linux-kernel - Re: [PATCH v2] block: document blk-plug

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Message-Id: <20110829144843.c8e9d397.akpm@linux-foundation.org>
Date:	Mon, 29 Aug 2011 14:48:43 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	sjayaraman@...e.de
Cc:	Jens Axboe <axboe@...nel.dk>, LKML <linux-kernel@...r.kernel.org>,
	Shaohua Li <shaohua.li@...el.com>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v2] block: document blk-plug

On Mon, 29 Aug 2011 16:58:21 +0530
Suresh Jayaraman <sjayaraman@...e.de> wrote:

> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -863,17 +863,23 @@ struct request_queue *blk_alloc_queue_node(gfp_t, int);
>  extern void blk_put_queue(struct request_queue *);
>  
>  /*
> + * blk_plug allows to build up a queue of related requests by holding the I/O
> + * fragments for a short period. This allows merging of sequential requests
> + * into single larger request. As the requests are moved from per-task list to
> + * the device's request_queue in a batch, this results in improved
> + * scalability as the lock contention for request_queue lock is reduced.
> + *
>   * Note: Code in between changing the blk_plug list/cb_list or element of such
>   * lists is preemptable, but such code can't do sleep (or be very careful),
>   * otherwise data is corrupted. For details, please check schedule() where
>   * blk_schedule_flush_plug() is called.

What does the older part of this comment mean?  If a code section is
preemptible then it *will* sleep.  That's what preemption does.

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