Re: [Kernel-janitors] RE: [PATCH] cciss: replace schedule_timeout()

[Nish Aravamudan <nish.aravamudan@gmail.com>]

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uOn Fri, 23 Jul 2004 17:45:55 -0500, Miller, Mike (OS Dev)
<mike.miller@hp.com> wrote:
> You'll have to explain why msleep will guarantee the timeout period whereas 
> schedule_timeout may not. Maybe I'm missing something.

Well, there are two things to consider here:

As a reference, the way the code was written before: 
-               set_current_state(TASK_INTERRUPTIBLE);
-               schedule_timeout(HZ / 10); /* wait 100ms */

1) Since the state is set to TASK_INTERRUPTIBLE, although a 100ms
delay is requested, if a signal is received, then the delay ends
prematurely and the task will resume after the schedule_timeout()
call. Now, if this was the desired effect, then the the patch should
not be applied.

2) However, if a true delay was desired, then TASK_INTERRUPTIBLE
should not have been the set state, but TASK_UNINTERRUPTIBLE should
have been. This would have made the code:
                 set_current_state(TASK_UNINTERRUPTIBLE);
                 schedule_timeout(HZ / 10);
msleep() achieves this exact purpose, but also:
  a) allows the parameter time to be in msecs, which is far clearer IMO; and
  b) guarantees that the task will not continue execution until after
the timeout has expired. Confusingly (at least to me), there are
certain conditions -- the details of which I will have to defer to
Greg Kroah-Hartman -- in which TASK_UNINTERRUPTIBLE'd
schedule_timeout()s may still return before the timeout. The desired
delay is achieved by wrapping the schedule_timeout(timeout) with a
while(timeout) loop. Thus, as long as timeout is non-zero, the task is
deferred. (see msleep()s definition for more details).

Does that help some? Please mail again if you have more questions, or
if something I wrote was unclear.

-Nish

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