Wrong use of MAX_JIFFY_OFFSET?

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);
}

Hi guys,

I saw a couple of places in current kernel have this kind of code:

Quoted 9 lines

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?

-----邮件原件-----
发件人: Andrew Morton [mailto:[email protected]] 发送时间: 2012年4月7日 3:05
收件人: Liu Yu
抄送: [email protected]
主题: Re: Wrong use of MAX_JIFFY_OFFSET?

Quoted 15 lines · On Wed, 28 Mar 2012 15:12:24 +0800 "Liu Yu" <[email protected]> wrote:

Hi guys,

I saw a couple of places in current kernel have this kind of code:

Quoted 9 lines

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

Quoted 6 lines

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

Quoted 7 lines

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.