Re: [PATCH] block: __elv_next_request() shouldn't call into the elevator if bypassing

[Jens Axboe <axboe@kernel.dk>]

On Wed, Jan 29 2014, Tejun Heo wrote:
> request_queue bypassing is used to suppress higher-level function of a
> request_queue so that they can be switched, reconfigured and shut
> down.  A request_queue does the followings while bypassing.
> 
> * bypasses elevator and io_cq association and queues requests directly
>   to the FIFO dispatch queue.
> 
> * bypasses block cgroup request_list lookup and always uses the root
>   request_list.
> 
> Once confirmed to be bypassing, specific elevator and block cgroup
> policy implementations can assume that nothing is in flight for them
> and perform various operations which would be dangerous otherwise.
> 
> Such confirmation is acheived by short-circuiting all new requests
> directly to the dispatch queue and waiting for all the requests which
> were issued before to finish.  Unfortunately, while the request
> allocating and draining sides were properly handled, we forgot to
> actually plug the request dispatch path.  Even after bypassing mode is
> confirmed, if the attached driver tries to fetch a request and the
> dispatch queue is empty, __elv_next_request() would invoke the current
> elevator's elevator_dispatch_fn() callback.  As all in-flight requests
> were drained, the elevator wouldn't contain any request but once
> bypass is confirmed we don't even know whether the elevator is even
> there.  It might be in the process of being switched and half torn
> down.
> 
> Frank Mayhar reports that this actually happened while switching
> elevators, leading to an oops.
> 
> Let's fix it by making __elv_next_request() avoid invoking the
> elevator_dispatch_fn() callback if the queue is bypassing.  It already
> avoids invoking the callback if the queue is dying.  As a dying queue
> is guaranteed to be bypassing, we can simply replace blk_queue_dying()
> check with blk_queue_bypass().

Thanks Tejun, will queue up right after Linus has merged the previous
requests. And will add Franks tested-by.

-- 
Jens Axboe