Re: scheduler: IRQs disabled over context switches
From: Davide Libenzi (davidel_at_xmailserver.org)
Date: 05/24/04
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Date: Sun, 23 May 2004 16:04:39 -0700 (PDT) To: Russell King <rmk+lkml@arm.linux.org.uk>
On Sun, 23 May 2004, Russell King wrote:
> Not quite - look harder. They use spin_unlock_irq in finish_arch_switch
> rather than prepare_arch_switch.
Hmm, they do indeed. Hmm, if we release the rq lock before the ctx switch,
"prev" (the real one) will result not running since we already set
"rq->curr" to "next" (and we do not hold "prev->switch_lock"). So another
CPU might see "prev" as free-to-pull, while we're still playing with its
fields. Even in UP, we will have a window of time where "rq->curr" is
different from "current", with IRQ enabled (time tick). IMO we have two
problems in releasing the "rq->lock" and enabling IRQs. One is that "prev"
will result free-to-steal from another CPU (after "rq->curr" is set to
"next"). The other one is the timer tick, that might screw us up while
switching. OTOH we cannot enable IRQs while holding the "rq->lock" for
obvious reasons. Maybe something like below. With preempt, we already have
the preempt_disable() at the beginning of schedule(), so the timer tick
will not issue a reschedule on return.
- Davide
Index: kernel/sched.c
===================================================================
RCS file: /usr/src/bkcvs/linux-2.5/kernel/sched.c,v
retrieving revision 1.303
diff -u -r1.303 sched.c
--- a/kernel/sched.c 21 May 2004 20:17:47 -0000 1.303
+++ b/kernel/sched.c 23 May 2004 22:20:29 -0000
@@ -220,6 +220,8 @@
prio_array_t *active, *expired, arrays[2];
int best_expired_prio;
atomic_t nr_iowait;
+ task_t *prev;
+ atomic_t in_ctx_switch;
#ifdef CONFIG_SMP
struct sched_domain *sd;
@@ -243,13 +245,20 @@
#define task_rq(p) cpu_rq(task_cpu(p))
#define cpu_curr(cpu) (cpu_rq(cpu)->curr)
+#define rq_switching(rq) atomic_read(&rq->in_ctx_switch)
+
/*
* Default context-switch locking:
*/
#ifndef prepare_arch_switch
-# define prepare_arch_switch(rq, next) do { } while (0)
-# define finish_arch_switch(rq, next) spin_unlock_irq(&(rq)->lock)
-# define task_running(rq, p) ((rq)->curr == (p))
+# define prepare_arch_switch(rq, next) \
+do { \
+ rq->prev = current; \
+ atomic_inc(&rq->in_ctx_switch); \
+ spin_unlock_irq(&(rq)->lock); \
+} while (0)
+# define finish_arch_switch(rq, next) atomic_dec(&rq->in_ctx_switch)
+# define task_running(rq, p) ((rq)->curr == (p) || (rq_switching(rq) && rq->prev == (p)))
#endif
/*
@@ -1966,6 +1975,9 @@
runqueue_t *rq = this_rq();
task_t *p = current;
+ if (rq_switching(rq))
+ return;
+
rq->timestamp_last_tick = sched_clock();
if (rcu_pending(cpu))
@@ -3915,6 +3927,8 @@
INIT_LIST_HEAD(&rq->migration_queue);
#endif
atomic_set(&rq->nr_iowait, 0);
+ atomic_set(&rq->in_ctx_switch, 0);
+ rq->prev = NULL;
for (j = 0; j < 2; j++) {
array = rq->arrays + j;
-
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