Re: [Linux-ia64] Help with Ingo scheduler on IA64

David Mosberger
2002-01-17T01:42:30Z

In case you're still fighting with Ingo's scheduler on 2.4.xx, you
might want to try the attached patch. It's a gross hack to work
around a race condition in set_cpus_allowed(). Without this hack, the
kernel would almost always die as soon as it tried to migrate
ksoftirqd_CPU1 from CPU 0 to CPU 1. With the patch, it boots up to
the point where the serial line driver gets initialized, which is
progress, I suppose.

The fundamental problem is that current set_cpus_allowed() lets the
same task run on two CPUs for a brief period of time. This violates
scheduling assumptions made by the kernel and hence results in race
conditions.

--david

--- linux-2.5.2/kernel/sched.c Tue Jan 15 22:10:27 2002
+++ lia64-kdb/kernel/sched.c Wed Jan 16 17:24:39 2002
@@ -230,7 +230,7 @@
spin_unlock_irq(&this_rq()->lock);
}

-static inline void context_switch(task_t *prev, task_t *next)
+static inline void context_switch(task_t *prev, task_t *next, int dont_clear)
{
struct mm_struct *mm = next->mm;
struct mm_struct *oldmm = prev->active_mm;
@@ -245,7 +245,8 @@
switch_mm(oldmm, mm, next, smp_processor_id());

if (!prev->mm) {
- prev->active_mm = NULL;
+ if (!dont_clear)
+ prev->active_mm = NULL;
mmdrop(oldmm);
}

@@ -519,7 +520,7 @@
rq->nr_switches++;
rq->curr = next;
next->cpu = prev->cpu;
- context_switch(prev, next);
+ context_switch(prev, next, 0);
/*
* The runqueue pointer might be from another CPU
* if the new task was last running on a different
@@ -721,7 +722,7 @@
this_rq->nr_switches++;
this_rq->curr = this_rq->idle;
this_rq->idle->need_resched = 1;
- context_switch(current, this_rq->idle);
+ context_switch(current, this_rq->idle, 1);
barrier();
spin_unlock_irq(&this_rq()->lock);
}