* Wrong use of MAX_JIFFY_OFFSET?
@ 2012-03-28 7:12 Liu Yu
2012-04-06 19:05 ` Andrew Morton
0 siblings, 1 reply; 3+ messages in thread
From: Liu Yu @ 2012-03-28 7:12 UTC (permalink / raw)
To: linux-kernel
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?
Thanks,
Yu
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Wrong use of MAX_JIFFY_OFFSET?
2012-03-28 7:12 Wrong use of MAX_JIFFY_OFFSET? Liu Yu
@ 2012-04-06 19:05 ` Andrew Morton
2012-04-09 2:18 ` Liu Yu
0 siblings, 1 reply; 3+ messages in thread
From: Andrew Morton @ 2012-04-06 19:05 UTC (permalink / raw)
To: Liu Yu; +Cc: linux-kernel
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.
^ permalink raw reply [flat|nested] 3+ messages in thread
* re: Wrong use of MAX_JIFFY_OFFSET?
2012-04-06 19:05 ` Andrew Morton
@ 2012-04-09 2:18 ` Liu Yu
0 siblings, 0 replies; 3+ messages in thread
From: Liu Yu @ 2012-04-09 2:18 UTC (permalink / raw)
To: 'Andrew Morton'; +Cc: linux-kernel
> -----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
^ permalink raw reply [flat|nested] 3+ messages in thread
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2012-03-28 7:12 Wrong use of MAX_JIFFY_OFFSET? Liu Yu
2012-04-06 19:05 ` Andrew Morton
2012-04-09 2:18 ` Liu Yu
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