Re: [PATCH] serial: 8250_omap: fix a timeout loop condition

Dan Carpenter
2021-04-30T11:41:27Z

On Fri, Apr 30, 2021 at 11:46:07AM +0300, Andy Shevchenko wrote:
> On Thu, Apr 29, 2021 at 04:02:15PM +0300, Dan Carpenter wrote:
> > On Thu, Apr 29, 2021 at 02:08:45PM +0300, Andy Shevchenko wrote:
> > > On Thu, Apr 29, 2021 at 10:19:22AM +0300, Dan Carpenter wrote:
> > > > This loop ends on -1 so the error message will never be printed.
> > > >
> > > > Fixes: 4bcf59a5dea0 ("serial: 8250: 8250_omap: Account for data in flight during DMA teardown")
> > > > Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> > >
> > > ...
> > >
> > > > poll_count--)
> > > > cpu_relax();
> > > >
> > > > - if (!poll_count)
> > > > + if (poll_count == -1)
> > >
> > > Why not to change poll_count-- to --poll_count?
> > >
> >
> > Either one is fine. I considered several different ways and wrote the
> > patch twice. The downside of --poll_count is that it's an off by one
> > in that the author clearly intended to loop 25 times. It doesn't really
> > matter if we only loop 24 but off by ones are aesthetically unpleasant.
>
> I didn't get. If you use --poll_count you get exactly 25 times and moreover,
> you may convert variable to unsigned type.
>

Here is a small test to show that it loops 24 times.

#include <stdio.h>

int main(void)
{
int i = 25;

while (--i)
printf("%d\n", i);

return 0;
}

gcc test.c
./a.out | tac

Why would I make it unsigned? As a static analysis developer,
pointlessly unsigned variables are one of the leading causes for the
bugs I see.

There are times where a iterator counter needs to be unsigned long, or
u64 but I have never seen a case where changing an iterator from
"int i;" to "unsigned int i;" solves a real life kernel bug. It only
introduces bugs.

regards,
dan carpenter