Messages in this thread | | | From | "Liu Yu" <> | Subject | re: Wrong use of MAX_JIFFY_OFFSET? | Date | Mon, 9 Apr 2012 10:18:23 +0800 |
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> -----邮件原件----- > 发件人: Andrew Morton [mailto:akpm@linux-foundation.org] > 发送时间: 2012年4月7日 3:05 > 收件人: Liu Yu > 抄送: linux-kernel@vger.kernel.org > 主题: Re: Wrong use of MAX_JIFFY_OFFSET? > > On Wed, 28 Mar 2012 15:12:24 +0800 > "Liu Yu" <liuyums@bwstor.com.cn> wrote: > > > Hi guys, > > > > I saw a couple of places in current kernel have this kind of code: > > > > > static inline unsigned int elapsed_jiffies_msecs(unsigned long start) > > > { > > > unsigned long end = jiffies; > > > > > > if (end >= start) > > > return jiffies_to_msecs(end - start); > > > > > > return jiffies_to_msecs(end + (MAX_JIFFY_OFFSET - start) + 1); > > > } > > > > As you know, jiffies has a type of unsigned long, so if we know which is the > > end and > > which is the start, then (end - start) can simply figure out how much > > jiffies flies, > > without worry about the overflow. > > > > Look at the code above, assume that there is just an overflow happening on > > jiffies: end=0 and start=~0UL. > > Since end < start, then the return value of the function is > > jiffies_to_msecs(MAX_JIFFY_OFFSET+2). > > But shouldn't the correct value be jiffies_to_msecs(1)? > > > > could someone tell me that am I missing anything? > > > > Seems right. The code should be > > static inline unsigned long elapsed_jiffies_msecs(unsigned long start) > { > return jiffies_to_msecs(jiffies - start); > } > > Note the return type. jiffies_to_msecs() currently returns unsigned > int. I think it should return unsigned long. Even then, it can still > overflow with valid inputs on HZ=100 32-bit machines.
Thanks a lot for your reply :)
I saw similar code in drivers/net/wireless/ipw2x00/libipw_wx.c net/wireless/scan.c include/linux/jbd2.h
hope someone can fix this. :)
Thanks, Yu
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