linux-kernel - Re: [PATCH] iocost: don't nest spin_lock_irq in ioc_weight

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Date:   Fri, 01 Nov 2019 11:39:06 -0400
From:   Jeff Moyer <jmoyer@...hat.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] iocost: don't nest spin_lock_irq in ioc_weight_write()

Dan Carpenter <dan.carpenter@...cle.com> writes:

> This code causes a static analysis warning:
>
>     block/blk-iocost.c:2113 ioc_weight_write() error: double lock 'irq'
>
> We disable IRQs in blkg_conf_prep() and re-enable them in
> blkg_conf_finish().  IRQ disable/enable should not be nested because
> that means the IRQs will be enabled at the first unlock instead of the
> second one.

Can you please also add a comment stating that irqs were disabled in
blkg_conf_prep?  Otherwise future readers will surely be scratching
their heads trying to figure out why we do things two different ways in
the same function.

Thanks!
Jeff

>
> Fixes: 7caa47151ab2 ("blkcg: implement blk-iocost")
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> ---
>  block/blk-iocost.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 2a3db80c1dce..a7ed434eae03 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c
> @@ -2110,10 +2110,10 @@ static ssize_t ioc_weight_write(struct kernfs_open_file *of, char *buf,
>  			goto einval;
>  	}
>  
> -	spin_lock_irq(&iocg->ioc->lock);
> +	spin_lock(&iocg->ioc->lock);
>  	iocg->cfg_weight = v;
>  	weight_updated(iocg);
> -	spin_unlock_irq(&iocg->ioc->lock);
> +	spin_unlock(&iocg->ioc->lock);
>  
>  	blkg_conf_finish(&ctx);
>  	return nbytes;