Re: [PATCH 2/4] block: move wbt allocation into blk_alloc_queue

On 5/25/21 1:04 AM, Ming Lei wrote:

wbt_init() calls wbt_alloc() which adds allocated wbt instance into
q->rq_qos. This way is very dangerous because q->rq_qos is accessed in
IO fast path.

So far wbt_init() is called in the two code paths:

1) blk_register_queue(), when the block device has been exposed to
usespace, so IO may come when adding wbt into q->rq_qos

2) sysfs attribute store, in which normal IO is definitely allowed

Move wbt allocation into blk_alloc_queue() for avoiding to add wbt
instance dynamically to q->rq_qos. And before calling wbt_init(), the
wbt is disabled, so functionally it works as expected.

I don't like this change since it is not generic - it only helps the WBT
implementation.

All rq-qos policies call rq_qos_add() and all these policies take effect
before rq_qos_add() returns. Does the q->rq_qos list perhaps have to be
protected with RCU? Would that be sufficient to fix the crashes reported
in the cover letter?

Thanks,

Bart.