linux-kernel - Re: [PATCH 5/5] block: revert back to synchronous request

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Date:   Tue, 14 Apr 2020 08:47:25 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     axboe@...nel.dk, viro@...iv.linux.org.uk, bvanassche@....org,
        gregkh@...uxfoundation.org, rostedt@...dmis.org, mingo@...hat.com,
        jack@...e.cz, ming.lei@...hat.com, nstange@...e.de,
        akpm@...ux-foundation.org, mhocko@...e.com, yukuai3@...wei.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Omar Sandoval <osandov@...com>,
        Hannes Reinecke <hare@...e.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH 5/5] block: revert back to synchronous request_queue
 removal

On Tue, Apr 14, 2020 at 04:19:02AM +0000, Luis Chamberlain wrote:
> Commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") merged on
> v4.12 moved the work behind blk_release_queue() into a workqueue after a
> splat floated around which indicated some work on blk_release_queue()
> could sleep in blk_exit_rl(). This splat would be possible when a driver
> called blk_put_queue() or blk_cleanup_queue() (which calls blk_put_queue()
> as its final call) from an atomic context.
> 
> blk_put_queue() decrements the refcount for the request_queue
> kobject, and upon reaching 0 blk_release_queue() is called. Although
> blk_exit_rl() is now removed through commit db6d9952356 ("block: remove
> request_list code"), we reserve the right to be able to sleep within
> blk_release_queue() context. If you see no other way and *have* be
> in atomic context when you driver calls the last blk_put_queue()
> you can always just increase your block device's reference count with
> bdgrab() as this can be done in atomic context and the request_queue
> removal would be left to upper layers later. We document this bit of
> tribal knowledge as well now, and adjust kdoc format a bit.
> 
> We revert back to synchronous request_queue removal because asynchronous
> removal creates a regression with expected userspace interaction with
> several drivers. An example is when removing the loopback driver and
> issues ioctl from userspace to do so, upon return and if successful one
> expects the device to be removed. Moving to asynchronous request_queue
> removal could have broken many scripts which relied on the removal to
> have been completed if there was no error.
> 
> Using asynchronous request_queue removal however has helped us find
> other bugs, in the future we can test what could break with this
> arrangement by enabling CONFIG_DEBUG_KOBJECT_RELEASE.
> 
> Cc: Bart Van Assche <bvanassche@....org>
> Cc: Omar Sandoval <osandov@...com>
> Cc: Hannes Reinecke <hare@...e.com>
> Cc: Nicolai Stange <nstange@...e.de>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Michal Hocko <mhocko@...nel.org>
> Cc: yu kuai <yukuai3@...wei.com>
> Suggested-by: Nicolai Stange <nstange@...e.de>
> Fixes: dc9edc44de6c ("block: Fix a blk_exit_rl() regression")
> Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
> ---
>  block/blk-core.c       | 19 ++++++++++++++++++-
>  block/blk-sysfs.c      | 38 +++++++++++++++++---------------------
>  include/linux/blkdev.h |  2 --
>  3 files changed, 35 insertions(+), 24 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 5aaae7a1b338..8346c7c59ee6 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -301,6 +301,17 @@ void blk_clear_pm_only(struct request_queue *q)
>  }
>  EXPORT_SYMBOL_GPL(blk_clear_pm_only);
>  
> +/**
> + * blk_put_queue - decrement the request_queue refcount
> + *
> + * Decrements the refcount to the request_queue kobject, when this reaches
> + * 0 we'll have blk_release_queue() called. You should avoid calling
> + * this function in atomic context but if you really have to ensure you
> + * first refcount the block device with bdgrab() / bdput() so that the
> + * last decrement happens in blk_cleanup_queue().
> + *
> + * @q: the request_queue structure to decrement the refcount for
> + */
>  void blk_put_queue(struct request_queue *q)
>  {
>  	kobject_put(&q->kobj);
> @@ -328,10 +339,16 @@ EXPORT_SYMBOL_GPL(blk_set_queue_dying);
>  
>  /**
>   * blk_cleanup_queue - shutdown a request queue
> - * @q: request queue to shutdown
>   *
>   * Mark @q DYING, drain all pending requests, mark @q DEAD, destroy and
>   * put it.  All future requests will be failed immediately with -ENODEV.
> + *
> + * You should not call this function in atomic context. If you need to
> + * refcount a request_queue in atomic context, instead refcount the
> + * block device with bdgrab() / bdput().

I think this needs a WARN_ON thrown in to enforece the calling context.

> + *
> + * @q: request queue to shutdown

Moving the argument documentation seems against the usual kerneldoc
style.

Otherwise this look good, I hope it sticks :)