Messages in this thread | | | Date | Fri, 6 Apr 2012 12:05:21 -0700 | From | Andrew Morton <> | Subject | Re: Wrong use of MAX_JIFFY_OFFSET? |
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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.
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