linux-kernel - Re: [PATCH 1/2] staging: zram: minimize `slot_free

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Date:	Fri, 06 Sep 2013 16:42:07 +0200
From:	Jerome Marchand <jmarchan@...hat.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Minchan Kim <minchan@...nel.org>, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] staging: zram: minimize `slot_free_lock' usage

On 09/06/2013 03:47 PM, Sergey Senozhatsky wrote:
> Calling handle_pending_slot_free() for every RW operation may
> cause unneccessary slot_free_lock locking, because most likely
> process will see NULL slot_free_rq. handle_pending_slot_free()
> only when current detects that slot_free_rq is not NULL.
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> 
> ---
> 
>  drivers/staging/zram/zram_drv.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
> index 91d94b5..17386e2 100644
> --- a/drivers/staging/zram/zram_drv.c
> +++ b/drivers/staging/zram/zram_drv.c
> @@ -532,14 +532,15 @@ static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
>  {
>  	int ret;
>  
> +	if (zram->slot_free_rq)
> +		handle_pending_slot_free(zram);
> +

Calling handle_pending_slot_free() without holding zram->lock?
That's racy.

Jerome

>  	if (rw == READ) {
>  		down_read(&zram->lock);
> -		handle_pending_slot_free(zram);
>  		ret = zram_bvec_read(zram, bvec, index, offset, bio);
>  		up_read(&zram->lock);
>  	} else {
>  		down_write(&zram->lock);
> -		handle_pending_slot_free(zram);
>  		ret = zram_bvec_write(zram, bvec, index, offset);
>  		up_write(&zram->lock);
>  	}
> 
> --
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