# HG changeset patch
# User Jan Beulich <jbeulich@xxxxxxxxxx>
# Date 1301043797 0
# Node ID a65612bcbb921e98a8843157bf365e4ab16e8144
# Parent 941119d58655f2b2df86d9ecc4cb502bbc5e783c
x86/hpet: eliminate cpumask_lock
According to the (now getting removed) comment in struct
hpet_event_channel, this was to prevent accessing a CPU's
timer_deadline after it got cleared from cpumask. This can be done
without a lock altogether - hpet_broadcast_exit() can simply clear
the bit, and handle_hpet_broadcast() can read timer_deadline before
looking at the mask a second time (the cpumask bit was already
found set by the surrounding loop).
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>
Acked-by: Gang Wei <gang.wei@xxxxxxxxx>
---
diff -r 941119d58655 -r a65612bcbb92 xen/arch/x86/hpet.c
--- a/xen/arch/x86/hpet.c Fri Mar 25 09:01:37 2011 +0000
+++ b/xen/arch/x86/hpet.c Fri Mar 25 09:03:17 2011 +0000
@@ -34,18 +34,6 @@
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 @@
/* 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 @@
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 @@
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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