Re: [PATCH 8/9 v8] sunrpc: New helper cache_delete_entry for deleting cache_head directly

[NeilBrown <neilb-IBi9RG/b67k@public.gmane.org>]

On Mon, 27 Jul 2015 11:10:45 +0800 Kinglong Mee <kinglongmee-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
wrote:

> A new helper cache_delete_entry() for delete cache_head from
> cache_detail directly.
> 
> It will be used by pin_kill, so make sure the cache_detail is valid
> before deleting is needed.

I cannot see any justification for validating the cache_detail.

When this gets called, the cache_head has not yet been freed (though it
probably will be soon) so the cache_detail must still be around.

However it is possible for this to race with cache_clean() which could
have already removed the cache_head from the list (and decremented
->entries), but which hasn't called cache_put() yet.

The use of cache_list_lock is not enough to protect against that race.

So I think you should drop the use of cache_list_lock, drop the check
that detail is still in the list, and after getting ->hash_lock, check
hlist_unhash() and only unhash if that failed.

Thanks,
NeilBrown

> 
> Because pin_kill is not many times, so the influence of performance
> is accepted.
> 
> v8, same as v6.
> 
> Signed-off-by: Kinglong Mee <kinglongmee-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> ---
>  include/linux/sunrpc/cache.h |  1 +
>  net/sunrpc/cache.c           | 30 ++++++++++++++++++++++++++++++
>  2 files changed, 31 insertions(+)
> 
> diff --git a/include/linux/sunrpc/cache.h b/include/linux/sunrpc/cache.h
> index 03d3b4c..2824db5 100644
> --- a/include/linux/sunrpc/cache.h
> +++ b/include/linux/sunrpc/cache.h
> @@ -210,6 +210,7 @@ extern int cache_check(struct cache_detail *detail,
>  		       struct cache_head *h, struct cache_req *rqstp);
>  extern void cache_flush(void);
>  extern void cache_purge(struct cache_detail *detail);
> +extern void cache_delete_entry(struct cache_detail *cd, struct cache_head *h);
>  #define NEVER (0x7FFFFFFF)
>  extern void __init cache_initialize(void);
>  extern int cache_register_net(struct cache_detail *cd, struct net *net);
> diff --git a/net/sunrpc/cache.c b/net/sunrpc/cache.c
> index 4a2340a..b722aea 100644
> --- a/net/sunrpc/cache.c
> +++ b/net/sunrpc/cache.c
> @@ -454,6 +454,36 @@ static int cache_clean(void)
>  	return rv;
>  }
>  
> +void cache_delete_entry(struct cache_detail *detail, struct cache_head *h)
> +{
> +	struct cache_detail *tmp;
> +
> +	if (!detail || !h)
> +		return;
> +
> +	spin_lock(&cache_list_lock);
> +	list_for_each_entry(tmp, &cache_list, others) {
> +		if (tmp == detail)
> +			goto found;
> +	}
> +	spin_unlock(&cache_list_lock);
> +	printk(KERN_WARNING "%s: Deleted cache detail %p\n", __func__, detail);
> +	return ;
> +
> +found:
> +	write_lock(&detail->hash_lock);
> +
> +	hlist_del_init(&h->cache_list);
> +	detail->entries--;
> +	set_bit(CACHE_CLEANED, &h->flags);
> +
> +	write_unlock(&detail->hash_lock);
> +	spin_unlock(&cache_list_lock);
> +
> +	cache_put(h, detail);
> +}
> +EXPORT_SYMBOL_GPL(cache_delete_entry);
> +
>  /*
>   * We want to regularly clean the cache, so we need to schedule some work ...
>   */

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