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SubjectRe: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs
On 12/18/2011 11:31 PM, Srivatsa S. Bhat wrote:
> 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!

We started seeing this same problem last week. I've come up with almost
the same solution but you beat me to the list!

> 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); \

Don't you want to call {get,put}_online_cpus() outside the
preempt_{disable,enable}()? Otherwise you are scheduling while atomic?

With that fixed

Acked-by: Stephen Boyd <sboyd@codeaurora.org>

but I wonder if taking the hotplug mutex even for a short time reduces
the effectiveness of these locks? Or is it more about fast readers and
slow writers?


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