Jimmy,
I know I had asked about this before, prompting you to add an
extensive comment. Nevertheless, looking over the (few) rwlock
users, I'm now puzzled by the need for a lock here.
According to the comment in struct hpet_event_channel, this is to
prevent accessing a CPU's timer_deadline after it got cleared from
cpumask. I wonder, however, whether the whole thing couldn't
be done without lock altogether - hpet_broadcast_exit() could
just clear the bit, and handle_hpet_broadcast() could read
timer_deadline before looking at the mask a second time (the
cpumask bit was already found set by the surrounding loop, and
by the time "mask" and "next_event" get actually used they may
have become stale also with the current implementation).
Draft patch below.
Jan
--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -34,18 +34,6 @@ struct hpet_event_channel
int shift;
s_time_t next_event;
cpumask_t cpumask;
- /*
- * cpumask_lock is used to prevent hpet intr handler from accessing other
- * cpu's timer_deadline after the other cpu's mask was cleared --
- * mask cleared means cpu waken up, then accessing timer_deadline from
- * other cpu is not safe.
- * It is not used for protecting cpumask, so set ops needn't take it.
- * Multiple cpus clear cpumask simultaneously is ok due to the atomic
- * feature of cpu_clear, so hpet_broadcast_exit() can take read lock for
- * clearing cpumask, and handle_hpet_broadcast() have to take write lock
- * for read cpumask & access timer_deadline.
- */
- rwlock_t cpumask_lock;
spinlock_t lock;
void (*event_handler)(struct hpet_event_channel *);
@@ -199,17 +187,18 @@ again:
/* find all expired events */
for_each_cpu_mask(cpu, ch->cpumask)
{
- write_lock_irq(&ch->cpumask_lock);
+ s_time_t deadline;
- if ( cpu_isset(cpu, ch->cpumask) )
- {
- if ( per_cpu(timer_deadline, cpu) <= now )
- cpu_set(cpu, mask);
- else if ( per_cpu(timer_deadline, cpu) < next_event )
- next_event = per_cpu(timer_deadline, cpu);
- }
+ rmb();
+ deadline = per_cpu(timer_deadline, cpu);
+ rmb();
+ if ( !cpu_isset(cpu, ch->cpumask) )
+ continue;
- write_unlock_irq(&ch->cpumask_lock);
+ if ( deadline <= now )
+ cpu_set(cpu, mask);
+ else if ( deadline < next_event )
+ next_event = deadline;
}
/* wakeup the cpus which have an expired event. */
@@ -602,7 +591,6 @@ void __init hpet_broadcast_init(void)
hpet_events[i].shift = 32;
hpet_events[i].next_event = STIME_MAX;
spin_lock_init(&hpet_events[i].lock);
- rwlock_init(&hpet_events[i].cpumask_lock);
wmb();
hpet_events[i].event_handler = handle_hpet_broadcast;
}
@@ -729,9 +717,7 @@ void hpet_broadcast_exit(void)
if ( !reprogram_timer(per_cpu(timer_deadline, cpu)) )
raise_softirq(TIMER_SOFTIRQ);
- read_lock_irq(&ch->cpumask_lock);
cpu_clear(cpu, ch->cpumask);
- read_unlock_irq(&ch->cpumask_lock);
if ( !(ch->flags & HPET_EVT_LEGACY) )
{
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