Hi,
I feel the following patch is a better fix for 2 reasons:
1. As Al Viro pointed out, if we do for_each_possible_cpus() then we might
encounter unnecessary performance hit in some scenarios. So working with
only online cpus, safely(a.k.a race-free), if possible, would be a good
solution (which this patch implements).
2. *_global_lock_online() and *_global_unlock_online() needs fixing as well
because, the names suggest that they lock/unlock per-CPU locks of only the
currently online CPUs, but unfortunately they do not have any synchronization
to prevent offlining those CPUs in between, if it happens to race with a CPU
hotplug operation.
And if we solve issue 2 above "carefully" (as mentioned in the changelog below),
it solves this whole thing!
diff --git a/include/linux/lglock.h b/include/linux/lglock.h
index f549056..583d1a8 100644
--- a/include/linux/lglock.h
+++ b/include/linux/lglock.h
@@ -126,6 +127,7 @@
int i; \
preempt_disable(); \
rwlock_acquire(&name##_lock_dep_map, 0, 0, _RET_IP_); \
+ get_online_cpus(); \
for_each_online_cpu(i) { \
arch_spinlock_t *lock; \
lock =&per_cpu(name##_lock, i); \
@@ -142,6 +144,7 @@
lock =&per_cpu(name##_lock, i); \
arch_spin_unlock(lock); \
} \
+ put_online_cpus(); \
preempt_enable(); \
} \
EXPORT_SYMBOL(name##_global_unlock_online); \