[Kernel-janitors] Re: [PATCH] usb/auerswald: replace

[Nishanth Aravamudan <nacc@us.ibm.com>]

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On Wed, Jul 28, 2004 at 08:23:23PM +0200, Wolfgang M?es wrote:
> Hello Nishanth,
> 
> On Wednesday 28 July 2004 01:12, Nishanth Aravamudan wrote:
> > I would appreciate any comments from the janitors list.
> >
> > Applys-to: 2.6.7
> >
> > Description: Use msleep() instead of schedule_timeout() to
> > guarantee the task delays for the desired time.
> >
> > Signed-off-by: Nishanth Aravamudan <nacc@us.ibm.com>
> >
> >
> > --- linux-vanilla/drivers/usb/misc/auerswald.c	2004-06-16
> 
> I am the author of this driver.
> 
> Can you give me the rationale of the proposed change?
> In what sense is msleep() better/other than schedule_timeout?

Certainly. Some of this is going to be a repetition from previous posts
to the list, but oh well.

In this particular case, the rationale is quite simple. There are
certain situations -- the details of which I will have to defer to Greg
Kroah-Hartman for -- in which a code snippet such as:

set_current_state(TASK_UNINTERRUPTIBLE);
schedule_timeout(some_time);

will not delay as desired, i.e. the task resumes *before* some_time has
elapsed, instead of after. To prevent this condition from occuring,
msleep() wraps the schedule_timeout() in a while-loop on the timeout
value. Thus, as long as the timeout retains a positive value the task
gives the CPU back up and delays again. And once the timeout is 0, the
task is resumed. For the details, I recommend consulting the source
(kernel/timer.c).

Also, and this is less technical, but msleep() allows code to be written
in terms of milliseconds instead of jiffies, which is far clearer to a
person, IMO. All rounding issues are taken care of by msleep() as well,
as it uses msecs_to_jiffies() to calculate the correct delay value for
schedule_timeout().

Does that help?

Thanks,
Nish

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