Messages in this thread |  | | | Date | Thu, 29 Sep 2005 22:55:01 +0200 | | From | Willy Tarreau <> | | Subject | Re: [PATCH 1/3] 2.6.14-rc2-mm1: fixes for overflow msec_to_jiffies() |
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On Thu, Sep 29, 2005 at 12:52:07PM -0700, Andrew Morton wrote: > Willy Tarreau <willy@w.ods.org> wrote: > > > > Thanks Andrew, > > > > I'm very sorry because I have verified the code with gcc-2.95.3, > > gcc-3.3.6 and gcc-3.4.4 on x86 and alpha to ensure that everything > > went smooth on archs where sizeof(long) > sizeof(int). But I've > > tested all the combinations in user-space for obvious ease of > > validation. I believe I forgot to use -Wall. What architecture > > gave you this, and with which compiler please ? I'm willing to > > fix this as soon as I can understand the root of the problem. > > > > http://www.zip.com.au/~akpm/linux/patches/stuff/top-posting.txt
Guess what ? I hate it too when others do it, but I often think that my mail will be a one-liner which will be easier to read this way, and of course I'm wrong. Nice FAQ BTW.
> > On Thu, Sep 29, 2005 at 02:43:12AM -0700, Andrew Morton wrote: > > > Willy Tarreau <willy@w.ods.org> wrote: > > > > > > > > +#if HZ <= MSEC_PER_SEC && !(MSEC_PER_SEC % HZ) > > > > +# define MAX_MSEC_OFFSET \ > > > > + (ULONG_MAX - (MSEC_PER_SEC / HZ) + 1) > > > > > > That generates numbers which don't fit into unsigned ints, yielding vast > > > numbers of > > > > > > include/linux/jiffies.h: In function `msecs_to_jiffies': > > > include/linux/jiffies.h:310: warning: comparison is always false due to limited range of data type > > > include/linux/jiffies.h: In function `usecs_to_jiffies': > > > include/linux/jiffies.h:323: warning: comparison is always false due to limited range of data type > > > > > This was a ppc64 build, gcc-3.3.3, CONFIG_HZ=250
OK, I have a free account on a ppc64 machine in case I cannot reproduce on anything else.
> Look a the value which MAX_MSEC_OFFSET will take (it's 2^63 minus a bit). > Comparing that to an unsigned int will generate the always-true or > always-false warning.
I see it. I've done the ulong magic only on the constant computation, while in theory, I should have casted m to ulong before the comparison because m is implicitly casted to ulong in the return (common type). In practise, it should be better to cast MAX_MSEC_OFFSET to unsigned int in the comparison, but there's still a risk of warning if MAX_MSEC_OFFSET becomes equal to ~0.
I'll makes a few tests and check that gcc is smart enough to remove the cast code when unneeded if I cast m to ulong.
Thanks for the details, Willy
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