linux-kernel - Re: [PATCH v2] cfi: fix deadloop in cfi_cmdset_0002.c do_write

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Date:   Fri, 22 Feb 2019 11:28:46 +0530
From:   Vignesh R <vigneshr@...com>
To:     Liu Jian <liujian56@...wei.com>, <dwmw2@...radead.org>,
        <computersforpeace@...il.com>, <bbrezillon@...nel.org>,
        <marek.vasut@...il.com>, <richard@....at>,
        <joakim.tjernlund@...inera.com>, <ikegami@...ied-telesis.co.jp>,
        <keescook@...omium.org>
CC:     <linux-mtd@...ts.infradead.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] cfi: fix deadloop in cfi_cmdset_0002.c do_write_buffer



On 20/02/19 2:17 AM, Liu Jian wrote:
> In function do_write_buffer(), in the for loop, there is a case
> chip_ready() returns 1 while chip_good() returns 0, so it never
> break the loop.
> To fix this, chip_good() is enough and it should timeout if it stay
> bad for a while.
> 
> Fixes: dfeae1073583 ("mtd: cfi_cmdset_0002: Change write buffer to check correct value")
> Signed-off-by: Yi Huaijie <yihuaijie@...wei.com>
> Signed-off-by: Liu Jian <liujian56@...wei.com>
> Reviewed-by: Tokunori Ikegami <ikegami_to@...oo.co.jp>
> ---
> v1->v2:
> change git log, put the Fixes tag on a single line
> 
>  drivers/mtd/chips/cfi_cmdset_0002.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/mtd/chips/cfi_cmdset_0002.c b/drivers/mtd/chips/cfi_cmdset_0002.c
> index 72428b6..818e94b 100644
> --- a/drivers/mtd/chips/cfi_cmdset_0002.c
> +++ b/drivers/mtd/chips/cfi_cmdset_0002.c
> @@ -1876,14 +1876,14 @@ static int __xipram do_write_buffer(struct map_info *map, struct flchip *chip,
>  			continue;
>  		}
>  
> -		if (time_after(jiffies, timeo) && !chip_ready(map, adr))
> -			break;
> -
>  		if (chip_good(map, adr, datum)) {
>  			xip_enable(map, chip, adr);
>  			goto op_done;
>  		}
>  
> +		if (time_after(jiffies, timeo))
> +			break;
> +

It is quite possible that this thread might be pre-empted just after
chip_good() check but before before time_after(). If the thread, then
resumes execution after timeo has elasped then, this code will wrongly
indicate write failure.

To avoid this case, you should add one more check for check_good() even
when time_after() returns true. Something like:

	if (time_after(jiffies, timeo)) {
		if (chip_good(map, adr, datum)) {
			xip_enable(map, chip, adr);
			goto op_done;
		}
		break;
	}



>  		/* Latency issues. Drop the lock, wait a while and retry */
>  		UDELAY(map, chip, adr, 1);
>  	}
> 

-- 
Regards
Vignesh