On Mon, Dec 13, 2010 at 6:25 PM, Eric Dumazet <eric.dumazet@xxxxxxxxx> wrote:
> Le lundi 13 décembre 2010 à 13:42 +1100, Nick Piggin a écrit :
>> + */
>> +static inline void add_mnt_count(struct vfsmount *mnt, int n)
>
> maybe name it __add_mnt_count() (should be called with preempt off)
>
>> +{
>> +#ifdef CONFIG_SMP
>> + (*per_cpu_ptr(mnt->mnt_count, smp_processor_id())) += n;
>
> __this_cpu_add(mnt->mnt_count, n);
>
>> +#else
>> + mnt->mnt_count += n;
>> +#endif
>> +}
>
> and define a preempt safe version :
>
> static inline void __add_mnt_count(struct vfsmount *mnt, int n)
> {
> #ifdef CONFIG_SMP
> __this_cpu_add(mnt->mnt_count, n);
> #else
> mnt->mnt_count += n;
> #endif
> }
>
> static inline void add_mnt_count(struct vfsmount *mnt, int n)
> {
> #ifdef CONFIG_SMP
> this_cpu_add(mnt->mnt_count, n);
> #else
> preempt_disable();
> mnt->mnt_count += n;
> preempt_enable();
> #endif
> }
That looks good, thanks.
>> +
>> +static inline void set_mnt_count(struct vfsmount *mnt, int n)
>> +{
>> +#ifdef CONFIG_SMP
>
>
>> + preempt_disable();
>> + (*per_cpu_ptr(mnt->mnt_count, smp_processor_id())) = n;
>> + preempt_enable();
>
> last 3 lines can be replaced by :
>
> this_cpu_write(mnt->mnt_count, n);
Yep, thanks.
>> +#else
>> + mnt->mnt_count = n;
>> +#endif
>> +}
>> +
>
>> #ifdef CONFIG_SMP
>> int __percpu *mnt_writers;
>> + int __percpu *mnt_count;
>> #else
>> int mnt_writers;
>> + int mnt_count;
>> #endif
>
> You could use a struct and one per cpu allocation to use one cache line
> for both objects :
>
> struct mnt_counters {
> int writers;
> int count;
> };
>
> ...
>
> #ifdef CONFIG_SMP
> struct mnt_counters __percpu *mnt_counters;
> #else
> struct mnt_counters mnt_counters;
> #endif
>
> This would use one pointer instead of two in SMP
Yes that's a good point too.
Thanks,
Nick
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