[Q] jiffies overflow & timers.

[Ihar 'Philips' Filipau]

Hello!

   [ ver 2.4.18/22 ]

   [ I feel strongly that is FAQ - so any ptr(RTFM)!=0 apreciated. ]

   [ ldd2 covers this very sparsely - overflow case is not covered at 
all - just mentioned. Google - looks like I cannot find right keywords 
for this ... ]

   I'm trying to find correct solution for case of jiffies overflow and 
standard kernel timers (./kernel/timer.c).

   My module has to maintain list of timers. I cannot reuse directly 
struct timer_list - since it uses jiffies and jiffies do wrap on overflow.

   I decided to use struct timeval & do_gettimeofday(). But still I 
have to handle case when next timer to expire will happend after jiffies 
will overflow.

  So my question - how to detect that jiffies had overflown?

  Is the following code is sufficient?
  (Assuming that I will not try to set timer longer than (~0UL/(HZ)) 
seconds)

unsigned long
tv_get_next_expiring_jiffies( struct timeval *target_tv )
{
  struct timeval curr_tv, timeout;
  ulong dif_jif;

  do_gettimeofday( &curr_tv );

  /* timeout = curr_tv - target_tv */
  tv_sub( &timeout, &curr_tv, target_tv );

  dif_jif = tv_to_jiffies( &timeout );   /* assumption above. */

  if (jiffies > ~0UL - dif_jif) {
      /* will overflow
       * so wait for overflow, then just reschedule
       */
      return ~0UL;
  } else {
      /* will not */
      return jiffies + dif_jif;
  }
}

Is (~0UL) "safe harbour"? in other words - will this value reached? /me 
cannot find where jiffies is incremented... Dumd grep -r jiffies gives 
no results (no assignment, no taking of pointer, literally no matches in 
.S files - what I'm missing?) Because if this value will be reached and 
I will detect that jiffies == ~0UL I will have to "while(jiffies == 
~0UL);" wait for overflow.

--
Ihar 'Philips' Filipau  / with best regards from Saarbruecken.
--                                                           _ _ _
 "... and for $64000 question, could you get yourself       |_|*|_|
   vaguely familiar with the notion of on-topic posting?"   |_|_|*|
                                -- Al Viro @ LKML           |*|*|*|

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