[Xen-devel] [PATCH] cpuidle: add comments for hpet cpumask_lock usage

["Wei, Gang" <gang.wei@xxxxxxxxx>]

cpuidle: add comments for hpet cpumask_lock usage

Signed-off-by: Wei Gang <gang.wei@xxxxxxxxx>

diff -r 764e41b09017 xen/arch/x86/hpet.c
--- a/xen/arch/x86/hpet.c       Thu Jun 17 08:53:12 2010 +0100
+++ b/xen/arch/x86/hpet.c       Fri Jun 18 11:41:28 2010 +0800
@@ -34,6 +34,17 @@ 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_start/end after the other cpu's mask was cleared --
+     * mask cleared means cpu waken up, then accessing timer_deadline_xxx 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_xxx.
+     */
     rwlock_t      cpumask_lock;
     spinlock_t    lock;
     void          (*event_handler)(struct hpet_event_channel *);

Attachment: comment-hpet-cpumask_lock.patch
Description: comment-hpet-cpumask_lock.patch

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