want to take back my suggestion about > > adding rcu_read_lock() / rcu_read_unlock() in blk_mq_tagset_busy_iter() > > since it is not allowed to sleep inside an RCU read-side critical > > section, since blk_mq_tagset_busy_iter() is used in request timeout > > handling and since there may be blk_mq_ops.timeout implementations that > > sleep. > > Yes, that's why I was going with atomic, rather than some synchronization > primitive which may sleep. > > > > > Ming's suggestion to serialize blk_mq_tagset_busy_iter() and > > blk_mq_free_rqs() looks interesting to me. > > > > So then we could have something like this: > > ---8<--- > > -435,9 +444,13 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, > busy_iter_fn *fn, > if (!blk_mq_hw_queue_mapped(hctx)) > continue; > > + while (!atomic_inc_not_zero(&tags->iter_usage_counter)); > + > if (tags->nr_reserved_tags) > bt_for_each(hctx, tags->breserved_tags, fn, priv, true); > bt_for_each(hctx, tags->bitmap_tags, fn, priv, false); > > + atomic_dec(&tags->iter_usage_counter); > } Then it is just one spin_lock variant, and you may have to consider lock validation. For example, scsi_host_busy() is called from scsi_log_completion()<-scsi_softirq_done(), which may be run in irq context, then dead lock can be triggered when the irq is fired during freeing request. thanks, Ming
