linux-kernel - Re: generic_make

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Message-ID: <20130430060829.GF9563@kernel.dk>
Date:	Tue, 30 Apr 2013 08:08:29 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	remaper <yp.fangdong@...il.com>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: generic_make_request not locked

On Tue, Apr 30 2013, remaper wrote:
> /*
>  * We only want one ->make_request_fn to be active at a time,
>  * else stack usage with stacked devices could be a problem.
>  * So use current->bio_list to keep a list of requests
>  * submited by a make_request_fn function.
>  * current->bio_list is also used as a flag to say if
>  * generic_make_request is currently active in this task or not.
>  * If it is NULL, then no make_request is active.  If it is non-NULL,
>  * then a make_request is active, and new requests should be added
>  * at the tail
>  */
> void generic_make_request(struct bio *bio)
> {
> ...
>     if (current->bio_list) {
>         /* make_request is active */
>         bio_list_add(current->bio_list, bio);
>         return;
>     }
> ...
> }
> 
> current->bio_list is a global variable, and generic_make_request would
> run on multithread environment, so, i don't understand why here(
> bio_list_add ) not lock?  thx everybody.

'current' refers to the task currently executing task. So
current->bio_list isn't shared, hence there's no need for any locking.

-- 
Jens Axboe

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