On Fri, 2 Mar 2012, Mel Gorman wrote:
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index e9eaec5..ba6d217 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -92,38 +92,25 @@ extern void cpuset_print_task_mems_allowed(struct task_struct *p);
> * reading current mems_allowed and mempolicy in the fastpath must protected
> * by get_mems_allowed()
> */
> -static inline void get_mems_allowed(void)
> +static inline unsigned long get_mems_allowed(void)
> {
> - current->mems_allowed_change_disable++;
> -
> - /*
> - * ensure that reading mems_allowed and mempolicy happens after the
> - * update of ->mems_allowed_change_disable.
> - *
> - * the write-side task finds ->mems_allowed_change_disable is not 0,
> - * and knows the read-side task is reading mems_allowed or mempolicy,
> - * so it will clear old bits lazily.
> - */
> - smp_mb();
> + return atomic_read(¤t->mems_allowed_seq);
> }
>
> -static inline void put_mems_allowed(void)
> +/*
> + * If this returns false, the operation that took place after get_mems_allowed
> + * may have failed. It is up to the caller to retry the operation if
> + * appropriate
> + */
> +static inline bool put_mems_allowed(unsigned long seq)
> {
> - /*
> - * ensure that reading mems_allowed and mempolicy before reducing
> - * mems_allowed_change_disable.
> - *
> - * the write-side task will know that the read-side task is still
> - * reading mems_allowed or mempolicy, don't clears old bits in the
> - * nodemask.
> - */
> - smp_mb();
> - --ACCESS_ONCE(current->mems_allowed_change_disable);
> + return seq == atomic_read(¤t->mems_allowed_seq);
> }
Use seqlock instead of the counter? Seems that you are recoding much of
what a seqlock does. A seqlock also allows you to have a writer that sort
of blocks the reades if necessary.
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