linux-kernel - Re: [PATCH v2 04/10] block: revert back to synchronous request

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Message-ID: <749d56bd-1d66-e47b-a356-8d538e9c99b4@acm.org>
Date:   Sun, 19 Apr 2020 15:23:31 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Luis Chamberlain <mcgrof@...nel.org>, axboe@...nel.dk,
        viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        rostedt@...dmis.org, mingo@...hat.com, jack@...e.cz,
        ming.lei@...hat.com, nstange@...e.de, akpm@...ux-foundation.org
Cc:     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 v2 04/10] block: revert back to synchronous request_queue
 removal

On 4/19/20 12:45 PM, Luis Chamberlain wrote:
> +/**
> + * blk_put_queue - decrement the request_queue refcount
> + *
> + * @q: the request_queue structure to decrement the refcount for
> + *

How about following the example from 
Documentation/doc-guide/kernel-doc.rst and not leaving a blank line 
above the function argument documentation?

> + * Decrements the refcount to the request_queue kobject, when this reaches
                               ^^
                               of?
> + * 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().
> + */

Is calling bdgrab() and bdput() an option from a context in which it is 
not guaranteed that the block device is open?

Does every context that calls blk_put_queue() also call blk_cleanup_queue()?

How about avoiding confusion by changing the last sentence of that 
comment into something like the following: "The last reference must not 
be dropped from atomic context. If it is necessary to call 
blk_put_queue() from atomic context, make sure that that call does not 
decrease the request queue refcount to zero."

>   /**
>    * blk_cleanup_queue - shutdown a request queue
> + *
>    * @q: request queue to shutdown
>    *

How about following the example from 
Documentation/doc-guide/kernel-doc.rst and not leaving a blank line 
above the function argument documentation?

>    * 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().

Surrounding blk_cleanup_queue() with bdgrab() / bdput() does not help. 
This blk_cleanup_queue() must not be called from atomic context.

>   /**
> - * __blk_release_queue - release a request queue
> - * @work: pointer to the release_work member of the request queue to be released
> + * blk_release_queue - release a request queue
> + *
> + * This function is called as part of the process when a block device is being
> + * unregistered. Releasing a request queue starts with blk_cleanup_queue(),
> + * which set the appropriate flags and then calls blk_put_queue() as the last
> + * step. blk_put_queue() decrements the reference counter of the request queue
> + * and once the reference counter reaches zero, this function is called to
> + * release all allocated resources of the request queue.
>    *
> - * Description:
> - *     This function is called when a block device is being unregistered. The
> - *     process of releasing a request queue starts with blk_cleanup_queue, which
> - *     set the appropriate flags and then calls blk_put_queue, that decrements
> - *     the reference counter of the request queue. Once the reference counter
> - *     of the request queue reaches zero, blk_release_queue is called to release
> - *     all allocated resources of the request queue.
> + * This function can sleep, and so we must ensure that the very last
> + * blk_put_queue() is never called from atomic context.
> + *
> + * @kobj: pointer to a kobject, who's container is a request_queue
>    */

Please follow the style used elsewhere in the kernel and move function 
argument documentation just below the line with the function name.

Thanks,

Bart.