Re: [PATCH 4/5] blk-mq-debugfs: Show busy requests

Bart Van Assche
2017-05-30T17:24:39Z

On Sat, 2017-05-27 at 08:54 +0800, Ming Lei wrote:
> On Thu, May 25, 2017 at 04:38:09PM -0700, Bart Van Assche wrote:
> > Requests that got stuck in a block driver are neither on
> > blk_mq_ctx.rq_list nor on any hw dispatch queue. Make these
> > visible in debugfs through the "busy" attribute.
>=20
> The name of 'busy' isn't very explicit about this case, and I
> guess you mean the requests are dispatched to hardware, so
> 'dispatched' or sort of name may be more accurate.

Hello Ming,

There is already a debugfs attribute with the name "dispatch". I'm afraid
having one attribute with the name "dispatch" and another with the name
"dispatched" would be confusing.

> [ ... ]
> During this small window, the request can be freed and reallocated
> in another I/O path, then use-after-free is caused.

A similar remark applies to all request queue debugfs attributes: the queue
state can change immediately after having queried the state so that's not
unique to this attribute. Regarding the "use-after-free": the memory that i=
s
allocated for requests is only freed after the debugfs attributes have been
removed so the code that implements this attribute will read the contents o=
f
a struct request. It is up to the person who reads the contents of this
attribute to decide how to interpret the contents.

> But the new fixed blk_mq_quiesce_queue() can be used before calling
> blk_mq_tagset_busy_iter() to avoid the race.

That would be overkill. The "busy" attribute is intended as a debugging hel=
p.
The implementation of this function should trigger any crashes. But it was
not my intention to avoid data races when I implemented this function.

Bart.=