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Subject: Re: [PATCH v2] cfi: fix deadloop in cfi_cmdset_0002.c do_write_buffer
To:     Liu Jian <liujian56@huawei.com>, <dwmw2@infradead.org>,
        <computersforpeace@gmail.com>, <bbrezillon@kernel.org>,
        <marek.vasut@gmail.com>, <richard@nod.at>,
        <joakim.tjernlund@infinera.com>, <ikegami@allied-telesis.co.jp>,
        <keescook@chromium.org>
CC:     <linux-mtd@lists.infradead.org>, <linux-kernel@vger.kernel.org>
References: <1550609234-40861-1-git-send-email-liujian56@huawei.com>
From:   Vignesh R <vigneshr@ti.com>
Message-ID: <e9923a1f-aefc-fdd9-97d8-5c0012f09726@ti.com>
Date:   Fri, 22 Feb 2019 11:28:46 +0530
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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@huawei.com>
> Signed-off-by: Liu Jian <liujian56@huawei.com>
> Reviewed-by: Tokunori Ikegami <ikegami_to@yahoo.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