Re: [PATCHv3] blk-mq: pop cached request if it is usable

[Jens Axboe <axboe@kernel.dk>]

On 5/21/26 1:02 PM, Keith Busch wrote:
> From: Keith Busch <kbusch@kernel.org>
> 
> When submitting a bio to blk-mq, if the task should sleep after peeking
> a cached request, but before it pops it, the plug flushes and calls
> blk_mq_free_plug_rqs, freeing the cached_rqs. This creates a
> use-after-free bug. Fix this by popping the cached request before any
> possible blocking calls if it is suitable for use.
> 
> Popping this request first holds a queue reference, so avoid any
> serialization races with queue freezes and can safely proceed with
> dispatching that request to the driver. This potentially increases a
> timing window from when a driver wants to freeze its queue to when
> requests stop being dispatched. That scenario is off the fast path
> though, and drivers need to appropriately handle requests during a
> freeze request anyway.
> 
> The downside is the popped element needs to be individually freed when
> we performed a bio plug merge. The cached request would have had to be
> freed later anyway, but this patch does it inline with building the plug
> list instead of after flushing it.

Keith and I had a side-bar about this earlier today, and yes this is so
much better imho than both v2 or Christoph's sledge hammer.

I've already tested this one too, and as expected, it's not regressing
performance. It's also saner in that there's no gap between peek and pop
anymore, so no risk of further blocking screwing with the list on
unplug.

-- 
Jens Axboe