Messages in this thread | | | Date | Fri, 25 Jul 2008 23:17:03 +0100 | From | Ben Dooks <> | Subject | Re: recent IDE regression |
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On Fri, Jul 25, 2008 at 09:37:25AM -0700, Linus Torvalds wrote: > > > On Fri, 25 Jul 2008, Ben Dooks wrote: > > > > personally, i would much prefer to see the loop being less evil > > like: > > > > for (p = s; p < end; p += 2) > > be16_to_cpus((u16 *)p); > > Well, in this case, the code actually depends on 'p' being back at the > start of the buffer by the end of it all, so it would need some more > changes than that.
I'm not adverse to the loop running backwards, it is just that we end up evaluating macro with side-effects to the pointer itself, which is "just asking for trouble".
for (p = end ; p != s; p -= 2) be16_to_cpus((u16 *)p);
This removes the side-effect of be16_to_cpus(), which means that the change to p is moved back into the for statement; After all, the for construct has three code blocks for a reason!
I just wonder what would happen if be16_to_cpus(x) evaluated 'x' more than once... it would be difficult to find problems introduced if this happened as it would fail to hang, just not do the 'right' thing...
> But yes, I applied David's patch, but I _also_ suspect that we would be > better off without code that does horrid things like casts and assignments > inside the function arguments. > > So it would be nice to re-code that loop to be more readable. But due to > the reliance of 'p' being 's' after the loop, the minimal patch would be > something like the appended.
As noted above, I don't have any problems with the looping going backwards, just the problems with potential side-effects.
> Bartlomiej - take this or not, I'm not going to commit it - I haven't > tested it, nor do I even have any machines that would trigger it. So this > is more a "maybe something like this" than anything else. > > Linus > > --- > drivers/ide/ide-iops.c | 8 +++++--- > 1 files changed, 5 insertions(+), 3 deletions(-) > > diff --git a/drivers/ide/ide-iops.c b/drivers/ide/ide-iops.c > index 8aae917..ae03151 100644 > --- a/drivers/ide/ide-iops.c > +++ b/drivers/ide/ide-iops.c > @@ -506,14 +506,16 @@ void ide_fix_driveid (struct hd_driveid *id) > > void ide_fixstring (u8 *s, const int bytecount, const int byteswap) > { > - u8 *p = s, *end = &s[bytecount & ~1]; /* bytecount must be even */ > + u8 *p, *end = &s[bytecount & ~1]; /* bytecount must be even */ > > if (byteswap) { > /* convert from big-endian to host byte order */ > - for (p = end ; p != s;) > - be16_to_cpus((u16 *)(p -= 2)); > + for (p = s ; p != end ; p += 2) > + be16_to_cpus((u16 *) p); > } > + > /* strip leading blanks */ > + p = s; > while (s != end && *s == ' ') > ++s; > /* compress internal blanks and strip trailing blanks */
-- Ben
Q: What's a light-year? A: One-third less calories than a regular year.
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