Re: [PATCH] sched: fix never executed code due to expression always false

[Jesper Juhl <juhl-lkml@dif.dk>]

On Fri, 15 Apr 2005, Nick Piggin wrote:

> Jesper Juhl wrote:
> > On Fri, 15 Apr 2005, Nick Piggin wrote:
> > 
> > 
> > > Jesper Juhl wrote:
> > > 
> > > > There are two expressions in kernel/sched.c that are always false since
> > > > they
> > > > test for <0 but the result of the expression is unsigned so they will
> > > > never
> > > > be less than zero. This patch implement the logic that I believe is
> > > > intended
> > > > without the signedness issue and without the nasty casts.
> > > > <disclaimer>patch is compile tested only</disclaimer>
> > > > 
> > > This is not *quite* the intended behaviour. It is OK for prev->timestamp
> > > to be '0 - a bit' and now to be '0 + a bit' in the case of wrapping.
> > > 
> > > Although considering they're 64-bit values, I'm not sure how much we care.
> > > 
> > 
> > How do you propose to fix this then?  As the code is now the expressionsa
> > are always false - should we just remove the them?  Or do you have a
> > sensible definition of "a bit" ?  or ome other suggestion alltogether?
> > 
> 
> Make it a signed comparison?
> 
As per this patch perhaps? : 

Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>

--- linux-2.6.12-rc2-mm3-orig/kernel/sched.c	2005-04-11 21:20:56.000000000 +0200
+++ linux-2.6.12-rc2-mm3/kernel/sched.c	2005-04-15 02:21:34.000000000 +0200
@@ -2697,9 +2697,10 @@ need_resched_nonpreemptible:
 	schedstat_inc(rq, sched_cnt);
 	now = sched_clock();
 	if (likely((long long)now - prev->timestamp < NS_MAX_SLEEP_AVG)) {
-		run_time = now - prev->timestamp;
-		if (unlikely((long long)now - prev->timestamp < 0))
+		if (unlikely(((long long)now - (long long)prev->timestamp) < 0))
 			run_time = 0;
+		else
+			run_time = now - prev->timestamp;
 	} else
 		run_time = NS_MAX_SLEEP_AVG;
 
@@ -2776,7 +2777,7 @@ go_idle:
 
 	if (!rt_task(next) && next->activated > 0) {
 		unsigned long long delta = now - next->timestamp;
-		if (unlikely((long long)now - next->timestamp < 0))
+		if (unlikely(((long long)now - (long long)next->timestamp) < 0))
 			delta = 0;
 
 		if (next->activated == 1)


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