linux-kernel - Re: [PATCH] bcache: Fix a sleep-in-atomic bug

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Date:   Tue, 30 May 2017 23:52:10 -0800
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Jia-Ju Bai <baijiaju1990@....com>
Cc:     shli@...nel.org, axboe@...com, mchristi@...hat.com,
        git@...ux.ewheeler.net, colyli@...e.de, akpm@...ux-foundation.org,
        wangyijing@...wei.com, mingo@...nel.org, mhocko@...e.com,
        linux-bcache@...r.kernel.org, linux-raid@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bcache: Fix a sleep-in-atomic bug

On Wed, May 31, 2017 at 03:23:40PM +0800, Jia-Ju Bai wrote:
> The driver may sleep under a spin lock, and the function call path is:
> journal_wait_for_write (acquire the lock by spin_lock)
>   closure_sync
>     schedule --> may sleep

This patch is incorrect, you've introduced a double unlock.

Did you actually observe a sleep in atomic?

> 
> To fix it, the lock is released before "closure_sync", and the lock is 
> acquired again after this function.
> 
> Signed-off-by: Jia-Ju Bai <baijiaju1990@....com>
> ---
>  drivers/md/bcache/journal.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/drivers/md/bcache/journal.c b/drivers/md/bcache/journal.c
> index 1198e53..ad47c36 100644
> --- a/drivers/md/bcache/journal.c
> +++ b/drivers/md/bcache/journal.c
> @@ -724,6 +724,7 @@ static struct journal_write *journal_wait_for_write(struct cache_set *c,
>  			btree_flush_write(c);
>  		}
>  
> +		spin_unlock(&c->journal.lock);
>  		closure_sync(&cl);
>  		spin_lock(&c->journal.lock);
>  		wait = true;
> -- 
> 1.7.9.5
> 
>